Hi,
I don't quite get it.. Why do some lenses foces fatser than others?? even if both use the USM one lens will focus faster. why is that??
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Hi,
I don't quite get it.. Why do some lenses foces fatser than others?? even if both use the USM one lens will focus faster. why is that??
I think USM can even have different implementations on different lens models.
Plus, different lens' USM will need to move different set(s) of glasses to autofocus. For very fast lenses, it takes a lot of glasses and the motor might need to move the heavier glasses (depending on implementation). Similarly, different zoom range and largest aperture the lens has will present different work load to the USM.
It's the aperature rating of the lens. The smaller the aperature or f5.6 the slower the lens which means less light is permitted into the lens and thus more time is required to expose the image on the sensor, whereasfast lenses; f1.2 for example, have a very wide maximum aperture and so shorter time periods are required to expose the image sensor.
This is why you will see lenses labeled for example 70-200 f2.8 IS USM vs 70-200 f4 IS USM. The 2.8 lets in 2x more light than the f4 lens. One has to think in opposites when referring to aperatures.
Hope this helps or at least answers your query?!
[:)]
i'm sorry but you seem to be referring to lens apertures.
my question was about lens autofocus, and why some lens focus faster than others.
like the 85 1.2 which is a "fast" lens since it does let in more light. That part I do get. But it tends to focus slower, now why is that??
Several factors influence AF speed.
- Optical design of the lens. A lens whose focusing group is large/heavy (e.g. EF 85/1.2L II), or whose optical formula is such that focusing requires a longer travel (EF 100/2.8L macro IS), will focus more slowly.
- Maximum aperture of the lens. An f/2.8 or wider-aperture lens will generally focus faster than an f/4 or slower lens, primarily due to the AF sensor's added sensitivity at f/2.8 or faster; but this is not always the case.
- USM design. AFD motors are slower and noisier than USM motors. The larger the USM motor, the more torque it generates, and the faster it can move the focusing group.
- Ambient light level. AF performance is impaired at very low light levels in the absence of any AF assist beam.
- Subject contrast. Low subject contrast, or contrast in a direction opposite to the AF sensor's sensitivity, inhibits AF acquisition.
- Subject distance. Close focusing is generally harder than distant focusing, due to shallower DOF.
- Optical stability. The stability of the image presented to the AF sensor affects its performance.
I may have missed a few other factors as well, but these tend to be the main ones. Of the ones that are relevant to the lens itself, you can see how it really comes down to a combination of factors, and not just one. It depends on how Canon builds the lens. The 85/1.2L II is a good example of a relatively slow-focusing lens, mainly because its focusing group is HEAVY--a large, thick aspherical element needs to be moved.
Well, partly it is aperture - f/2.8 and faster lenses let in enough light to activate the higher-sensitivity central autofocus point, meaning both more accurate and faster determination of proper focus.
But, mostly the autofocus speed is due to the lens design - and again, aperture plays a role here. Longer focal length lenses that have wide apertures mean more glass to move than a shorter, equally wide-aperture lens. That's because the f number is a ratio of focal length to aperture diameter (and the glass elements must be as wide as the aperture). A 200mm f/2 lens must have a 100mm diameter aperture (200/100=2) and thus glass elements that are also 100mm in diameter, whereas a 200mm f/2.8 lens only requires a 72mm diameter. More glass means slower to move. Zoom lenses have additional elements that must be moved to maintain parfocality (the lens remains in focus as it's zoomed), again meaning slower focusing.
Also, not all USMs are created equal. Older lenses (based on year of release) generally have slower USM motors, because technological advances have led to faster motors.
Finally, there are special cases, like the EF 85mm f/1.2L that you mention. Although that lens has a USM motor, it's also a front-focusing lens (the front of the lens extends during focusing). Internal or rear focusing lenses generally focus faster.
<p align="left"]<span style="font-family: Arial, Helvetica, sans-serif;"]This one particular lens that you mention, the depth of field is almostpaper-thin, so at f/1.2 almost nothing is in focus. Your camera's autofocus system will be taxed to its limit and you'll have a tough time getting your intended subject in focus if you're worrying about lab-grade sharpness.
