Originally posted by weaponx525 did not expect that the variable aperature would be affected.
Ignore Mr. Northrup. Constant aperture compared to variable aperture is a red herring, Northrup's point about the amount of light hitting the sensor with different image sizes is just plain wrong. My A 70-210 f4 lens is not f6 on my K-30, other than depth of field
if comparing to photos taken with the same lens on a FF body, using a FF lens on an APS-C camera doesn't affect the image within the APS-C field of view. The light hitting each pixel doesn't change. That's what the camera has to expose for.
A FF lens has to cover a larger sensor with the same registration distance (flange focal distance) as an APS-C only lens on the same mount; depending on the focal length, the FF lens may need to have a bigger diameter and a different combination of glass elements, which will tend to make FF lenses heavier, larger and less convenient to handle than a lens designed to work only with APS-C sensors. That's why the FF 28-105 has similar dimensions to the APS-C 18-135 lens, even though the focal length range is shorter. By the way, the aperture range of the 28-105 is the same as the 18-135 over their respective focal length ranges. If Northrup was right, why wouldn't the 28-105 have an aperture range of f2.3-3.7?