AF Nikkor 50mm f/1.8D for D5100?
Lens is 50mm FX format, when I use in my D5100 which DX. Like how would be this mm? Some of my friend said it will be like 70mm. Is that true?
The crop factor is 1.6
So your lens would give the field of view (FOV) of 1.6x50mm=80mm lens on a 35mm format.
A 50mm lens is 50mm on any format. The field of view would change according to the size of the sensor.
A DX lens would also have the same issues.
Not strictly speaking - the focal length doesn't change just because you mount it on a different camera - a 50mm lens is a 50mm lens.
What does happen, is that because consumer DSLR's have a smaller sensor than old, film SLR's (which were 35mm or ''full frame), the field of view is cropped - and it will give you the equivalent of the field of view of a 75mm lens in full frame terms, that's all.
The crop factor for Nikons is 1.5 (not 1.6 which is Canons).
The only use of this is to give those of us who are/were used to shooting 35mm film some reference as to field of view when using DSLR's. If you've never shot 35mm it has no relevance whatsoever.
You do know don't you that this lens will not autofocus on your D5100? You will need to manually focus it or buy the AF-S version (more expensive).
The focal length of a lens never changes. What changes when you use a 50mm lens on your D5100 is the equivalent angle of view.
On a 35mm film camera or full-frame DSLR a 50mm lens is considered a "normal" lens since its angle of view approximates that of the human eye.
On your D5100 with its 1.5x "crop factor" a 50mm lens has the equivalent angle of view of a 75mm lens on a 35mm film camera or full-frame DSLR. The "crop factor" causes the 50mm lens to become a medium length telephoto. This makes a 50mm lens on your D5100 a good portrait lens.
In case you aren't aware of this, the Nikkor AF 50mm f1.8D will NOT auto focus on your D5100. If you want auto focus you'll have to spend more money for the Nikkor AF-S 50mm f1.8G lens. This is because your D5100 doesn't have a focusing motor in the camera body, thus requiring a lens with its own focusing motor. Nikkor AF lenses don't have an in-lens focusing motor. Nikkor AF-S lenses do.
Focal length doesn't change. However, because the sensor size is smaller than 35mm film, it simulates the field of view of a 75mm lens on a 35mm film size.
It's a D so no, not FX. On the D5100, the 50mm f/1.8D will still be 50mm. The angle of view will be like that of a 75mm lens on a full-frame camera.
The 50mm f/1.8D lens will not autofocus with your camera. Get the 50mm f/1.8 AF-S version. Just like the D version, the 50mm f/1.8 AF-S will also be exactly a 50mm lens on your D5100 and behave like a 75mm lens on a 35mm film camera.