[wim]

I did some calculatiosn a few years ago, with a known focusing distance, known object width, and known size on the sensor of the object.

This way you can calculate everything necessary to figure out true FL.

 

For the Nikon 70-200 F/2.8 VR, I think the VR1, forgot to write that down, I found the following:

 

1. focused at 373.0 inch, 9.47 m, the calculated FL was 194 mm (193.5) - that is within tolerance (say +/- 5%).

2. focused at 178.0 inch, 4.52 m, the calculated FL was 192 mm (192.4) - still within tolerance.

3. focused at 44.6 inch, 1.13 m, the calculated FL was 169 mm (168.8) - way out of tolerance, and very visible.

 

This is one of the reasons why I do not like IF-mechanisms, as they generally do show this type of focus breathing. Macro lenses with IF are generally very good examples of this behaviour, and often easy to calculate, based on MFD and magnification factor.

The Canon EF 100 F/2.8 USM macro at MFD is only 70 mm, the EF-S 60 F/2.8 macro is 50 mm, and the EF 180L at MFD is something in the order of 120 mm, if not less.

 

Generally speaking, Canon's L-zooms are actually very good in this regard, none to very little focus breathing, last I checked anyway.

 

Kind regards, Wim