<p align="left"]<span style="font-family: Arial;"]<span style="font-family: Arial, Helvetica, sans-serif;"]For the 85 1.2 II lens, the system needs to be much more precise than other lenses to ensure accurate focus at f/1.2. Depth of field at f/1.2 is a few inches (5 cm) at one hundred feet (30m), so extreme AF accuracy is required to make real use of the f/1.2 aperture. <span style="font-family: Arial, Helvetica, sans-serif;"]The lens needs to be positioned mechanically within distances measured in wavelengths of light, so this takes a while longer than less precise, slower f/stop lenses.
<p align="left"]<span style="font-family: Arial;"]The above information was taken from another site, however; <span style="font-family: Arial;"]I'm sure the other kind folk here would confirm or better explain this occurance with this as well as some of the other lenses.
<p align="left"]<span style="font-family: Arial;"]
<p align="left"]<span style="font-family: Arial;"]
<p align="left"]<span style="font-family: Arial;"]
LOL, looks like others have beat me to the 'punch'!
[;)]
Quote:
Originally Posted by elmo_2006
I'm afraid this is not quite correct. No 35mm AF system that I know of has a baseline wider than f/2.8--not Canon, not Nikon. While it is true that an f/1.2 lens will admit several times more light than an f/2.8 lens, it is only in very low light conditions where this might make any difference, and I'm not even sure if it does.
However, because the AF sensor itself has a maximum sensitivity of f/2.8--whether that is horizontal, vertical, cross, or diagonal--that means that there is no added precision conferred by using a lens with a larger maximum aperture, because the sensor itself only sees the f/2.8 image. If you mount an f/5.6 lens on the body, the f/2.8 sensors don't work at all because the light is blocked, much in the way that a split-level prism focusing screen goes dark when you stop down. To extend the analogy, if you made that split-level prism larger--i.e., it occupied a larger area of the focusing screen--then you would be able to focus even more precisely, but the cost of that extra precision is that it goes dark at even larger apertures. The same is true for AF sensors--if Canon put in a sensor with AF @ f/1.4, it would only be useful for lenses that are f/1.4 or faster, and would not give any added precision between an f/1.2 lens and an f/1.4 lens.
In summary, the real reason why the 85/1.2L II has slow AF is related to the weight of the focusing group and the power of the USM motor to drive its movement, because in actuality, the AF system is really only trying to attain focus within 1/3rd of the f/2.8 depth of focus. It just so happens in most cases that this is good enough for a lens that is relatively free of focus shift.
I will say, Wicker, you are providing me with an extensive education on this [8-|] Very nice read.
brendan [H]
yes WP... I do believe I am starting to get a clearer picture as to why focus is slower on some lenses.
THANKS!!!
I've had this <span style="text-decoration: line-through;"]argument [wait, this isn't dpreview, we can stay friendly :)] discussion before. It's a fun one to have.
Now, just to muddy the waters a bit:
Quote:
Originally Posted by wickerprints
Well, low light and low contrast situations. Contrast can be thought of as (brightest bright - the darkest dark) / (the brightest bright + the darkest dark). Unless you're washing out (not the situation we're concerned with), black stays black, so if you let in less light, you necessarily have less contrast in all situations. So very low light scenes are de facto low C, but a low contrast scene with plenty of light on it would also benefit from a faster lens.
Quote:
Originally Posted by wickerprints
So, I think you're probably right that for various reasons (DOF, mechanical, so on) the benefit to AF from wider lenses is not linear. That is, it's not as if going wider maps perfectly to faster AF. However, whatever is functionally true with today's lenses and technology-- optically, the wider the lens the better the AF could perform. As I mentioned above, the brighter the scene, the higher the contrast ratio can be. A larger lens collects more light, so more contrast information has to be potentially available. Functionally, what this must means is that given the same camera body (the same AF settings) a faster lens has the ability to find focus on a darker or less contrasty scenes-- these would be the ones right on the edge of AF functionality. This is because those scenes, viewed through the wider lens, are brighter and consequently have a higher potential contrast for AF sensors to seek.
Quote:
Originally Posted by wickerprints
No argument at all here.
LOL...I'm feeling the backlash from the last entry that was pulled from another website relating to this lens andthe query- as mentioned. I guess I'll be posting an update to that other site....thatMr. Rockwell character he has nothing on WP!...LOL!
[:P]
Quote:
Originally Posted by elmo_2006
Hehe we've pretty much established how much faith we have in KR inanother thread. Kind of confirms things a bit doesn't it.
asmodai, I think you are mistaken; wickerprints did an excellent job explaining the situation, but I'll see if my two cents can help.
