Lenses for this kind of work fit into a number of categories...

1. Normal Lenses
2. Macro Lenses
3. Microscope Objectives
4. Darkroom/Enlarger Lenses
5. Stacked lenses (Reversing a normal lens on the end of a telephoto lens)
6. Video camera lenses

Normal Lenses

These can be great cheap lenses to start you off if you have a bellows. I started doing this with an ancient manual focus Olympus OM 50mm/f1.8 standard lens, purchased from eBay for an obscenely low price. I still use this lens reversed for low magnification bellows work.

If your system is non Olympus, you should be able to find an equivalent lens for your system of choice. Just make sure it will fit onto the front of your bellows, preferably reversed. You may need a reversing adapter to achieve this, as usual take a look on eBay for these... It should be a pre-digital manual focus lens.

The Olympus OM bellows has the neat feature that you can reverse a lens without a reversing adapter by simply reversing the lens mount on the bellows focus rail.

Note this kind of lens generally works best when stopped down by a couple of f-stops. I never use the 50/1.8 at a larger aperture than f4, and more usually I use f5.6.

Probably the easiest, and almost certainly the cheapest way to get some high magnification is a cheap old manual focus 50/1.8 lens from ANY manufacturer (Don't pay more than $20!) which will stop down the lens when you adjust the aperture even when not fitted to a camera. Add to this a reversing ring for your camera mount and an extension tube or two and you should be able to shoot happily up to around 4:1 (with sufficient extension, you'll need around 200mm of extension to achieve 4:1)

Macro Lenses

The next step up, in terms of quality (and a few steps up in terms of price!), true macro lenses should work well on the front of a bellows without reversing, if used for their intended magnification range. Here I'm talking about real macro lenses - primes, not these 'macro zooms' you can get these days which are not true macro lenses, merely conventional zoom lenses with 'close up' focussing. These generally will not be suitable for use on a bellows.

There are a large range of lenses in available this category, from relatively low power 100mm macro lens to the high powered bellows only macro lenses like the OM 38/2.8 and OM 20/2.0, or their Canon, Minolta, Nikon or Leitz equivalents. The latter are much more suitable for use on a bellows.

Some of these lenses can be very expensive, and will only work at their best for a small range of magnifications.

Most decent macro lenses will work well wide open, or in some cases 1/2 or 1 stop down from there. Using them wide open will obviously result in thinner 'slices' and more shots to get the same range of focus, but you should end up with a final image containing more detail.

Microscope Objectives

For high magnification stacked work, the macro lenses mentioned above struggle to keep up with some of the best microscope objectives as this forum post shows.

The most commonly used (and probably the best available for this kind of work) are the Nikon CF PLAN objectives, non infinity corrected, RMS mount. The Nikon M PLAN CF 10x objective is a great place to start. That is the lens used for most of my high-magnification (7-15x) stack work at the moment.

These objectives generally do not have aperture control, and are quite fast. This together with the high magnification factors leads to very thin DOF

Darkroom/Enlarger lenses

I confess I don't know much about these, just that many use them as low-magnification lenses for this kind of work. The 50/2.8 EL-Nikkor seems to be well liked...

There are several discussions about these sorts of lenses over at thePhotoMacrography forum

Stacked Lenses

I don't know much about this either, but the principle is bung a reversed 50mm standard lens on the front of a 200mm telephoto lens and you should get a roughly 4:1 macro combination. Whether it will work well or not depends on the lenses and there are too many combinations for much to be known on this subject. If you have some suitable sounding lenses then it should be worth splashing out a few pounds on a reversing adapter and some step rings if required.

Video lenses

Another kind of lens which could well be suitable for this kind of work has recently come to my attention. A chap called Linden has shot some spectacular stacks of Sunset moth wing scales using a lens which came from a video camera - "Canon TV lens JF16mm 1:1.4". He is using it reverse mounted on extension tubes.

Some examples of the results he is achieving can be found on flikr.