ETA: Just realized I replied to a very old post. It's been a while since I was here. The facts are as true as they ever were, even if the info is too late to help, so I'll let it stand.
Quote:
Originally Posted by suzzer99
I sold my gf my D-80 (DX, crop sensor). She hinted that she wants a 50mm prime for Xmas. I ordered her this as I'm 95% sure her effective focal length is going to be 50mm. But it concerns me why Nikon would make a DX 35mm and not just call it a 50mm if it's meant for DX cameras.
That's probably the right lens to get her. It is a "normal lens", which means it has a focal length similar to the diagonal dimension of the sensor of the camera on which it will be used. This gives it an angle of view that seems natural. Most people think they need a 50mm lens because it was what nearly all cameras came with for about 50 years. But those cameras were 35mm film cameras, and on them, 50mm was a normal lens. The other choice would be the AF-S 28mm f/1.8G. It is even closer to true normal, but that lens is larger, more expensive and not as sharp.
They call it a 35mm lens because that's what it is. The focal length of a lens is the distance between the optical centre of the lens and the image plane when the lens is focused at infinity.
Mounted on a Nikon 'DX' camera (which is a camera with an F-mount for its lens but a sensor that measures about 24mm x 16mm instead of the the 36mm x 24 mm 'FX' full frame sensor the same size as 35mm film for which F-moint was originally made), the 35mm f/1.8 lens gives about the same angle of view as a 53mm lens would on an FX camera.
It is an f/1.8 lens but on DX it gives the same Depth of Field and diffraction blur as an f/2.7 lens would give on FX.
As a result, we could say that a 35mm f/1.8 lens mounted on a DX camera is equivalent to a 53mm f/2.7 lens mounted on FX.
Generally companies that make full frame cameras are not in the game of passing off their lenses as something they're not. Companies that make mostly cameras with small sensors like to report the focal lengths of their lenses only in Full Frame equivalent terms, because it makes them sound bigger. But they don't like to report the equivalent aperture because it makes them sound slower.
Last edited by DoTheMath; 10-29-2017 at 10:21 PM.
Reason: Can't read posting dates