DLA is Nonsense! It should be deleted from the reviews.

Everybody knows small aperture means strong diffraction, but there is no limit from the pixel size and density.


According toDigital SLR Camera Reviews here, so-called DLA (Diffraction Limited Aperture) is the aperture where diffraction begins to visibly affect image sharpness at the pixel level. It says as sensor pixel density increases, the narrowest aperture we can use to get perfectly pixel sharp images gets wider.


This theory assumes the image is a perfect dot and nothing else, and compares its airy disk with pixel size. If the airy disk is larger then pixel size, then the diffraction is visible.
<p class="last"]But actually, the photo image is not a dot, but a continuous surface. When diffraction occurs, the whole the image, every part, every dot gets some kind of blur. So each pixel is definitely affected by some degree of diffraction. There&rsquo;s no such thing called perfectly pixel sharp images. Diffraction could be visible at any aperture, any pixel size theoretically. The size of pixel doesn&rsquo;t limit anything. Digital sensor and film have no difference here.

Hello, Poop...this is Fan.


Fan...this is Poop.





I'm ducking!

Are you advocating that we all use pinhole cameras with f/800 lenses? [:P]

This has been discussed at length here before. Here is some light reading on the subject.


LOL


Mark

I think what DLA needs is a little more understanding and kindness, not agression and hate.

Read the 1st sentence and understand it.

Quote:

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Originally Posted by Jon Ruyle

I think what DLA needs is a little more understanding and kindness, not agression and hate.






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I do not see it, aggression and hate that is.

Quote:

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Originally Posted by pin008

Read the 1st sentence and understand it.
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pin008 - We're generally a very friendly and helpful forum community here, and if there's a misunderstanding about anything (or disagreement), those who know generally try to help and weigh in. However, your first post seems a bit aggressive and your second post a bit sarcastic. If you'd like to have a meaningful discussion about DLA, I'm sure those that understand it best will be glad to explain its generally accepted significance in greater detail (or else link to information that has already been covered, as above).


Try to keep an open mind, and if you still disagree with something, explain your position in a respectful manner.

If you think I have some aggressive or sarcastic attitude, I hereby formly apologize! i didnt mean it.


as a non-native speeker, to express my debate is already hard. I can hardly arrange noble words.


Pleasefollow my debates, ignore my words and expressioins.





[:P]

Quote:

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Originally Posted by clemmb

Quote:

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Originally Posted by Jon Ruyle

I think what DLA needs is a little more understanding and kindness, not aggression and hate.
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I do not see it, aggression and hate that is.

---

Well, if someone said you were nonsense and suggested *you* be deleted, wouldn't you feel someone was acting agressively toward you? Might you even feel that someone hated you? Poor, misunderstood DLA. What did it ever do to deserve this kind of treatment?


(I might add that I was, and still am, being a little silly. I don't actually think the OP was being hateful.)


Though, on a (slightly) more serious note, I personally find DLA useful and would not like to see it purged, deleted, stricken from reviews, or otherwise persecuted, oppressed, or marginalized. Sure, opening up wider than the DLA doesn't mean diffraction is impossible to detect. Does that mean it is nonsense? if I stop down way past the DLA, I figure diffraction will have an effect on sharpness. When I do astrophotography, I feel there is little point using a barlow if the f/ number of my scope is greater than the DLA of my ccd.


It's a rule of thumb. Do what you will with it (even if that's nothing).

Never seen someone come ablaze overdiffractionlimitedaperture.

Daniel better chime in here before this thread gets out of hand...or perhaps it is already too late.


In any case, diffraction is an intrinsic physical phenomenon of passing a wave through a hole. It's there no matter how big or small the hole or wavelength is.


But the ability to observe the extent of diffraction depends strongly on the resolution of the device that is used to measure it. While it is true that a real-world image is not composed of discrete, collimated point light sources, it is also irrelevant. If one acknowledges the two basic principles that (1) diffraction exists, and (2) the degree of diffraction is inversely proportional to the size of the aperture, then it doesn't matter what the object space looks like. Your ability to see the diffraction basically depends on the highest spatial frequency that your imaging device is able to capture.


