Might Be a Dumb Question

[nathanphillips]

I have been thinking quite a bit about auto focus capability, and extenders / which combination will work on a full frame or 1.3/1.6 frame (EOS 20D).
Knowing that the 20D will only auto focus with f5.6 or better...
It has just sort have popped into my head, how does autofocus work then?

To some people, this may seem really stupid, but to me...just learning this stuff....it seems kinda amazing I figured it out (I think...please tell me if this is correct!). This is my understanding of auto focus now: the camera focuses with wide open f....then it quickly changes to the desired f of the shot. The camera never auto focuses at higher f than wide open...it just temp changes to your desired f when actually taking the shot. ;)

Is this right?

[Jon]

That's 100% correct. Except while you're actually stopped down to take the picture or using the DoF preview button, the lens is wide open.

In a (vasty oversimplified) nutshell, the AF sensors are off to the side of the regular sensor, and get "masked" when the available lens aperture is slower than about f/8 on a 1-series, or f/5.6 on anything else. When they're masked, they can't get a good focus lock, and will hunt. The camera electronics cut off the AF when they detect this situation, so you don't burn the motors out. When you tape the TC, you block that info from reaching the camera, so it tries to AF even with a TC.

[nathanphillips]

Thanks for the reply!
Feel pretty great...Im starting to understand more! :cool: