Originally posted by simons-photography yea i understand the crop factor thing and i tend to reason in terms of 50 mm as the "normal" but now I have to start thinking in terms of 33 mm as being the normal.
if the image is croped by 1.5 (and 1.5 X 1.5 = 2.25) then its receiving proximately half the light isn't it ? as the light outside the sensor area wich is half the area of a film frame does not come into the image anymore. erm maybe I'm talking jiberish actually = bed time after I've read those links
While the image is receiving half the "light" in total, with the croppped sensor, the intensity of light per square mm does not change, that is why your exposure does not change.
You need to simply consider focal length is always focal length, it is the distance (for a simple lens) that the lens is away from the plane of focus, for an object at infinite distance. Because this is a physical property of the lens, it cant change, it is independant of the sensor size.
F Stop similairly is a fixed parameter of a lens. Again consider a simple lens, it is the diameter of the lens divided by the focal length. Again this cant change and is independant of the sensor size. The F-Stop of a lens really is only an indication of how bright an image is, and this brightness is based upon the intensity of light per square mm.
With respect to the crop factor and focal length, it is better to forget all about "equivelent focal length" what really changes is the field of view, that is all. an ASP C sensor crops out the middle of a 35 mm frame, hence with the same lens it is clear that the angle of view on an ASP-C sensor is less than a 35mm or full frame sensor. In terms of printing, to print up to a specific size image, the smaller sensor requires more magnification in processing.