What do we mean by being able to observe the diffraction? After all, if some degree of diffraction occurs at any f-number, then there is some even at f/1.0 (assuming the lens is otherwise ideal). But what does it mean to have a sensor with such high resolution that we can "see" the effect? How do we know when we do see it? One way to think about it is to say that one is diffraction limited at the spatial frequency at which increasing the sensor resolution further confers no additional advantage, yet reducing the resolution causes you to lose detail. But that's counter-intuitive because we don't usually think of sensors as being of variable resolution.


Thus, we usually phrase DLA in terms of loss of sharpness at a fixed sensor resolution when stopping down further; i.e., a particular sensor "starts to show" a loss of sharpness at the DLA and this softness increases as f-number increases. Opening the aperture faster than the DLA doesn't make it sharper, either (again, assuming an ideal lens).


But a word to the OP: the reason why your post came across as "aggressive" is that your first post basically called the diffraction-limited aperture model nonsense, and dismissed it without providing any real evidence to support your claim. You didn't even introduce yourself or engage in those usual polite pleasantries that are considered essential to demonstrate good social skills.

<p class="MsoNormal" style="margin: 0cm 0cm 0pt;"]<span lang="EN-US"]<span style="font-family: Calibri; font-size: small;"]Let me prove it.
<p class="MsoNormal" style="margin: 0cm 0cm 0pt;"]<span lang="EN-US"]<o:p><span style="font-family: Calibri; font-size: small;"]</o:p>
<p class="MsoNormal" style="margin: 0cm 0cm 0pt;"]<span lang="EN-US"]<span style="font-family: Calibri; font-size: small;"]A is a peace of image at any f-number. It may already be diffracted anyway. It forms a pixel signal as A&rsquo;.
<p class="MsoNormal" style="margin: 0cm 0cm 0pt;"]<span lang="EN-US"]<o:p><span style="font-family: Calibri; font-size: small;"]</o:p>
<p class="MsoNormal" style="margin: 0cm 0cm 0pt;"]<span lang="EN-US"]<span style="font-family: Calibri; font-size: small;"]When f-number increases, no matter how much, Image A will became B, and forms a pixel signal as B&rsquo;.
<p class="MsoNormal" style="margin: 0cm 0cm 0pt;"]<span lang="EN-US"]<o:p><span style="font-family: Calibri; font-size: small;"]</o:p>
<p class="MsoNormal" style="margin: 0cm 0cm 0pt;"]<span lang="EN-US"]<span style="font-family: Calibri; font-size: small;"]B&rsquo; is definitely differenced from A&rsquo;, and it will show in the digital image. It happens at any apertures and any pixel sizes. Pixel size doesn&rsquo;t limit its influence to be visible or not.
<p class="MsoNormal" style="margin: 0cm 0cm 0pt;"]<span lang="EN-US"]<span style="font-family: Calibri; font-size: small;"]
<p class="MsoNormal" style="margin: 0cm 0cm 0pt;"]<span lang="EN-US"][img]/cfs-file.ashx/__key/CommunityServer.Components.UserFiles/00.00.00.46.61/B.jpg[/img]

I don't know much about the subject, but read the other thread that clemmb linked because the topic was discussed at length there.


The basic idea that I got from reading there is that yes diffraction occurs equally regardless of pixel size or recording method, but the idea of DLA is that as your pixels size gets smaller / density gets higher, the camera can resolve more detail and that detail becomes noticeably diffracted earlier. Thus the higher the pixel density the sooner the diffraction can be noticed. That is DLA. No? At f/11 big pixels won't notice the diffraction because they couldn't have resolved the detail that is being lost via diffraction anyway, but smaller pixels would have been able to and therefore the loss is noticed. This isn't to say that the diffraction is worse with greater pixel density.


To me an analogy that makes sense is lens quality. Viewing images at 100% from a low pixel density camera can and does look sharper than the same setup (lens, sensor size, etc) with higher pixel density, because you are resolving more detail and can notice more problems. However if you take that higher pixel density image and lower the resolution or view it at the same size as the other image, it will look the same or better, not worse.


Basically if your sensor is better, it more quickly stops being the bottleneck on image quality as diffraction sets in or when using a lower quality lens. This is true of almost anything, it's just a matter of finding the weakest link in a chain of making high quality images in this case.


Somebody correct me if I'm wrong here.

