He would like to increase the depth of field when photographing, for example, spiders. He was wondering if the Mirex tilt-shift adapter may help solve this problem. It should also be noted that he photographs with a full-frame Nikon digital SLR, a 200 mm macro lens and has plenty of other Nikon lenses.
Although this is not necessarily a purchasing problem, I think the problem may be interesting for other readers and my reply is nevertheless an appeal for moderation. Here are some thoughts and opinions in relation to the spider photograph problem:
How to maximize depth of field in macro photographs
If it is the goal to maximize depth of field in a macro photograph, I would recommend to align the camera sensor parallel to the subject that should appear sharp (e.g., parallel to the spider net). Besides this, the depth of field can, for example, be increased by using a lens with a shorter focal length and by photographing with a smaller aperture. However, by doing so, the background of your macro photographs will likely become very irregular and uneven, which I find really annoying and distractive.
Tilt-shift and depth of field
It is correct that tilting the lens can be used to increase the (apparent) depth of field. The "normal" situation for this application is a landscape photograph where everything from the foreground all the way to the background should be rendered sharp. Tilting the lens downward tilts the focal plane so that it becomes (more) parallel to the landscape and therefore the depth of field appears larger. However, as far as I understand, tilting a lens achieves this effect by moving the focal plane, not by increasing the width of the area that appears sharp.
Tilt-shift and macro photography
As far as I understand, many people use tilt movements in macro photography not to maximize depth of field, but rather to be able to photograph with an open aperture, to dissolve the background and to place the (shallow) focal plane as desired. At least this is how I use the tilt-shift adapter for macro photography.
Tilt-shift adapter for full-frame Nikon DSLRs
The tilt-shift adapter that I have described and use is designed for the micro-four thirds mount (it could possibly be used on Sony NEX cameras via an adapter) and Canon EF lenses (Nikon lenses could be mounted via an adapter). In order to allow the tilt and shift movements, the lens must cover a larger image than the size of the camera sensor. A tilt-shift adapter for a Nikon (or another full-frame DSLR) must thus have a mount for medium or large format lenses. The best Nikon tilt-shift solutions (although expensive) are the excellent Nikon tilt-shift lenses. However, these will likely not solve the spider photograph problem, because in my opinion tilting is not necessary in this case.
I am afraid that I do not have a real solution for the spider photograph problem described above. I would definitely not recommend buying another camera in order to use the Mirex tilt-shift adapter, because I do not think that this would solve the problem. If I was challenged to photograph spiders in their nets I would most likely try with a 100 mm macro lens (in full frame terms) because I think this provides the best compromise between working distance, nice rendering of the background and sharpness (and because this is what I have/had and am experienced with). Approaching the spiders will be more difficult than with a 200 mm macro lens, but I would try to overcome this by photographing as early in the morning as possible, when the spiders are likely slower. There are people who photograph lions with wide angle lenses, so with patience and perseverance it should be possible to capture spiders with a 100 mm lens!