My personal macro and tele lens comparison

Hi Chris,

Many thanks for posting your useful tests and experiences. I will be back with some reports on the equipment I have personally been considering for "macro" use (Oly 75/1.8, Oly 40-150/4-5.6 and Pany 100-300/4-5.6) together with my Canon 500D close-up lens and/or my newly acquired Fotga extension tubes. But first a few words about a problem you mentioned already in an earlier thread and mention again here: that of backfocusing.

This is a problem I have also encountered in my little "macro" tests and when I first did, it took me by surprise. After all, CDAF systems, unlike PDAF, are thought not to have any issues with systematic front- or back-focusing. So what was really going on?

Well, there are a couple of optical problems that can fool any AF system, whether CDAF or PDAF. One of them, which I was already aware of, is known as focus shift and implies that the best focus point varies with the f-stop used. If you focus wide open, whether by AF or MF, and then stop down for the actual exposure, the focus will be slightly suboptimal.

However, there is a second possibility too, which hasn't been all that clear to me until now, namely that the point a CDAF system (possibly a PDAF system too, I don't have a PDAF camera to test with any more) considers maximally sharp, i.e., the point of maximum contrast, is not the same as the one our own eyes would say is maximally sharp. During the little tests I performed in order to get to know my own equipment, I encountered some unusually clear examples of this problem, which I thought it might be a good idea to share so that others interested in doing "macro" will know what's going on and what to look out for.

The samples below are 100 percent crops from two images shot with my E-M5 and the Pany 100-300/4-5.6 wide open (f/5.6) at 300 mm with my high-quality Canon 500D (a +2 achromatic close-up lens) at the front end (fitted with a 67-58 mm step-down ring since my 500D is 58 mm). The first crop in each pair is maximally sharp according to the AF system, the second maximally sharp according to my own eyes. As you would guess, the second has focus set slightly closer to the camera than the point chosen by the AF system.

As I hope you can see, the AF system isn't really wrong given the criterion it is working with. The first crop in each pair undoubtedly has higher contrast. Yet, we (or at least I) perceive it as blurrier than the second, which is less contrasty but has more well-defined contours.

The known cause of focus shift is spherical aberration and I am pretty sure that the same aberration is responsible for what we see in these crops as well. Although we are likely to encounter this aberration in quite modest amounts when we use our native MFT lenses without add-ons like close-up lenses or extension tubes, it can, as these examples show, take on very significant magnitudes when we try to go closer than the lens ordinarily allows. In fact, based on what I have seen so far, this is the most significant problem we are likely to encounter when we try to use non-macro lenses for "macro". The problem, of course, is not only one of focusing. The MTF values (resolution/microcontrast) will suffer as well.