Originally posted by stevebrot All internal focus lenses work by reducing the effective focal length when focusing down from infinity. The most obvious effect for still photography is less magnification for a given subject distance than one would expect from an external/block focus lens at the same distance and nominal focal length. This is not something that most of us will notice unless we are doing a focus pull in video or doing some very involved focus stacking for near/far landscape or macro*. For example, I had been using my Sigma 17-70/2.8-4.0 for over a year before I noticed that what I saw in the viewfinder at 70mm and 1.0m appeared wider than what I knew it should be from experience. I was mildly peeved, but once I figured out was the compromise to allow cool stuff like lightning fast focus I was good with it.
Nice succinct description Steve.
There have been a few threads in which people have said that lens A is better than lens B for pseudo macro (ie at or close to the MFD) because lens B has a lot of focus breathing. Common nominees for the lens B category have been the DA 18-135 and the DA 55-300 PLM. The short answer is just to look at the specs: maximum focal length, MFD and maximum magnification. The screwdriven DA-L and DA 55-300mm f4-5.8 models (which are not IF lenses) have a maximum magnification of 0.28x and an MFD of 1.4m. The DA 55-300mm f4.5-6.3 (an IF lens) has a maximum magnification of 0.3x and an MFD of 0.95m. (I once did a field comparison of focus breathing between the DA-L and the PLM:
One or the other need help on a 55-300 zoom lens?? - PentaxForums.com) Which is better for pseudo-macro? I'd take the PLM, not only because of the slightly higher magnification, but because the shorter MFD gives more options for framing the subject. True that if you move from 1.4m to 0.95m, the butterfly is more likely to fly off, but it is also more likely that you will be able to get a clear view from closer in. For things that don't fly off and don't bite, 0.95m is much more convenient than 1.4m.
Since the DFA 70-210 f2.8 has a maximum magnification of 0.13x, pseudo macro is rather academic, as Paul suggested. But the thread topic invites wider consideration.