Quote:

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Originally Posted by wickerprints

Daniel better chime in here before this thread gets out of hand...or perhaps it is already too late.


In any case, diffraction is an intrinsic physical phenomenon of passing a wave through a hole. It's there no matter how big or small the hole or wavelength is.


But the ability to observe the extent of diffraction depends strongly on the resolution of the device that is used to measure it. While it is true that a real-world image is not composed of discrete, collimated point light sources, it is also irrelevant. If one acknowledges the two basic principles that (1) diffraction exists, and (2) the degree of diffraction is inversely proportional to the size of the aperture, then it doesn't matter what the object space looks like. Your ability to see the diffraction basically depends on the highest spatial frequency that your imaging device is able to capture.


What do we mean by being able to observe the diffraction? After all, if some degree of diffraction occurs at any f-number, then there is some even at f/1.0 (assuming the lens is otherwise ideal). But what does it mean to have a sensor with such high resolution that we can "see" the effect? How do we know when we do see it? One way to think about it is to say that one is diffraction limited at the spatial frequency at which increasing the sensor resolution further confers no additional advantage, yet reducing the resolution causes you to lose detail. But that's counter-intuitive because we don't usually think of sensors as being of variable resolution.


Thus, we usually phrase DLA in terms of loss of sharpness at a fixed sensor resolution when stopping down further; i.e., a particular sensor "starts to show" a loss of sharpness at the DLA and this softness increases as f-number increases. Opening the aperture faster than the DLA doesn't make it sharper, either (again, assuming an ideal lens).


But a word to the OP: the reason why your post came across as "aggressive" is that your first post basically called the diffraction-limited aperture model nonsense, and dismissed it without providing any real evidence to support your claim. You didn't even introduce yourself or engage in those usual polite pleasantries that are considered essential to demonstrate good social skills.
<div style="CLEAR: both"]</div>

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You gentlman, i respect your opinion "that phrase DLA in terms of loss of sharpness at a fixed sensor resolution when stopping down further; i.e., a particular sensor "starts to show" a loss of sharpness at the DLA and this softness increases as f-number increases. Opening the aperture faster than the DLA doesn't make it sharper, either (again, assuming an ideal lens)."


but my opinion may be for a real lens, large aperture brings more geometry aberration. narrow down the aperture increases the sharpness in this aspact when lost sharpness by diffraction. the best IQ comes at the balance.


And for ideal lens, the only loss of sharpness came from diffratction, then it "starts to show"at the largest aperture, as the graph shows above. And and softness increases as f-number increases.





This is the traditional theory on film camera. I think itapplys to digtal camera too. DLAmay notbe caculated by pixel density.

Quote:

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Originally Posted by StapledPhoto

I don't know much about the subject, but read the other thread that clemmb linked because the topic was discussed at length there.


The basic idea that I got from reading there is that yes diffraction occurs equally regardless of pixel size or recording method, but the idea of DLA is that as your pixels size gets smaller / density gets higher, the camera can resolve more detail and that detail becomes noticeably diffracted earlier. Thus the higher the pixel density the sooner the diffraction can be noticed. That is DLA. No? At f/11 big pixels won't notice the diffraction because they couldn't have resolved the detail that is being lost via diffraction anyway, but smaller pixels would have been able to and therefore the loss is noticed. This isn't to say that the diffraction is worse with greater pixel density.


To me an analogy that makes sense is lens quality. Viewing images at 100% from a low pixel density camera can and does look sharper than the same setup (lens, sensor size, etc) with higher pixel density, because you are resolving more detail and can notice more problems. However if you take that higher pixel density image and lower the resolution or view it at the same size as the other image, it will look the same or better, not worse.


Basically if your sensor is better, it more quickly stops being the bottleneck on image quality as diffraction sets in or when using a lower quality lens. This is true of almost anything, it's just a matter of finding the weakest link in a chain of making high quality images in this case.


Somebody correct me if I'm wrong here.






<div style="CLEAR: both"]</div>

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ye, I somehow agree with you.


but in my opinion, diffraction shows not only as on dot becomes a airy disk, or on single line became wider these kind of details, but the change of contrast in every single detail among the image. And alse, airy disk is only 84%energy aera (i don't know how to strict define it in english), the actual diffraction spread far larger.


