Macro and effective focal lenght,  a case history

A thing that I rarely find in lens reviews, is a that the use of floating elements or IF design, while enhancing quality troghout the whole focusing range, changes the effective focal length of the lens with the focusing distance,  so that it corresponds to the nominal length only near infinity. Noticeably, Bjorn Roerslett quotes this in his excellent website, especially in the section on macro lenses.

It is common knowledge, for instance, that at minimum focusing distance the nikkor 180 2.8 af is a 150 mm and the 300 f4 af (NOT the AFS)   is roughly a 250 mm.

This shift is usually acceptable and it is not  a big problem apart for two cases: macro lenses and wide to tele zooms with macro options, where the shift in focal length may be dramatic.

Let's see an extreme example:

The micro nikkor 105 f4 Ai reached a reproduction ratio (RR)  of 1:2 at 47mm while the nikon 28-200mm  afG  zoomed at 200mm reaches a RR of 1:3.1 at 44mm. How can it happen that a 200mm reaches a lower  RR at a shorter distance with respect to the 105?

The answer is simple, the 28-200 at minimum focusing distance never attains 200mm. Indeed its effective focal length at 44cm is about 80 mm. You won't find this on brochures and press releases I'm afraid. As far as I remember, only  the brochure of the 24-200 Tokina warned that the area framed by zooms may not be the same  that of equivalent primes at same focusing distance. Rather cryptic but at least they wrote something.

This not only means that you have to go closer to the subject (obvious) but also  that the frame area and depth of field are different from one could expect, and  that there is no way to  isolate the subject by completely blurring the background, a feature which renders so pleasant  macro shots taken with a telephoto.

Is it possible to calculate the effective focal length of a lens at a given focusing distance?

Yes, the algorithm is also easy

                                    Focusing distance (in mm)

                                  _________________________  = effective focal length

                                           1 /RR    + RR +2

RR (Reproduction Ratio) goes as percentual

For the 28-200 G we have 440/0.3+0.33+2= 440/5.4=ca 80mm at minimum focusing distance.

 Since I am mostly interested in macro, I built up a list of effective focal length (approximated for the decimals) of some macro lenses, which I report here below for those interested.

 With an APS sized digital slr the 1.5 crop factor helps to bring back good focusing (and working) distance and things go better. I was disappointed of the 70-180 (due to the length of the barrel its working distance on film cameras was a mere 13cm ) and sold it. Now, owing the D70,  I would have it again in my bag...

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