Does bigger aperature help autofocus?

[jeffcable]

Originally Posted by Alan_Ackoff

Chris,

I was having focus problems in low light with my 5D and my Canon dealer suggested setting Custom Function 4-1 which gives you a focus button (the asterisk button) and leaves the camera in manual focus otherwise.

This gives you both manual and autofocus available all the time. It's eliminated 90 percent of my autofocus problems. Give it a try and see what you think.

Alan

The value of that function for me, Alan, is that you then don't have the lens trying to find another point of focus, once you have established your focus point, when you take up half pressure with the shutter release and are just about ready to trip the shutter and it is a lot less clumsy that switching the lens to manual mode.

Regards,
Jeff

[Bruce_Stenman]

With a long telephoto the difference is quite apparent with a faster lens or conversely adding a 2x teleconverter can really slow AF done.

The Nikon D2h and D2x will autofocus in low light without IR assist better than the Mark II or the 5D and naturally the 30D. I do not know about the Mark III which I have recently acquired and has just come back from Canon's service center.

I have found that the greatest difference in AF is the camera. The D2x will AF much faster (about 1/4 the time) of the D200 in very low light situations. The lens is also a big factor with some lenses focusing much better than others in low light.

I have tried using f1.4 primes in very low light and find they do not AF any better than my Nikon 17-55mm f.28. There are other variables at play as one sees with the different AF performance of two different manufacturers lenses under normal lighting conditions.

[David_Buzzard]

Faster apertures definately makes a difference in Autofocus. 1) They see better in low light, and that helps the sensors, and 2) they have shallower depths of field, and that makes the sensors more sensitive.

Nikon, alas, doesn't make any AF-S high speed primes. However, I have no problems with the D2x and the f2.8 zooms focusing in very low light.

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