Originally posted by atnbirdie I finally did a test to clarify the issue and found that if I set my two APS-C camera zooms (a Pentax and a Tamron) at 50mm and take pictures, the result looks identical to one I then take on the same camera using an old Pentax SMA 50mm prime. That surprised me, but not so much as I thought about it.
Yes, because they all still use the same focal length measurements. So even though your 18-55mm zoom is designed for APSC, the 50mm marker on it is still the same as a standard 50mm lens. They all project the same image regardless, just less or more of it is captured by the different sized sensors, that's all. Which is your crop factor.
This
wikipedia image illustrates it the simplest - same size image projected by the lens, just capturing a smaller segment of it. If both are captured at exactly the same resolution, eg 4000 pixels wide, it will give the illusion the smaller image is actually closer up or zoomed in, hence a 50mm on APSC being an 'equivalent' of 75mm on full frame. It's the same as cropping and enlarging in photoshop.
So your APSC camera is simply not capturing that extra image lying outside the APSC sensor size area which the old 50mm lens is projecting, but a full frame camera could.
A lens designed for APSC is just basically so they can make it smaller and lighter by using smaller diameter glass to only project the required smaller (but still same size image) coverage onto the smaller sensor area only. Which in turn means this lens will only project onto the middle of a full frame sensor instead of giving full coverage.