@dpworkin is correct. With traditional 35mm full frame camera, the 55mm focal length is considered the proper 1:1 ratio with the human eye for perspective and distortion. However, this is for infinity focus only. For distances of approximately 20’ and closer, an 85mm is the proper distortion. Closer still, at 100mm. The best macro work is 100–200mm.
No one makes a 55mm lens any longer, so you’d have to find a used one or buy zoom. Minolta disagreed with the formulation, and made their normal lens a 58mm.
Medium format requires a 75mm, or 80mm, or 90mm for normal depending upon if you’re shootiing 6×4.5, 6×6, or 6×7 respectively.
But you asked about digital. And that also depends upon the format.
Nikon, Canon, Sony all make full frame cameras where the 55mm lens formula would fit. Some people, including myself, have the original Minolta Rokkor 58mm modified to fit on the Sony. But these manufactures, and Pentax, among others, also make smaller chip DSLR’s that vary between a 1.3x or 1.5x crop factor. So you would have to get something like a 30mm to 35mm lens to act as a normal lens. This Sigma 30mm f1.4 is the typical answer for those wanting a high quality low light normal lens for their cropped sensor DSLR.
OK then there is Olympus, Panasonic, Samsung, and others who have adopted what’s called the 4/3’rds chip. These cameras double the focal length, so a 25mm acts like a 50mm, an 8mm acts like a 16mm, a 400mm like an 800mm and so on.
They also have what’s called a Micro 4/3’rds, which acts the same, but takes a different series of lenses that move the rear element closer to the chip. You can’t use Micro 4/3rds on regular 4/3rds, but you can use the regular on the Micro with an adapter.
Then we have the Sigma system with a 1.7x crop factor. You can do the math on that one.
Then we have the new series of pseudo full chip point and shoot cameras. They are generally half frame at 1.5x, so a 28mm setting on the zoom would be closer to the normal 56mm on full frame.
Then we have the standard mini chip point and shoot cameras. They may advertise 10mp – 15mp, but that’s accomplished by cramming smaller photo sites into a smaller chip size, which increases noise levels. Their chips vary wildly in size and crop factors depending upon manufacturer, so only the brochures or instruction manuals would know the answer to your question.
Then we have camera phones…