As diffraction influence all the details, how could the size of pixel matter.


Actual the <span class="trans"]sensitivity to the contrast ofeach pixel would influence the visiblilty of the diffraction.

Hmmm I don't really see your point and all this technical stuff isn't helping me understand it all without reading it a couple times[:P]


"DLA is nonsense! It should be deleted from the reviews."


Alright...first of have you tried the images sharpness at different apertures? So could you notice a decrease of sharpness beyond the DLA?


I personally do see the difference and I always try to keep the aperture at the DLA at maximum, even with macro where a larger DOF is welcome. I like to get a sharper image over a softer but with greater DOF image.


Anyhow, I don't think it should be deleted, because it proved it's use to me and I thank Bryan for putting it in it's reviews. I think I'm not the only one. And I believe 1 extra column in a list doesn't hurt anyone... perhaps 1 now[:P]


But please continue the great debate on whether it is good or not or right or wrong, I might be reading it all more carefully if I have more time later this week[A]


Jan

My experiences with the current high res cameras (50D, 5D2) is that DLA isn't something to worry about in real world use. I shoot with confidence at f16 on the 50D and f22 on the 5D2 and have no problems. If you need to stop down, go for it.

Quote:

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Originally Posted by wickerprints

Daniel better chime in here before this thread gets out of hand...or perhaps it is already too late.

---

After your excellent post, I don't think there's anything I could possibly add to the discussion. Of course, that's never stopped me before. [:D]

Quote:

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Originally Posted by pin008

DLA is Nonsense! It should be deleted from the reviews.

---

I respectfully disagree. It is not nonsense, and I like having it on the reviews. It tells me what f-numbers are capable of the highest sharpness (depending on the lens, of course) as well as the point of diminishing returns (in the context of a multi-camera comparison).

Quote:

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Originally Posted by pin008

there is no limit from the pixel size and density.

---

Yes, there is. Every time I try to set my 24mm L II to f/32, a deep voice booms "thou shalt not stop down!" and a horde of crows materialize out of nowhere, pecking at me until I set it back to f/1.4. [:D] While there's nothing to physically restrain the photographer from using any particular f-number that they are capable of (and I don't mean to imply that you said there was), the sharpness of the resulting photo does have a limit, as well as a range of f-numbers over which the sharpness will be limited.

Quote:

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Originally Posted by pin008

This theory assumes the image is a perfect dot and nothing else, and compares its airy disk with pixel size. If the airy disk is larger then pixel size, then the diffraction is visible.

---

As wickerprints said, it also applies to continuous surfaces.

Quote:

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Originally Posted by pin008

There&rsquo;s no such thing called perfectly pixel sharp images.

---

I think there is. The most extreme definition of "perfectly pixel sharp" that I can think of would be 95% MTF at Nyquist with no sharpening, and that can be achieved with sensors that are designed improperly (without OLPF) at low spatial frequencies. But I'm sure Bryan had in mind a more typical (and reasonable) definition (e.g. no drop in contrast at Nyquist due to diffraction that is visibly noticeable after a small amount of sharpening) -- which is what others said in the thread.

Quote:

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Originally Posted by pin008

The size of pixel doesn&rsquo;t limit anything. Digital sensor and film have no difference here.

---

Say someone is shooting f/64 macro photos on their 6 MP rebel and upgrades to a 7D in order to take advantage of the smaller pixel size. They will be in for a shock when they see that the modern 18 MP images are no more detailed than their ancient camera.





Contrast that with the example of a photographer who shoots portraits at f/5.6. When he upgrades from the 6 MP Rebel to the 7D, the linear resolution can be almost doubled.





Between the two examples is someone who shoots at f/22. There will be an increase in resolution, but since f/22 is narrower than the DLA of the 7D, returns will be diminished.
<div></div>

According to the DLA theory which should be like graph A. pictures lose sharpness quickly after DLA, so DLA is an important critical value.


but what i see islike graph B.





[img]/cfs-file.ashx/__key/CommunityServer.Components.UserFiles/00.00.00.46.61/C.gif[/img]

the graph is a sketch map, but i think you know what i mean.

Quote:

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Originally Posted by Daniel Browning

Quote:

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Originally Posted by wickerprints

Daniel better chime in here before this thread gets out of hand...or perhaps it is already too late.

