You may ask, what is "standard" about this lens? Why is the 50mm lens referred to as the "standard" lens? Aside from the fact that in the past a 50mm lens has customarily accompanied 35mm SLR cameras on purchase, there are several other reasons. Technically, a normal lens is defined as one whose focal length closely approximates the diagonal dimension of the picture frame. A 50mm lens' focal length is closest to the 43.2 mm diagonal of the 35 mm camera's 24 x 36 mm frame. Another is that its field of coverage (40° horizontally, 46° diagonally) is roughly equal to what one human eye can view with relative clarity. Even though their focal length are somewhat longer than the 43mm diagonal of the 24 x 36mm format, simply, the 50 mm standard lens gives an "honest" image because of its perspective yields, which is extremely close to faithfully close to human vision. Combine this with the fact that this lens is often successful in creating the same image of the scene that the photographer has in mind, and it's easy to see that the 50mm lens is a good basic lens.
Extracted from: Nikon's Nikkor 50mm standard / normal lenses
Lưu ý rằng trên máy DSLR, focal length thực sự là focal length ghi trên ống kính nhân với crop factor của body (là tỉ lện diện tích của sensor máy đó so với film chuẩn 24 x 36mm). Ví dụ về crop factor của một số dòng máy phổ biến là:
1.3x – Canon EOS 1D/1D MkIIN
1.5x – Nikon D40/D40x/D50/D70/D70s/D80/D200/D2X/D2Hs, Minolta 7D, Fuji S3 Pro, Pentax *istDS/K100D/K110D/K10D
1.6x – Canon EOS 300D/350D/400D/450D/500D/550D/10D/20D/30D/40D
2.0x – Olympus E-400/E-500/E-300/E-1
