when looking at FX lenses for DX sensor...

[nemesis256]

... should I look at the actual focal length or the effective focal length? In other words, if I have a 200 mm DX lens, will a 200 mm FX lens on a DX sensor have the same field of view? Or would it have a field of view of a 300 mm DX lens?

[Sterling]

Millimeters are millimeters. If you have a DX sensor then any type of lens will work - there is no difference between a 200mm lens labelled DX or a 200mm lens labeled FX. They will both have the same field of view. When it comes to lenses, DX or FX designations don't matter if you have a DX sensor.

It is only if you have an FX sensor that you want lenses made for FX but it has nothing to do with differences in focal length. The difference between DX and FX lenses is that DX lenses are designed for a smaller image circle to work with the smaller DX sensor.

Now it is true that a 200mm lens on a DX sensor will have a similar field of view as a 300mm lens on an FX sensor (because of the 1.5X crop factor). But that only matters when comparing sensors not lenses. So, actual focal length is all that really matters.

[kirbinster]

The above is all good information, with one minor exception. When you use a lens that is designed for FX on a DX body you are only using roughly the middle two thirds of the diameter of the lens. This is important as most glass degrades image quality as you move toward the outer edges. Thus, a lens that has edge image issues on an FX body may perform much better when used on a DX body.

This is in part why Nikon replaced the great 70-200 with the newer version.