I remember a question being asked of Kevin MacDonnell in Photography magazine, many years ago Basically, the writer was doing a project and said can you send me full details of what cameras are available and how good they are. The reply was he could spend all day phoning around (it would be Googling perhaps today) and collecting all this together, but he wouldn't want to deny the writer the privelidge of doing it themselves. Amusing and maybe even completely made up, who knows, but it is true that finding out for ourselves is the best way to learn.
So, lest the spirit of Kevin MacDonnell should haunt me, yesterday I set about seeing which lens would be best for close up shots of small objects. I had to hand my Vivitar 55mm f/2.8 Macro, a manual fopcus lens that I have reviewed for EPZ and found to be excellent. I also had the Pentax AF 50mm and 100mm macro lenses. Also, I had the Pentax 18-135mm WR zoom that is an excellent general purpose lens on APS-C format. I settled on using my APS-C Pentax K-3 II, which offers 24MP and gives excellent results itself. For the target I chose a page out of Island Years by F Fraser Darling (Readers Union, London, 1952) and as far as possible selected the same small part of that page for each lens, first at open aperture and then at f/11. I know that f/11 would not be the sharpest aperture, but at close distances depth of field will be a major consideration so a macro lens should perform well at all apertures.
First up was the Vivitar 55mm f/2.8 Auto Macro
Second was the SMC Pentax-D FA 50mm f/2.8 Macro
Third was the SMC Pentax-D FA 100mm f/2.8 Macro
Fourth was the SMC Pentax-DA 18-135mm f/3.5-5.6 WR. We know that with zoom lenses the focal length is only even remotely accurate at infinity and that it shortens when used closer, but I did not expect the enormous shift with this lens. Set at 100mm following the 100mm macro, the zoom lens does not even come close to the same field of view, nor does it when zoomed to 135mm. So it needs to be used much closer than expected. Because of limited aperture, the zoom lens was tested at f/5.6 and f/11.
The conclusion is that all the macro lenses show very sharp central performance at f/2.8, dropping off a little at the edges. By f/11 they are crisp all over the frame and clearly the best choice, particularly for flat subjects. The zoom is not the marked focal length close up and needs to be much closer than anticipated, but it does perform well at f/11 and is very usable. Any lack of sharpness at the dges wide open will not be helped by curvature of field, and on normal 3D subjects the results will be much better. The moral of the tale is if we do our own testing then we know for ourselves if a particular lens fits our requirements or not.