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Basic(ish) Question about Aperture
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
Dave George
Marketing Director -
1 Reply
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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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