Originally posted by Digitalis It pays to be careful, there have been quite a few misinformed users on the forums that-shall-not-be-named that have said that a full frame sensor by virtue of its greater area receives more light from the lens and smaller APS-C sensors that have a smaller area receive less light.
You are wrong and those other "misinformed users" are correct. The aperture specifies the illuminance, which is the luminous flux that makes it to the sensor
per unit area. For a given f-stop this is in fact the same across camera systems (ignoring transmittance loss, which we seem to do in still photography, though it's darned important).
Thus
a larger sensor does in fact collect more light -- simply multiply the constant flux by the larger area. This is why, all other factors being equal, larger sensors produce better images (define "better" in terms of signal to noise, dynamic range, whatever).
Originally posted by Digitalis there isn't any cropping going on here - cropping is what a photographer does to a photograph after it is taken. APS-C is is simply a different format, to use the term "cropping" is misleading.
On the contrary, take exactly the same photo with an APS-C camera and a 35mm camera. By "same" I mean perspective, focal length, aperture etc. The result is equivalent to cropping the larger image in the same proportion as the sensor sizes. Using a smaller sensor is like throwing away the extra parts of the image that do not fit into the crop. The term cropping is the simplest way to explain this relationship and is is hence entirely appropriate.
Originally posted by Jüri So to answer the inquiry of original poster, full frame 50mm lens does not replace a 75mm APS-C lens one to one. When both are compared on a Pentax dSLR, the latter will give narrower prespective.
No, perspective is only a matter of where you stand relative to your subject. It has nothing to do with sensors or lenses. I believe you mean "narrower field of view".
Originally posted by billtin59 To me, all this talk of "crop factors" is a waste of time.
Except for those who wish to understand. In this day and age people are swapping lenses between different sensors more than ever. Hence it is important to understand equivalency. I wrote
an article on the topic but apparently no-one reads it, or any that preceded them by other authors.
I applaud the OP for trying the experiment themselves.