A.J.R. Loc: Devon, UK

Not given this much thought before but wonder if anyone can answer this question.
Although f10 is not a regular f stop for ease of explanation I will use it in this example.

If the focal length of a zoom lens is set on 50mm and an f stop of f10 is selected the diameter of the aperture will be 5mm. If the lens is zoomed to 100mm (focal length), keeping the f stop on f10 the aperture should change to 10mm.

Is this change of f stop diameter made by clever mechanisms within the lens that alter the diameter as well as the focal length when zooming, or is there another explanation?

Your answers would be much appreciated.

Mac Loc: Pittsburgh, Philadelphia now Hernando Co. Fl.

The f/stop is the ratio of the diameter of the aperture to the focal length. So with a constant f/stop the diameter will change as the focal length changes to maintain the ratio.

CO

There's a lever on the face of the lens mount that controls that aperture. Some of Nikons new lenses are "E" lenses that have an electronically controlled aperture. They say those lenses can make changes to the aperture faster which is useful when in continuous mode.

Apaflo Loc: Anchorage, Alaska

It's clever magic, or smoke and mirrors!

Actually the ratio of the physical aperture to the focal length only works for a "simple" lens, and zooms are clearly not simple! The actual effective aperture is determined by the "entrance pupil" which is magnified by the optics as the lens is zoomed. With less expensive designs the magnification of the entrance pupil does not track 1:1 with the zoom magnification, and hence the aperture is variable over the zoom range. If care is taken to design the lens to closely match the zoom magnification to the exit pupil magnification the lens will have a constant aperture over the zoom range.

Another effect that is apparent even with fixed focal length lenses is that the effective f/stop starts changing dramatically as the image magnification becomes greater than perhaps 0.1X. The formula is
Effective Fstop = Fstop * (1 + Magnification). Hence with any given lens if the aperture is set to a marked f/5.6, but the image is focused close enough to produce an equal size image on the sensor, the effective aperture is f/11. If focused close enough to have a 5X macro, that marked f/5.6 is really an effective aperture of f/32. Note that DOF and diffraction depend on the effective aperture, not the marked aperture (which would be correct if the lens is focused at infinity).

Technical minutia that seems insignificant for average photography, but can really bite one hard under some specific circumstances if it is not known and taken into account.

mrtobin Loc: North East Ohio

The lens on the camera is wide open (at it's largest aperture) so you can see what you are taking a picture of and so the camera can focus. It gets stopped down after you press the shutter button. So yes"the change of f stop is made by a clever mechanisms"

A.J.R. Loc: Devon, UK

Thank you for your excellent explanation. Yes I understood the concept when using a macro lens but had never thought about the relationship between aperture and focal length when using a zoom.

A.J.R. Loc: Devon, UK

Yes that's the simple bit. See Apaflo's reply as to how it works with a zoom lens.