# F/number on a fixed aperture zoom lens?



## prodigy2k7 (Jul 26, 2008)

EF 70-200mm F/4 USM
The F/4 is constant. But the focal length changes. Does that mean the physical size of the aperture change?
Ex: 70/4 or 200/4

Isn't the physical size of the aperture the focal length divided by the aperture number?


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## prodigy2k7 (Jul 26, 2008)

ha... Almost 20 views no replies!!!

Either all the smarties are offline or they are lazy tonight


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## Alex_B (Jul 26, 2008)

prodigy2k7 said:


> EF 70-200mm F/4 USM
> The F/4 is constant. But the focal length changes. Does that mean the physical size of the aperture change?



I never looked too much into lens internals, but it should, should it not?

but then again, when the lens is wide open (f/4), aren't the aperture blades all retracted and out of the way? I guess then the aperture is only defined by the entrance pupil as given by the physical dimension of the lens front element?


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## compur (Jul 26, 2008)

As Bob Shell said in 2003:


> "An f-stop is the ratio between the focal length of the lens and the
> *apparent* *size of the lens opening as viewed through the front*. It must
> take into account the magnification factor of all lens elements in front of the
> diaphragm, because it is the size of the opening that the light "sees" as it
> ...


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## nymtber (Jul 26, 2008)

the f/number is a ration of focal length:apurture size (both in mm). So YES the ACTUAL size of the opening does change, but the same amount of light is let through. 

yea I learnt something in that _Understanding Exposure_ book already 

Edit: Compur and I were typing at the same time apparently!


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## Helen B (Jul 26, 2008)

There are a few threads about this. Here's one that prodigy started recently. The answer is in that thread - a constant aperture zoom lens may have a constant physical aperture (known as the aperture stop or sometimes as the iris) and rely solely on the change in magnification of the iris by the front elements; or it may have a variable physical aperture to complement the change in magnification. My Nikkor 17-35 f/2.8 is one of those designs that uses both properties to maintain its aperture.

The important thing is that the f-number is the focal length divided by the diameter of the entrance pupil, not by the diameter of the physical aperture.

Best,
Helen


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## epatsellis (Jul 26, 2008)

Thanks Helen, you saved me from retyping my reply in the other post...

erie


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## prodigy2k7 (Jul 27, 2008)

Thanks everyone


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