Max Aperture for Autofocus

[akr]

Hello all

This may be a stupid question but anyway....

I often see people saying if you have an f5.6 lens and use an extender then you will lose AF unless you have a 1 series body (canon wise).

Now if I set my 40d to f8 in say AV mode by this theory it should not autofocus, but it does.

I am clearly missing something :shrug:, could someone enlighten me.

Thanks

Al

 

[purpleclouds]

Your lens aperture doesn't actually close until you fire the shutter! All auto-focussing is done with the aperture fully open!

 

[towershot]

It is set to f/8 but does not stop down (close down from F/2.8 or whatever)Till you have released the shutter.

In the 60s when you set the F stop, it closed down there and then making it dark while looking through the viewfinder.

The lads in the lab came up with the automatic iris which closed as you took a pic,but stayed open for composing.


All badly explained ,but then I take pics and don't teach Muhahaha....

 

[akr]

So with an f5.6 lens and a 1.4 extender the min aper is f8 so will not autofocus as it cannot at f8, however with say an f4 lens it does it's AF at f4 and then changes the aperture after that to f8.

All cleared up! I can now sleep easy!

Thank for the quick responses

Al