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After your excellent post, I don't think there's anything I could possibly add to the discussion. Of course, that's never stopped me before.

Quote:

---

Originally Posted by pin008

DLA is Nonsense! It should be deleted from the reviews.

---

I respectfully disagree. It is not nonsense, and I like having it on the reviews. It tells me what f-numbers are capable of the highest sharpness (depending on the lens, of course) as well as the point of diminishing returns (in the context of a multi-camera comparison).

Quote:

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Originally Posted by pin008

there is no limit from the pixel size and density.

---

Yes, there is. Every time I try to set my 24mm L II to f/32, a deep voice booms "thou shalt not stop down!" and a horde of crows materialize out of nowhere, pecking at me until I set it back to f/1.4. While there's nothing to physically restrain the photographer from using any particular f-number that they are capable of (and I don't mean to imply that you said there was), the sharpness of the resulting photo does have a limit, as well as a range of f-numbers over which the sharpness will be limited.

Quote:

---

Originally Posted by pin008

This theory assumes the image is a perfect dot and nothing else, and compares its airy disk with pixel size. If the airy disk is larger then pixel size, then the diffraction is visible.

---

As wickerprints said, it also applies to continuous surfaces.

Quote:

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Originally Posted by pin008

There&rsquo;s no such thing called perfectly pixel sharp images.

---

I think there is. The most extreme definition of "perfectly pixel sharp" that I can think of would be 95% MTF at Nyquist with no sharpening, and that can be achieved with sensors that are designed improperly (without OLPF) at low spatial frequencies. But I'm sure Bryan had in mind a more typical (and reasonable) definition (e.g. no drop in contrast at Nyquist due to diffraction that is visibly noticeable after a small amount of sharpening) -- which is what others said in the thread.

Quote:

---

Originally Posted by pin008

The size of pixel doesn&rsquo;t limit anything. Digital sensor and film have no difference here.

---

Say someone is shooting f/64 macro photos on their 6 MP rebel and upgrades to a 7D in order to take advantage of the smaller pixel size. They will be in for a shock when they see that the modern 18 MP images are no more detailed than their ancient camera.





Contrast that with the example of a photographer who shoots portraits at f/5.6. When he upgrades from the 6 MP Rebel to the 7D, the linear resolution can be almost doubled.





Between the two examples is someone who shoots at f/22. There will be an increase in resolution, but since f/22 is narrower than the DLA of the 7D, returns will be diminished.

---

Thanks for your explanation. but i still hold opposite opinions.





if i understand correctly, you define DLA as





A.what f-numbers are capable of the highest sharpness (depending on the lens, of course)





B.the point of diminishing returns (in the context of a multi-camera comparison)





About A, you indicate that for an ideal lens without any geometrical aberration, the sharpness increases from the largest aperture to the DLA, and than decrease.


I don't think a ideal lens has the best sharpness at DLA. the largest aperture is clearly influenced less by the diffraction. any aperture smaller has more diffraction problems.







And B is just repeating A again. DLA means nothing in the real multi-camera comparison. because you can't use DLA to calculate when a 18MP sensor have the same resolution or same amount of sharpness loss as a 8MP sensor. the point of diminishing returns means no more than best sharpness point.

Quote:

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Originally Posted by pin008

if i understand correctly, you define DLA as





A.what f-numbers are capable of the highest sharpness (depending on the lens, of course)





B.the point of diminishing returns (in the context of a multi-camera comparison)

---

No. Those are two reasons why I like having DLA on the site. I did not say that they were any kind of definition of DLA. I did not think it was necessary to define DLA, because it is stated plainly on every review on this site and you yourself quoted portions of it.

Quote:

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Originally Posted by pin008

I don't think a ideal lens has the best sharpness at DLA. the largest aperture is clearly influenced less by the diffraction. any aperture smaller has more diffraction problems.

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No. The difference in the effects of diffraction at apertures wider than the DLA are so minute that it is practically imperceptible. It has no influence on resolution whatsoever, and the the difference in contrast is barely measurable. See for yourself in the comparison of these center crops (not the corners):


1D3 + 200mm f/2 L IS at f/2.8 vs. 1D3 + 200mm f/2 L IS at f/5.6.


