FX/DX lenses

1. FX lenses are made for FX bodies and DX lenses are made for DX bodies. So a 50mm lens behaves as a 50 mm lens on an FX body and it behaves as a 75 mm lens on a DX body. Why does a 50mm DX lens designed for a DX body behave like a 75mm lens on a DX body? 2. Would the use of FX lenses in a DX body, compared to DX lenses in a DX body, yield much better results? 3. Which one is a better upgrade for a Nikon D7000 user--- D7100 or D600 (for Portrait and Travel)? Amar Saha, via email Great questions! 1. When you use a FX lens on a DX body, the periphery of the image is cut off because the DX body has a smaller sensor. Hence the manufacturer saves on cost and weight by producing a smaller and lighter DX lens that just covers the DX sensor. Mind you, the focal length is not changed, only the size and weight is. So obviously, when this DX lens is attached to the DX body, it too behaves like a 75 mm lens. By that we mean that the 50 mm DX lens provides an angle of coverage that a 75 mm lens would have offered in the 35mm format. Just for your knowledge, the size of the sensor also determines the angle of view. 2. When you use a FX lens on a DX body, only the central portion of the lens is used to pass the light through. This results in better edge sharpness on the DX sensor (sharpness at the periphery of a lens is always less compared to the middle. Using only the middle portion of the lens thus avoids loss of sharpness at the edges). It also helps to reduce corner darkening. DX lenses (and modern FX lenses) are designed differently. They are ‘telecentric’, meaning to say that the light coming out of the lens does not spread out. This performs a very important task – it makes the light hit the photo-diodes (on the imaging sensor) perpendicularly. This is an important requirement for the photo-diodes – it helps to avoid/reduce corner darkening and loss of sharpness at the edges. So to come back to your question – “Would the use of FX lenses in a DX body yield much better results?” – the answer is ‘no’. But if you do use an older version of an FX lens (one that is not telecentric), then you stand to gain some advantage. 3. As a better upgrade for D7000 (for ‘portrait and travel’), I would opt for the D600 over the D7100. Both are 24 megapixel, but the D600 is a fullframe ( FX) model. This means better/smoother tonalities, lesser digital noise (especially at higher ISOs), and better control over the depth of field.