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Basic(ish) Question about Aperture

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Dave George
Basic(ish) Question about Aperture
on Jan 30, 2012 at 12:27:00 am

There's something that confuses me about f/stop and aperture. If the f/stop is a ratio relative to focal length, why is it constant on a zoom lens? (Or is it?)

When I calculate the area of the aperture on, say, a 24mm lens at f/2.8, I get...

24/2.8/2=4.285mm (radius)
pi(r)^2 = 57.7 sq mm (area of aperture)

Right so far?

But I have a 24-70mm 2.8 lens and when I zoom from one focal length to another, the size of the aperture doesn't change. (Or does it?) Wouldn't it have to, if f/stop is a function of focal length?

Thanks for helping me out here!


Dave George
Marketing Director

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Francis Mead
Re: Basic(ish) Question about Aperture
on Apr 29, 2012 at 12:40:44 am

Hey Dave

I'm by no means a techie, but I discovered this recently too - I have a 24 - 104 mm Canon Lens - and basically it won't go wider than 4.0 - it's fixed - so I assume yours is the same - so it basically depends on the lens how wide it will go - and it's limited.

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