35mm lens and crop sensor?
If I were to buy the Nikkor 35mm f/1.8 DX lens, and use it on my D5100 with a crop sensor, would it effectively be a 52.5mm focal length, because of the cropped DX sensor, or does the fact that this is a DX lens mean that it is meant to be a 35mm focal length when used with a crop sensor?
Added (1). So with the differing angle of view and stuff, a 35mm would LOOK like a 50? I know focal lengths arent changed by the camera.
You may have misunderstood something.
A cropped sensor just uses part of the image being projected on the sensor when compared to a full frame sensor. This means that the angle of view is smaller when you use a cropped sensor camera, the focal length of lenses remain the same.
My 210 mm lens I use on my 4x5 view camera is a "normal" lens. If I were to use it on a full frame dSLR, it would become a telephoto lens, but the focal length stays the same. Placed on a cropped sensor camera, the lens sees even a smaller angle of view.
Sample angle of views:
210 mm on 4x5 view camera as an angle of view of 62°
210 mm on a full frame, sees 11°, 40 "
210 mm on a cropped sensor, sees. 7°, 40 "
The focal length is just as the lens says, 35mm.
But depending on the size of the sensor or film, a lens of a given focal length produces a different angle of view. A smaller sensor produces an angle of view that appears to be more zoomed in compared to a bigger sensor. That's because that smaller sensor basically covers a part of the area that the bigger sensor otherwise would occupy, and appears to "crop" a piece out of the image that would be produced by the bigger sensor. When this cropped part is displayed at the same size as an image produced by the bigger sensor, then it appears to be magnified compared to the image produced by the bigger sensor.
But still, the actual focal length of the lens remains the same. That crop factor stuff merely is used to determine what angle of view or apparent zoom level the lens produces compared to a camera that uses a full frame sensor.
This means that a 35mm lens when used on a DX camera with a 1.5 crop factor produces the same angle of view as a 52.5mm lens would produce when mounted on a full frame camera.
Note however that the crop factor comes from the camera/sensor, not from the lens.
A DX/FX designation on the lens only says if the lens can be used on the respective type of camera. FX lenses can be used on DX and FX, DX lenses should only be used on DX as they don't fill the whole frame on FX (albeit technically they can be used on FX). This designation on lenses does not affect the crop factor. The crop factor comes from the camera.
E.g.a FX lens, when mounted on a DX camera, would be subject to the 1.5 crop factor for the 'comparative' zoom level just like a DX lens is when it's mounted on a DX camera.
The 35mm lens will still be 35mm on the D5100. Due to the smaller sensor size of your camera, the 35mm lens will have an angle of view that is equal to a 52.5mm lens when mounted on a film or full frame camera. Focal length does not change. Only angle of view. If you don't work with full frame and film SLR's, there's no need to know the difference. You shoot with what you see.
The DX in the model designation of the lens indicates that the lens was made for crop sensor cameras. If you mount it on a full frame or film SLR, the lens will not be able to project an image to fill the whole digital sensor or frame of film. You would end up with a small picture with large dark fuzzy borders. This was done to make DX lenses cheaper.
The focal length is the focal length - it is a physical property of the lens. A 35mm lens is still a 35mm lens even when it it not mounted on a camera.
When you put a 35mm lens onto a cropped sensor APS-C Nikon camera it gives the same image coverage as a 52.5mm on a 35mm film camera or a full frame digital.
In reverse a 35mm lens on a 35mm or FF camera gives the same image coverage as a 23mm lens on a Nikon APS-C camera.
The focal length of a lens is always what it says it is. What you are dealing with is the change in the angle of view when a 35mm lens is mounted on a full-frame DSLR versus when its mounted on your 1.5x "crop sensor" D5100.
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. A 35mm lens on your D5100 will have the equivalent angle of view of a 52.5mm lens on a 35mm film camera or full-frame DSLR. So on your D5100 you'd use the 35mm as a "normal" lens when you wanted to photograph a scene close to how you saw it with your eyes.
http://www.sweeting.org/mark/lenses/nikon.php