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

[jks]

yes WP... I do believe I am starting to get a clearer picture as to why focus is slower on some lenses.





THANKS!!!

[asmodai]

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:

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.

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.

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.

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.

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.

No argument at all here.