When stacking a reversed lens on top of your other lens, here are the factors to consider:IMAGE LINK: http://www.flickr.com/photos/syamastro/6039630737/ TEST: DIY macro lens
- Magnification will be proportional to FL1/FL2, FL1 being FL for the on-camera lens, FL2 is for the reversed lens. So to gain maximum magnification you'd go for telephoto on-camera lens and widest possible reversed lens. In my experience, both lenses have to be fairly fast, to avoid any vignetting. (E.g., placing my Sigma 10-20 f4.0 reversed on top of my 135L resulted in terrible vignetting - only a tiny circle of light was visible).
- Going after too large magnifications, by using very wide reversed lens, will not result in better details resolution in my experience: placing 28mm f2.8 reversed on top of 135L did produce larger magnification than with reversed 50mm f2.0, but didn't resolve finer details (so you only get smaller field of view). I think 35mm (f2.8-3.5) is the widest you wanna go.
- It is very convenient to use MF reversed lens - because of the manual aperture control. You need wide open aperture during focusing, and then manually closing it down to f5.6-f/8 before shooting, to maximize DoF. (Changing the aperture of the on-camera lens doesn't make any difference, until the point when you start seeing sharp vignetting, when the aperture becomes too small. I think you should simply keep the on-camera lens wide open.)
- Many would say you don't need AF for macro, but at least sometimes (for slowly moving or still objects) this can be very helpful: one can run a sequence of shots with slowly changing focus of the on-camera lens, to be used for focus stacking (to greatly increase DoF). One can use a laptop tethered to the camera + automatic focusing software, or even more conveniently - alternative firmware Magic Lantern. You can get results like this (reversed 50mm on top of 135L):
, on Flickr