I met a lovely lady on Sunday. She is relatively fresh into the field of serious photography and was being frustrated in her efforts to get effective shots of coins without harsh shadows and full sharpness. She had gone into a camera store and the very helpful sales person suggested the Nikon 40mm Micro. This is a fine lens and very inexpensive. Unfortunately, it's a TERRIBLE choice for close up work.
While the lens is quite sharp and has a great aperture range to ensure depth of field, the idea of a macro (or as Nikon calls them - Micro) lens is to get lifesize captures. Simply this means that if you have a full frame sensor measuring 24mm x 36mm, you can get sufficient closeness to capture a subject that is 24mm x 36mm on the sensor. This is what the marketing people mean when they hurl 1:1 around in their documentation.
The challenge is how physically close you have to get as the photographer to get that 1:1 image. The shorter the focal length, the physically closer you can get. Sounds good, right?
Nope. The closer you must physically be, the greater the probability that you will become an impediment to getting a good image. If your subject is alive, you crowding in may cause it to leave. Or bite you. If your subject is static, you might end up blocking most or all of the light you need.
So, you may be thinking, ok smart guy, what does work? 100mm or longer. That's it. Start there. Crop sensor camera? Start around 100mm. Full frame sensor camera? Start around 100mm. If you shoot Nikon, the Nikkor 105 Micro is imho the most awesome lens that Nikon makes, amongst a family of awesome lenses. If you shoot Canon get the 100/2.8L. Both lenses are relatively expensive and both will last a lifetime. If you shoot Nikon, do NOT buy a lens built for the DX sensor. If you shoot Canon, do NOT buy an EFS lens for the crop sensor. Do yourself a BIG favour. Buy lenses built for full frame even if you only have a crop sensor. There's this thing called image circle and this other thing called internal vignetting. You want the most of the first and the least of the second.
I think I was able to help her out. By suggesting a much smaller aperture to optimize depth of field and recommending the use of a diffused daylight balanced CFL lamp with reflective white foam core, I expect that she will achieve the level of detail and soft light she needs without spending a couple of hundred dollars on softboxes and such. My biggest concern is that she won't be able to get the shot without getting in the way of the light because of the focal length requirement to be so close.