Lenses....some Focus faster than others...

[jks]

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??

[waving_odd]

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.

[elmo_2006]

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?!


[]

[jks]

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??

[wickerprints]

Several factors influence AF speed.

1. 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.
2. 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.
3. 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.
4. Ambient light level. AF performance is impaired at very low light levels in the absence of any AF assist beam.
5. Subject contrast. Low subject contrast, or contrast in a direction opposite to the AF sensor's sensitivity, inhibits AF acquisition.
6. Subject distance. Close focusing is generally harder than distant focusing, due to shallower DOF.
7. 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.

[neuroanatomist]

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.

[elmo_2006]

<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.
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[elmo_2006]

LOL, looks like others have beat me to the 'punch'!


[]

[wickerprints]

<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;"]
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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.

[Brendan7]

I will say, Wicker, you are providing me with an extensive education on this [8-|] Very nice read.


brendan [H]