This result is expected because the ratio between the combined intensity of the second minimum and the airy disk gives you an idea of the smallness of the difference in diffraction that can be seen between f-numbers below the DLA.

Quote:

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Originally Posted by pin008

DLA means nothing in the real multi-camera comparison. because you can't use DLA to calculate when a 18MP sensor have the same resolution or same amount of sharpness loss as a 8MP sensor.

---

Of course you can't use the DLA for either of those things. You also can't use DLA to find the sharpest f-number of an aberrated lens. Nor can you use it to find the diffraction cutoff frequency. Nor can it be used to do your laundry or predict winning lottery numbers. There are a million things you can't use the DLA for. That doesn't mean it "means nothing in the real multi-camera comparison."





One of the things the DLA is useful for is predicting which f-number is required in order to avoid diminishing returns of a camera upgrade. For example, if a forum member asks "What f-number do I need to use in order to avoid any blurring effect from diffraction when I upgrade from the 6 MP Rebel to the 18 MP 7D?" The answer is the DLA. Or if they ask "Why don't I get the full expected increase in resolution when I upgrad from 6 MP to 18 MP -- the lens has no aberration and I'm using f/22?". The answer is "because you are beyond the DLA and into the territory of diminished returns." Those are just two examples of when the DLA is useful in the context of multiple cameras.

It is not appropriate to conflate losses in image quality due to imperfections in the lens (i.e., optical aberrations, non-ideal transmission, color aberrations), with the losses due to diffraction, if the goal is to investigate how diffraction plays a role in image sharpness. In science, the goal of any good analysis is to isolate, investigate, and quantify individual factors that contribute to an observed phenomenon in order to explain the overall result. Sometimes, it is not possible to directly observe each individual effect, so one observes indirectly and uses other methods to deduce what would have happened if other interfering phenomena were idealized. This is also the case for diffraction in camera lenses.


The whole point of the discussion is to address your allegation that the notion of DLA is nonsense. As such, it is incorrect to then start talking about the relative contribution of other aberrations because what we are interested in is the theory of diffraction as a model for predicting how a lens would perform in the real world, and the usefulness of that model. Your claim is that this model should not depend on the pixel density of the recording medium. As I have mentioned already, that claim is incorrect.


I will now explain with your own diagram why you are wrong. If we suppose the real-world object is something like diagram A, and that diagram A represents the theoretical (infinite pixel density) image projected by an ideal lens at a wide aperture, say f/1.0, then diagram A' might represent the observed result by the sensor. It just so happens that the image aligns with the sensor lattice structure.


If diagram B represents the projected image of the real-world object at a small aperture, say f/16, then diagram B' would correctly represent the observed result by the same sensor with the same precise alignment as in diagrams A and A'.


But now suppose you increase the pixel density of the sensor further, by subdividing each of the squares in the diagrams into 100 smaller squares (so an increase of 10x linear density). Diagram A' would look no different, but diagram B' would be very different. Your ability to observe the effect and extent of diffraction is also changed--it has increased.


Now suppose that the pixel density of the sensor is decreased, so that the entire square of A' and B' is represented by a single pixel. Then in both cases, the value of that pixel is the same because the sensor has insufficient resolution to distinguish diagrams A and B--both have the same overall amount of black. Therefore, a sensor at that density has no ability to observe diffraction effects at f/16.


I have been more than patient and thoughtful in analyzing what you are trying to say. I don't think you have explained yourself well at all, and besides the apparent communication issues, your position is confusing and unscientific. I also think that you are unwilling or unable to consider the possibility that you are wrong, and therefore you do not read other people's responses--including mine--with a mindset of accepting what we say as valid. As a result, I don't think anyone will be able to persuade you, because you have basically made up your mind and are not interested in understanding why you are incorrect, but instead, have posted here only because you wish to force your viewpoint onto others, despite the overwhelming evidence to the contrary. My belief is supported by the fact that you come in here with no prior posting history, do not introduce yourself, and make a poorly-supported claim using an aggressive tone.


With this and my previous posts, I have now explained in detail everything that any reasonable person should need to know about the effect of diffraction on image sharpness and its relationship to pixel density; I do not have any further reason to discuss the matter further and consider it adequately addressed.

