Apaflo Loc: Anchorage, Alaska

Not true.

The f/stop is the ratio of Focal Length to Entrance Pupil. It is indeed well defined but is not necessarily constant in a zoom lens because the size of the Entrance Pupil changes as the lens is zoomed.

If all of the zoom is effected using only elements in front of the Entrance Pupil the f/stop ratio will remain constant as the focal length changes. If any of the zoom is effected using elements behind the Entrance Pupil that change will not equally magnify the Entrance Pupil and the ratio will not remain constant.

Read any text on optics that is current enough to discuss the theories that apply to zoom lenses.

John_F Loc: Minneapolis, MN

It seems quite possible to manufacture a zoom lens that can move the aperture leaves as the focal length changes. I would think such would not be a simple linking and would be very expensive. To my way of thinking one would not be getting much bang for the buck. So I would agree with Bebu.

mcveed Loc: Kelowna, British Columbia (between trips)

The f-stop is the ratio of the focal length to the effective diameter of the aperture. The "effective diameter" of the aperture is what the diameter would be if it was located in front of the lens. This convention is used so that the f-stop will remain constant no matter where in the lens barrel the manufacturer places the diaphragm. If the lens is set at f2.8 the size of the aperture must change when the lens is zoomed in order to maintain the ratio. So a lens which has a constant maximum f-stop of 2.8 can maintain that ratio no matter what the focal length. Less expensive lenses have a varying maximum apertures (f2.8 - 5.6 for example) which indicates that, so save costs, the lens elements are of smaller diameter and cannot maintain the f2.8 ratio when the focal length extends to the maximum range.

Apaflo Loc: Anchorage, Alaska

Read any text on optics to learn how it actually works.

"Effective diameter" of the aperture is not a convention, it is a measurable physical characteristic. It can be located in front of the lens (e.g. a pin hole lens), but rarely ever is.

Again f/stop is the ratio of the Focal Length to the Entrance Pupil. The size of the Entrance Pupil changes when the lens is zoomed. The change in size may or may not be constant with relation to the zoomed focal length (as has previously been described). The change is magnification and is unrelated to the physical size of lens elements. It is not accomplished by physically changing the diameter of the aperture opening, which is constant.

For another discussion see:

http://en.wikipedia.org/wiki/entrance_pupil

mcveed Loc: Kelowna, British Columbia (between trips)

Note the difference between "constant maximum aperture" and "aperture range". Constant maximum aperture of f2.8 means that you can set the aperture at 2.8 and shoot at 70mm with your lens. Less expensive lenses with smaller lens elements must stop down as the focal length increases. A less expensive lens might have a maximum aperture of f2.8 to f4. If you set this lens to 2.8 at 24mm the f-stop indicator in your viewfinder would gradually reduce the f-stop as you zoomed out. Whether the lens lengthens as you zoom is not relevant to the f-stop issue.

Apaflo Loc: Anchorage, Alaska

The physical size of the lens elements has absolutely no effect on f/stop. F/stop depends on the Entrance Pupil size, which only changes due to optical magnification as the focal length is changed by zooming.

See the Wikipedia article previously cited for a discussion of Entrance Pupil, f/stop, and how they are optically magnified as the lens is zoomed.

Here is that article again:

http://en.wikipedia.org/wiki/entrance_pupil

Apaflo Loc: Anchorage, Alaska

John I tried to entice you into a further discussion of Valasek's text by harshly stating it is not current. You have not responded, so I will be more precise.

Valasek in very plain English says exactly the same as I have said, and specifically that your definition of f-stop is incorrect.

See Chapter 5, Figures 1 (a) and 1 (b) for examples and read the text just before and after Figure 1 where f-stop is defined exactly as I have previously stated: Focal_Length / Entrance_Pupil.

That is the same as Focal_Length / Relative_Aperture. It specifically is NOT Focal_Length / Physical_ Aperture as you stated. The Relative Aperture (aka the Entrance Pupil) is magnified by the same factor as the zoom magification.

Valasek discusses the theory but does not apply it by name to constant-aperture as opposed to variable-aperture zoom lenses, simply because that was not significant in 1949. Today of course it is very significant.