EOS Re: 100-300mm f5.6L with Tamron 1.4x SF AF

[Henry Posner/B&H Photo-Video]

At 10:43 PM 01/16/2001, you wrote:
>OK, another newbie, to EOS anyway, question here. I'm having trouble
>visualizing how aperture affects AF.   We're always focusing with the
>lens wide open aren't we?  It's not like we have the DOF preview
>pressed while focusing. Then I could understand   Does the AF
>mechanism respond *as though* the dialed in aperture were effective?

Yes, you ARE always focusing with the lens wide open, but "wide open" means 
different things to different lenses. If you're focusing wide open with a 
50/1.4, far more light is passing through the lens to the AF sensor (and to 
your eye) than if you're wide open with a 100-300mm f/4.5-5.6 EF. I have a 
door in the front of my house and a garage door too. If I open both of them 
wide, they're both wide open. I can walk through either, but the car only 
passes through one.

AF sensors, mostly can't "see" well if the amount of light passing through 
them is equal to or less than the amount passed by an f/5.6 aperture.

--
regards,
Henry Posner
Director of Sales and Training
B&H Photo-Video, and Pro-Audio Inc.
http://www.bhphotovideo.com

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