Quote:

---

Originally Posted by Daniel Browning

No. The difference in the effects of diffraction at apertures wider than the DLA are so minute that it is practically imperceptible. It has no influence on resolution whatsoever, and the the difference in contrast is barely measurable. See for yourself in the comparison of these center crops (not the corners):


1D3 + 200mm f/2 L IS at f/2.8 vs. 1D3 + 200mm f/2 L IS at f/5.6.


This result is expected because the ratio between the combined intensity of the second minimum and the airy disk gives you an idea of the smallness of the difference in diffraction that can be seen between f-numbers below the DLA.

---

i don't understand why you think or how you can prove that the effects of diffraction are so minute that it is practically imperceptible at apertures wider.


in my diagram, since the pixels identify signal in 14bit, which is 4.4 trillion grey levels, the change is definitely measurable. and the diagram is showing a very tiny diffraction as an example. Imagining more diffraction effects happens here. Besides, the center crop has influence 8 times more pixels in B' than A'.


as it is impossible to distinguish the effects of diffraction from geometrical aberration, the example of 1D3 + 200mm f/2 L IS at f/2.8 vs. 1D3 + 200mm f/2 L IS at f/5.6 can't argue me down.



if Rayleigh criterion holds true in photography, and the highest resolution is limited by airy disk size, then 18MP sensor should have the same resolution as a 10MP sensor when the aperture is narrower than 10MP's DLA. But the truth is 7D + 200mm f/2 L IS at f/11 vs. 1000D + 200mm f/2 L IS at f/11.

to wickerprints,


my diagram is to show how tiny morediffractioninfluence large size pixel.

Quote:

---

Originally Posted by pin008

if Rayleigh criterion holds true in photography, and the highest resolution is limited by airy disk size, then 18MP sensor should have the same resolution as a 10MP sensor when the aperture is narrower than 10MP's DLA.

---

No, no. No one is saying that.


You seem to think the point of DLA is: if your aperture is larger than the DLA, you get no diffraction. If your aperture is smaller than DLA, more pixels would give you no gain at all.


No one (on this thread at least) has made such a claim. DLA is just the point at which airy discs are about the same size as pixels. That's all it means. Many of us find it useful to know when we've reached that point.


Daniel says diffraction is negligible when you're aperture us much wider than DLA, you say diffraction is still detectable. I'm not sure you're not both right. (I would say diffraction is detectable in theory at say, f/4 when DLA is say, f/11, but the effect is so tiny, who the heck cares?)


But weather or not diffraction is detectable or neglegable or whatever is beside the point. Even if you can still detect diffraction at f/1 when DLA is f/11, it does *not* mean that DLA isn't useful- at least to me. I know that if my f number is smaller than the DLA, diffraction will be so small that I, personally, don't care about it.


Similarly, if I have an f/8 lens and my DLA is f/6, it does *not* mean there will be no resolution gained by adding more pixels. But it *does* mean that diffraction is starting to have a major effect, and I won't be able to gain full advantage of added pixels.


In other words, showing that there is some diffraction at apertures wider than the DLA does not prove that DLA is useless. Likewise, showing there is some resolution gained when pixels are added to an already "diffraction limited" sensor does not mena DLA is useless. It just means that DLA does not mean what you think we think it means.

Learn how to communicate, otherwise your words are meaningless.

DLA is calculated under Rayleigh criterion(1.22&lambda;f/D). Rayleigh criterionsays an18MP sensor should have the same resolution as a 10MP sensor when the aperture is narrower than 10MP's DLA, which is not true. That's why I doubt DLA.

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Originally Posted by wickerprints

Learn how to communicate, otherwise your words are meaningless.
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Wow! that's truth. I'll remember it.


And learn to understand the others, try tofollow their idea, keep an open mind.

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Originally Posted by pin008

DLA is calculated under Rayleigh criterion(1.22&lambda;f/D).

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A particular lambda is used for DLA, but yes, that's about right.

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Originally Posted by pin008

Rayleigh criterionsays an18MP sensor should have the same resolution as a 10MP sensor when the aperture is narrower than 10MP's DLA, which is not true.

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It does *not* say that. Did you read my above post? Do you disagree? If so, why?

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Originally Posted by pin008

That's why I doubt DLA.

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DLA is not a statement. It can't be true or false. Saying "I doubt DLA" is like saying "I doubt table."


