F/number on a fixed aperture zoom lens?

[prodigy2k7]

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?

 

[prodigy2k7]

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

Either all the smarties are offline or they are lazy tonight

 

[Alex_B]

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?

 

[compur]

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
passes through the lens, not the actual physical diameter of the diaphragm
opening.

"It is this fact that allows companies to make constant aperture zoom lenses
which maintain a constant f-stop when the focal length changes, because
such lenses are designed so that the magnification factor (diopter value) of
all elements in front of the diaphragm changes as focal length is changed to
hold the aperture value constant."

 

[nymtber]

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!

 

[Helen B]

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

 

[epatsellis]

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

erie

 

[prodigy2k7]

Thanks everyone