Reverse Lens Technique Tips and Precautions
1. It’s best to use a wide aperture 50mm lens as the first lens. Again, these lenses are simple in construction and won’t lend their own artefacts to the image. That said, it's not a necessity and the technique will work with pretty much any two lenses.
2. Try to keep the lenses at their widest aperture to let the maximum light in. It might be tricky with the reversed lens, as its aperture is no longer under the camera’s control. You may need to manually hold the aperture lever at its widest. Unconventional methods like taping the lever or using a thick card paper to wedge it might be necessary. Please be very careful with this step, as there is a potential to damage your lens irreparably.
3. Automatic controls like auto-focus and auto-metering may not work. It's best to use manual focus. You also may have to physically move the camera back and forth to achieve focus. Be careful while doing that. Again, shoot multiple exposures and bracket a lot to get the exposure correct.
4. Always protect your lens with a UV filter, lest you scratch it.
5. If your first lens is 50mm, and you use a second lens with a focal length less than 50mm, you’ll end up with vignettes on your photo. There’s no solution for this except cropping that much out. Else use a really wide angled first lens.
6. Use external lighting, as flash lighting would in all probability be blocked by the second lens, causing its own shadow to fall on the subject.
7. A tripod will help a lot! You may have to restrict yourself to high shutter speeds and keep your hands rock-steady. A cable-release or using the timer would prove invaluable for sharp photos.
8. One really neat trick with the reverse lens technique is that with two simple wide angle lenses back to back, you have a powerful high-magnification macro lens! Beat that!