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

[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).

[nickds7]

Never seen someone come ablaze overdiffractionlimitedaperture.

[District_History_Fan]

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.

[Daniel Browning]

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. []

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).

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.

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.

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.

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.
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[pin008]

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.





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[pin008]

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

[pin008]

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.

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).

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.

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.

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.

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.

[Daniel Browning]

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.

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.

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.

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.

[pin008]

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.

[Jon Ruyle]

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.