Quote:
Originally Posted by asmodai
That's not how PDAF works. As wickerprints explained, the autofocus sensors can only see a certain part of the lens. No matter how much faster than f/2.8 the lens is, the contrast information available to the autofocus stays the exact same.
The only possible way for that to change is if a manufacturer somehow came out with the first ever f/1.4 or f/2 autofocus sensor. If that ever happened, it would be the only circumstance in which having lenses faster than f/2.8 makes any difference whatsoever.
Quote:
Originally Posted by Daniel Browning
Ahh! I think I understand the impasse here.
So, yes. it is true. Phase detection AF sensors become available in discrete intervals based on the lens they were designed for. No wider lens makes an already accessible af sensor more accessible. No argument there.
Moving beyond that, on to the AF sensor itself, onto the photosensitive surface area. They work by discerning light patterns (contrast patterns), and bringing them into alignment with one another by moving groups in the lens. Now, any given AF point represents some area in the circle of light thrown into the camera from the lens, some fixed point. The light cast on that static photosensitive area will be MORE INTENSE given identical scenes, identical focal lengths, using the same AF points, but with a wider lens capable of collecting more light. Static area, static position, but more photons hitting that surface area. More information HAS to be available. It's unavoidable, isn't it?
And in live view / contrast detect mode, forget about it. But I can imagine we'd disagree on that =]
I am open to being proven wrong if I am, so please, if I'm making some error here, let me know
http://www.cambridgeincolour.com/tutorials/camera-autofocus.htm
This is a really, really well written description of phase detection AF, and it's dependence on contrast. And as I mentioned, as as they touch on, contrast is dependent on sufficient light being gathered by the lens.
You are conflating AF acquisition with AF precision, which are two distinct concepts. It is one thing to ask the AF system to find focus, and another to ask the AF system how precisely it has found focus. The former requires a certain level of subject contrast across the AF region (and along the direction the sensor is sensitive), but the latter requires a certain baseline separation and therefore requires a lens with an aperture of a certain minimum diameter.
As I pointed out regarding the EF 85/1.2L II, the AF sensor is not slowed down because it has to find a more precise focus--the f/2.8 sensor simply isn't capable of discerning the f/1.2 image. And by this, I mean the change in AF target sharpness at f/1.2 when the focusing group is moved within +/- 1/3 of the f/2.8 depth of focus. That's what I am trying to explain. The slowness of AF due to insufficient light (and therefore insufficient subject contrast) is an entirely different aspect to AF performance.
If you think a bit about AF precision and how it pertains to the rated sensitivity of the AF sensor points, this makes perfect sense. If an f/2.8 sensor were able to discriminate changes in sharpness at an f/1.2 "level," then the same could be said for an f/5.6 sensor. There is a reason why the system is engineered with both sensitivities. The less sensitive points are needed because the more sensitive points become useless for slow lenses, and the more sensitive points are needed because the less sensitive points are, well, less sensitive! What goes for f/2.8 vs. f/5.6 also applies to f/1.2 vs. f/2.8.
Coming back to required contrast then, this is something that is correlated to but not strictly dependent on light level. Yes, you need a certain minimum EV for the AF to operate. But if it can operate, it doesn't get more precise--nor does it get faster--the more contrast you have.
As for Live View / image sensor-based contrast AF, this is a completely different creature from the phase-detect AF. The LV AF works by direct software processing of the pixel readout in the chosen area of interest, and is therefore working at whatever maximum aperture the lens has. In other words, if you enabled LV AF with the 85/1.2L II, you could in theory obtain precise focus of the f/1.2 image--in practice, the focus precision is a function of the camera's contrast-detection software algorithm.
Quote:
Originally Posted by asmodai
Not so. Here's how it works. Imagine you're inside the camera, standing on the image sensor. When you look out, you can see the entire lens aperture. At f/16, the light hitting you is not that intense. But as the f-number gets faster, you can still see the entire aperture, and the light gets brighter until you burn like an ant under a magnifying lens.
Now imagine you're standing on the autofocus sensor. There is a giant tube restricting you from seeing any part of the lens except the f/2.8 ring. At f/16 you see nothing. At f/2.8 you finally see something, but it's not that bright because it's only one part of the lens. Your not even burning at all. At f/2, it still looks the exact same, because the wider part of the aperture is completely blocked from your view. You don't notice at all when the f-number is changed to f/1.4, either.
