The technique I've landed on for photographing microsnails is reversing a prime focal length lens. After testing close-up filters I feel the quality and magnification are better by reversing a lens.
Note that this math seems to only hold true for reversing a prime lens (non-zoom). Also, the reversed lens should be set to it's widest aperture and both lenses focused at infinity.
The math for how much magnification you get reversing a lens is a simple forward lens focal length/reversed lens focal length. So, if you have a 200mm lens and you reverse a 50mm you are enlarging 4 times. It's handy-to-do, uncomplicated math.
Now I've got two choices as to where to reverse a lens. I own a Nikon D90 dslr camera and just purchased a Panasonic Lumix FZ150 digital camera. They are both 12 megapixels. This means the bigger sensor is less crowded for better images. Thus, the D90, being a dslr, has an edge on image quailty. Still, both produce usable images. The FZ150 I'd likely keep to ISO 100, maybe 200. The D90 I can shoot to 400, maybe 800.
The key, however, to enlarging these 1 to 4mm snails is sensor size. The D90 has a sensor size of 23.6mm x 15.8mm and the FZ150 has a 6.16mm x 4.62mm. So you can imagine which camera it's easier to fill with an enlarged portion of light projection.
The lens I own for the D90 is a 90mm macro. Forward it's a 1:1 lens. Useful for full frame if I'm photographing one of the larger polygyrids. With a 50mm lens reversed it's 1.8x enlargement. So let's say I want to photograph a 1.1mm Punctum minumissimum. Saying the snail is evenly round it projects a 1.98mm image onto a 23.6mm sensor. It's not very big, only 8% of the sensor height.
The FZ150 has a built-in lens ranging from 4.5mm to 108mm. This is equivalent to 24mm to 600mm in 35mm terms, but you can't use that measurement, only the real focal length. Reversing that same 50mm lens you get a 2.16x enlargement. It's not that much more than the dslr with a 90mm lens but the key is where it's being projected. Now the snail is 2.376mm on a 6.16mm sensor or 38.5% of the image height.
If I wanted to fill as much of the frame as I could with that same snail I'd just need to use the short measure to reverse engineer it. It needs to just about fill 4.62mm, so for easy math let's say I want to enlarge it to 4.4mm on the sensor (remember, it's 1.1mm), that's 95%. Good enough. Desired enlargement size/actual size is 4x magnification. Forward lens/ magnification is 27mm. I don't know of any 27mm lenses but 28mm lens are pretty common. If I were to get one my P. min. would now take up about 92% of the short measure of the image. The long side only gets about 70% but I would suppose an apical view with an even measure. That's still a lot of image on a 12 megapixel camera.
But what if I really wanted to use that D90 ? I could always use extension tubes. I've got about 58mm of extension tubes, add the 90mm lens for 148mm, reverse a 28mm lens is 5.28x enlargement. The snail is now 5.8mm out of the short measure (15.8mm) so a little over a third. I've never tried this but that seems like it would be the math. I could be wrong because of where the back of the forward lens is in. If I'm assuming I'm doing the math right, what if I added in a 2x teleconverter. It would go directly behind the forward lens but not between camera and tubes. It's now a 180mm lens and 58mm of tubes--238mm. Supposing this is all right, we're now at 8.5x magnification. Our snail is 9.35mm of projected image. I'll leave you to the math of what lenses/tubes/teleconverters/alchemy to use to try to fill the frame. Needless to say there are a ton of possibilities. But these possibilities generally add up to more of a cost that just the digicam and a prime lens or two.
As you see, it's easier to work with the small sensor for these small snails. Plus ultimately cheaper if you're starting from scratch. Another bonus is that the digital camera, although big as far as digicams go, is still lighter and compact thus more likely to go on a hike with me.
The biggest gotcha I've found with reversing a lens is that some lenses/combinations/focal lengths will be seen on the periphery of your image. I only have the 50mm lens and on the FZ150 I can use it from 76.5mm to 108mm without seeing any of the reversed lens. I'm not sure if a shorter lens would allow me to zoom out more or exacerbate the situation or keep it about the same. I'll have to give it a try if I can get my hands on a shorter lens.
One last thing. If you want to get really fancy you an just reverse the lens without a lens in front of it for various magnifications. I didn't do the math and I'm super tired but as a for instance I made a makeshift extension tube out of a Pringles can and then reversed a 50mm lens at the end of it. It would up giving me about 8x magnification (I shot this with the D90 since you can't remove the lens of the FZ150). Not bad for a 99¢ can of chips (although I find the inedible) and a lens I got on ebay for about $20. I already had a macro reverse ring to attach to a camera but you could also modify a body cap from the camera to connect it if one was so inclined.
Next headaches: Depth of Field, Lighting, Camera Shake.