[Brightcolours]

Lenses that change length during focussing do change focal length too at different distances. Kind of a strange myth that changing focal length has to do with internal focussing.



Some simple proof:



Lens / MDF:

Tamron 90mm f2.8 [color="#FF0000"](NOT IF)[/color] / 29cm

Canon 100mm f2.8 L USM[color="#2E8B57"] (IF)[/color]/ 30cm

Sigma 105mm f2.8 HSM OS [color="#2E8B57"](IF)[/color] / 31.2cm

Signa 105mm f2.8 [color="#FF0000"](NOT IF) [/color]/ 31.3cm

Tokina 100mm f2.8 [color="#FF0000"](NOT IF) [/color]/ 30cm

Canon 100mm f2.8 USM [color="#2E8B57"](IF)[/color] / 31cm

Nikon 105mm f2.8 [color="#FF0000"](NOT IF)[/color] / 31cm

Nikon 105mm f2.8 VR [color="#2E8B57"](IF)[/color] / 31cm

Pentax SMC-D FA 100mm f2.8 [color="#FF0000"](NOT IF)[/color] / 30cm

Pentax SMC FA 100mm f2.8 [color="#FF0000"](NOT IF)[/color] / 31cm



Or rather, I should say it differently.



Since focal length is calculated with the distance to the front element which does change depending on lens length... The thing that matters, field of view, does NOT depend on if a lens is IF or not IF. The field of view changes both with IF and non-IF lenses.

The focal length with the non-IF lenses will change more (due to its changing position of the front element), but as we can see above, the field of view remains the same to lenses with IF.



The problem with "focal length" definition is that it is not mapped 1:1 to field of view. It can be that a lens may lose some focal length when focussing close by, but that the field of view gets more narrow. We do tend to think of focal length saying everything about field of view, but it does not.



With my Canon 70-200mm f4 L USM (IF) for instance, it loses calculated focal length at MFD, but its field of view gets noticably more narrow.

My Sigma 18-50mm f2.8 (IF) loses calculated focal length too (more) at MFD, and its field of view gets noticably wider.



Basically, what we should be looking at is view angle or field of view, not focal length (as that is quite an abstract figure which we do not see in photos).



To summarize:

* Focal length and field of view are not linked 1:1.

* Non-IF lenses can have MORE focal length change than IF lenses.

* Non-IF lenses can have the SAME field of view at MFD as IF lenses.