The autofocus sensor gets light from only one part of the lens, so the intensity does not increase with faster f-numbers. Intensity only increases for the image sensor because the image sensors sees light coming from all angles of the aperture. But the autofocus sensor does not see all angles: it restricts itself to just one part of the aperture (as a necessary part of its design).
Quote:
Originally Posted by Daniel Browning
Now, how does that work? I'm a very small ring of photosensitive material with my view of the world occluded by an umbrella that covers the center 2*atan(1/2*1/2.8) angle of view. (Based on what depth?) Then what happens?
As the lens approaches focus, the light from the subject is scattered less and less. Its CoC on the sensor approaches zero. At some point, an edge detail will read more strongly on my neighbor than on me. So, by peaking the difference between my neighbors and myself, the AF system determines it found focus.
That's it? No weird integration or anything? Why occlude the center at all?
Quote:
Originally Posted by MikeWhy
The purpose is to get two or more different views of the same subject. My understanding is that it's more than just a central occlusion: each of the paired AF sensors gets light from an aerial image of a virtual subaperture at opposite sides of the f/2.8 annular ring. If it were designed to see more of the center, the accuracy would decrease in proportion with the decrease in baseline. If it were designed to see more of the outer aperture (e.g. f/1.4 lens), then it wouldn't work at all with f/2 or f/2.8 lenses (just as f/2.8 sensors don't work at all with f/4 or f/5.6 lenses). It only sees just that part of the lens that it's designed to (within certain tolerances... f/5.6 AF sensors will sometimes see a little of the f/8 ring, which is why they can kinda-sorta autofocus, though with a smaller baseline.)
Quote:
Originally Posted by Daniel Browning
I think I get it now. Instead of an actual ring, the paired sensors make up a portion of a virtual ring, the spacing being the same or at least analogous to the central occlusion. I'm quite sure I still don't understand why the ring or baseline distance improves accuracy (couldn't I just read adjacent pixels on the main sensor for contrast, for example?), but I already have a much clearer picture now of how the AF works. Thanks for the lucid explanations.
Quote:
Originally Posted by MikeWhy
Sure - in fact, that's exactly how contrast-detection AF works in LiveView on the newer bodies, and also how typical point-and-shoot cameras focus. It can be quite accurate - in fact, focusing a Canon dSLR in LiveView with a 10x magnification on the LCD is just about the most accurate way to focus a Canon camera. But if you've tried that, or used a P&S camera, you know that relatively speaking, contrast detection AF is very slow, sometimes taking nearly a full second to achieve focus. That's one reason that drives P&S users to dSLRs - it certainly was for me, too many missed shots of my fast-moving toddler!
You're welcome.
Quote:
Originally Posted by MikeWhy
Because the degree to which each paired AF sensor differs is directly proportional to the baseline distance. The further you get from the center of the aperture, the more that the light rays diverge. The f/5.6 AF sensors have a short baseline distance; their accuracy is 6 times worse than the f/2.8 AF sensors.
I have to say that I don't understandhow the focus speeds of similar f/5.6 lenses can be so great.
My 70-200 f/2.8L IS lens focuses almost instantly. Add a 2x extender in to turn it into a 140-400 lens thoughand focusing becomes painfully slow. People have said that this is because of the reduced aperture, but my prime 400mm f/5.6 lens also focuses almost instantly - even in poor light it's very quick.
This makes no sense to me. Since people have found that the tape-on-connectors trick to prevent lens reporting to the cameraincreases focus speed, Ican only assume that Canon have slowed down this when using the extender on purpose -perhaps to encourage people to buy the proper lenses.
Quote:
Originally Posted by Feanor
I don't know the reason for it (I would hope there's more to it than trying to 'force' people o buy more expensive lenses), but yes, it's by design. When used with the 1.4× extender, autofocus speed is reduced to a half. When used with the 2× extender, autofocus speed is reduced to a quarter.
Quote:
Originally Posted by Feanor
I don't know the reason for it (I would hope there's more to it than trying to 'force' people o buy more expensive lenses), but yes, it's by design. When used with the 1.4× extender, autofocus speed is reduced to a half. When used with the 2× extender, autofocus speed is reduced to a quarter.