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Old Apr 6, 2005, 6:18 AM   #1
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A question from someone new to photography.

When comparing two lenses (for example):

Canon EF 17-40mm F4.0L USM
Canon EF-S 10-22mm F3.5-4.5 USM

My understanding is that on the first lens an aperture value of F4 can be used throughout the focal length of 17-40mm. However, on the second lens the aperture can be set to F3.5 at the wide-end (10mm) and F4.5 at the telephoto-end (22mm). Is this correct? If not, what do the F-numbers mean?
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Old Apr 6, 2005, 7:05 AM   #2
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You are correct.
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Old Apr 6, 2005, 9:02 AM   #3
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Another thing to point out is that it isn't linear.

Taking the example you gave. The 10-22mm the max aperture might be f3.5 at 10mm, and then at 13mm the max is f4, at 20mm the max is f4.5, all the way to 22mm.

So all you can really say is that at 10mm it's f3.5 and at 22mm it's f4.5. You don't know when f4.5 becomes the max aperture.

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