[thxbb12]

Zoom lens design is extraordinarily complicated and there will actually be an article on it sometime down the road on photozone, but here's some basics.

 

Zoom lenses are designed with a set of "modules."  These are groups, but not in the way lens specs list groups (where a "group" is simply one set of lens(es) that only have two air-to-glass surfaces). 


Typically or classically a zoom uses 4 groups.  Fewer or more is possible, but it is very difficult optically to do it with fewer, and difficult mechanically to do it with more. 

 

The size of the entrance pupil is what determines the f number.  The size of the physical aperture does not matter.  If the zoom motion occurs only in front of the aperture stop, the lens will have a constant f number.  This forces the design into a very narrow solution space since there is extremely limited room to work with.  You have essentially thrown away a third (or more) of your degrees of freedom for correction in order to have a constant aperture. 


If you wish to allow zoom motions to happen on the rear side of the aperture as well, the focal length will be changed by movements which do not affect the entrance pupil, thus the f number does not stay constant with focal length change. 

 

The entrance pupil is the image of the physical aperture as seen from the front of the lens.  It is virtual but does have a location and apparent size.

 

<p style="color:rgb(40,40,40);font-family:helvetica, arial, sans-serif;">To paraphrase what I wrote to BC: in light of what you just wrote, designing a 16-70 f2.8-4 is easier than designing a 16-70 f4?

<p style="color:rgb(40,40,40);font-family:helvetica, arial, sans-serif;">If I follow the same logic, is designing a 16-70 f1.4-4 also easier than a 16-70 f4?

<p style="color:rgb(40,40,40);font-family:helvetica, arial, sans-serif;">That doesn't seem to make sense.

<p style="color:rgb(40,40,40);font-family:helvetica, arial, sans-serif;">Additionally: who would actually rather have a constant f4 lens over a f2.8-4 one?

<p style="color:rgb(40,40,40);font-family:helvetica, arial, sans-serif;">Likewise, who would rather have a 100-400 f4 vs a 100-400 f2.8-4?

<p style="color:rgb(40,40,40);font-family:helvetica, arial, sans-serif;">I know what I'd like better and I think I'm not the only one...