Nikon 24-70 2.8 aperture question

Helvegr

One thing I "thought" I understood about lenses what that the less expensive variable aperture lenses were variable largely because the length of the lens changes as you zoom in and out. The increase in the physical length of the lens mean that light has a longer way to travel, so although the actual aperture in the lens doesn't move, from an exposure perspective you get a different aperture.

I've rented a 24-70 lens are realized that while its a 2.8 aperture, the length of the lens isn't fixed. So I'm just curious how it maintains a fixed 2.8 when the length of the barrel is changing. Does something else happen in the lens to compensate?

G

ziggy53

The top zoom lenses are generally contsant aperture and internal focus (also sometimes internal zoom). The way that they maintain the constant aperture is best explained by Bob Shell:

Comments from Bob Shell (January 8, 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."