[dkpentax]

I tried staring through the 28mm while I focused and couldn't see anything moving around, anyone know which elements are supposed to move?

[Digitalis]

I tried staring through the 28mm while I focused and couldn't see anything moving around, anyone know which elements are supposed to move?

The only way to truly ascertain the movement of optical elements in a lens is by pulling it apart and examining the function of the internal cams/helicoids and any elements in a fixed position. I will point out that internally focusing lenses are distinct from those that incorporate floating elements, as a floating elements are often used to correct aberrations/extend focal range while a focusing group used in IF lenses is designed to move the focal plane in its entirely with the elements on either side of the focusing group remaining in a static position*.


I think the first lens to use floating elements was a 50mm f/2 constructed by Meyer Görlitz in 1950's - it was marketed as a macro/standard lens, in which the floating group could be switched to an alternate position and this optical configuration allowed for enhanced close focusing capability.

* Though in the case of some IF zoom lenses I have observed some movement of front or rear optical groups and sometimes both front and rear groups moved simultaneously with the focusing group remaining static - an example of this is the Sigma 8-16mm f/4.5-5.6 DC

[zapp]

Most zoom lenses accomplish their zoom functions by moving some lens elements around in relation to one another, and virtually all zoom lenses have varying performance across their zoom range.

The modern lens designs of using internal focus on prime lenses means they basically to the same thing. That implies that an internal focus prime lens is liable to have varying performance if you compare its infinity focus quality versus its minimum focus distance quality.

I have not noticed any such tests ever published, or any demonstration that this theory isn't correct.

You are mixing up the effect of different lens group movements trying to correlate this to performance. While all lenses will perform slightly different when changing lens positions, the idea is to compensate these effects. What is actually the idea or goal behind your reasoning? The quest for he best performing lens design? Having a dedicated focusing lens group, helps to keep performance up from near to far focus.