use of a FX lens on a DX (ie. cropped) camera

  Please excuse a naive question, but was wondering if someone could tell me if I use a FX lens at a particular aperture on my DX Nikon camera (the D7100),
would it be "darker" than if I use a DX lens at the same aperture, assuming everything else (shutter speed, ISO, etc) was set the same?  Or asked a different way,
do I have to use a longer exposure when I use my FX lenses at a particular aperture on my DX camera than when I use my DX lenses at the same aperture?
thanks for any advice,
richardo

The only difference between an FX lens and a DX lens is the size of the image circle it creates. FX lenses are designed to cover a minimum of 36mm x 24mm where DX lenses are designed to cover a minimum of 24mm x 16 mm. There are no other differences.

Yes so if the FX lens focuses the image on a larger surface area than a DX lens, and one is using a camera with the smaller sensor, wouldn't less light hit the sensor with the FX lens than a DX lens which focuses the entire image on a smaller area (ie, an area the same size as a sensor in the DX camera)? Or am I missing something?

Less total light yes because the sensor is smaller but not less light on the active area of the DX sensor compared to the same crop on the FX sensor. The center 24x16mm area of the FX sensor gets exactly the same light as the DX sensor does. So if you crop the FX sensor to the same 24x36, the same total amount of light hits that part of the chip. Now at a pixel level it could be different due to different pixel sizes but at the sensor level, the same 24x16 area gets the same amount of light.

Thanks EJ for taking the time to reply. So would you say I could use the same length of exposure when I use my FX lenses at a particular aperture on my DX camera than when I use my DX lenses at the same aperture?

ricardo00 wrote:Thanks EJ for taking the time to reply.  So would you say I could use the same length of exposure when I use my FX lenses at a particular aperture on my DX camera than when I use my DX lenses at the same aperture?

I'm not sure where you're trying to take this. A lens, set to F8 will provide light to the entire image circle at F8. It doesn't really matter what sensor size the camera has, it will still need to be set to F8 to expose for the light hitting the sensor as long as the image circle covers the sensor.

So a DX lens set at F8 will provide the equivalent amount of light as an FX lens set at F8. Both will produce an image circle of a certain size. As long as the sensor is covered by the image circle of the lens, there is no difference between the two in the amount of light placed on the sensor.

So the length of the exposure is no different in the case of the question above.

Thanks Tony for your input. I have been buying FX lenses in part because I am thinking I will someday switch to a
FX camera and also because in part the equivalent DX lenses are not available. Recently I started wondering if I am
in part losing out by doing this since the FX lens is focusing the image on a larger area than my current DX camera sensor covers. If so, I might be better off in low light situations using a DX lens with the same aperture as my FX lens, if I have the choice. That is what I have been struggling with.

ricardo00 wrote: So would you say I could use the same length of exposure when I use my FX lenses at a particular aperture on my DX camera than when I use my DX lenses at the same aperture?

Yes!