Originally posted by stevebrot Assuming you mean using a dSLR as a copy camera (scanning is different), the short answer is that you need a flat field lens with even even lighting corner to corner.
And although it sounds simple, it's not that easy to achieve in practice. It can take a lot of time to get it right, particularly if you scan negatives.
If you have many negatives, unless you need top IQ, it's probably easier and faster to get a photo scanner that have a batch scanning mode. Scanners like the Epson V550 are cheap, work well enough for family photo and will allow you to scan negatives band by band instead of shot by shot. Autoalign everything, takes care of scratches and dust, and reverse/balance the color. Way much faster than taking a macro shot of every photos, importing them in a computer, reverse the colors, correct the scratches...
I've tried both methods and for family photos, which don't have a stellar IQ anyway, the scanner method is good enough without much hassle compared to the macro lens method.