If you mean "I doubt DLA places hard limits on resolution", then you aren't alone. But no one is claiming that it does.

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Originally Posted by pin008

That's why I doubt DLA.

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Be careful what you doubt. DLAcan be fractally beautiful. DLA gives us snowflakes. DLAmay even be the reason we have a planet to live on. DLA might sue you. Worse yet, DLA might bomb you to oblivion.


But mostly, for me, it's a useful number that can help guide, but not dictate, mandate, or override creative influence on, the choice of aperture in photography. [:P]

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Originally Posted by Jon Ruyle

It just means that DLA does not mean what you think we think it means.

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I've been trying to follow this thread, and I think this line sums up the entire debate. This line is great!


Stephen

I think the real question is "Can you compose a pleasing image?"


Sorry....just trying tolighten ita bit. One must be able to "point - counterpoint" on a forum....othwerwise it's kinda stale.


I like the graphs....but the pragmatist in me says that my 85 1.2 L is tough to get sharpness wide open unless you practice (due to razor thin depth of field)....regardless of who's right on the technical explanation.


I guess though that's the point.....why? Maybe we should start a "light scattering physics" website as an offshoot of Brian's site. Meanwhile the rest of us schleps will go about composing, capturing, and pleasing.


Cheers,


Jeff

You agree that DLA is based on Rayleigh criterion. My textbook tells Rayleigh criterion determineslimiting resolution of a telescope set by diffraction. it is a hard limit on resolution (1.22&lambda;f/D).


If you don't think DLA places hard limits on resolution, please tell me why?

Count me as one of the schleps Jeff....this thread is like a discussion on religion, politics, camera brands (hmm, that has a familiar ring to it), etc.


Ithas continued to ad nauseum....just agree to disagree, appreciate Bryan's effortsandget back to shooting pix.


Finire


Bill

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Originally Posted by neuroanatomist

But mostly, for me, it's a useful number that can help guide, but not dictate, mandate, or override creative influence on, the choice of aperture in photography. [img]/emoticons/emotion-4.gif[/img]
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What I use for guideis the THE BEST APERTURE.

DLA is the aperture where diffraction begins to visibly affect image sharpness at the pixel level.








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Hi all.


I have to say, it's so difficult to figure out the "tone" everyone is using when there is no sound with the words ;)


My comment for this topic is that the other side of the Rayleigh equation needs to be stated in full:


sin(theta) = 1.22(lambda)/D


The DLA for each camera is calculated based on its angular resolution, which is different for each camera due to each camera having different sized pixels. The angular resolution can be defined as:


sin(theta) = L/f where L is the spatial resolution and f is the focal length of the lens.


So these are all lens parameters and have nothing to do with the sensor. But, if you use the definition that L = 1 pixel width (probably should be at least 2, but we'll leave that for another discussion), then the DLA becomes:


D/f = 1.22(lambda)/(L) which is f-# = (L)/(1.22(lambda))


So as L gets smaller (higher resolution for a given sensor size i.e. APS-C), the minimum f-# becomes smaller.


Remember, this is for an ideal lens with no aberrations. If you add aberrations, then all bets are off as it becomes difficult to separate the two phenomena without getting specific MTFs (shudder, shudder).


The DLA is useful for one thing and one thing only, at what aperture can you expect diffraction to become an important factor in image quality. It makes no statements on the quality of the lens (direct correlation to how well aberrations are controlled), or the actual focal length.


Last thing, I looked at the DLA before I got my 7D and all that it told me was that if I could stay below the DLA, I should try. But, fireworks, misty water, large depth of field and sunlight say use whatever f-# you need to get the image and DLA be damned ;)

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Originally Posted by pin008

What I use for guideis the THE BEST APERTURE.

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IMO, there is no such thing as a 'BEST' aperture, or any other setting. Choice of aperture is influenced bythe available aperture range of your lens,the desired exposure, the desired depth of field, the effect of diffraction, etc. Photography, like much of life, involves compromise. Sometimes I'll choose to use a very small aperture even though I know that will result in loss of sharpness from diffraction, because I want a deeper depth of field. Sometimes I choose a high ISO even though I know that will result in elevated noise, because I want a shutter speed fast enough to freeze motion.