Originally posted by vonBaloney That's all you need to know, and really you don't even need to know that if you can forget this "equivalence" nonsense.
The notion of "equivalence" is not nonsense. On the contrary, it is helpful to understand which lenses perform the same on different cameras with different sensor sizes.
For instance, a 50-135/2.8 lens on an APS-C camera produces the same images as a 75-200/4 lens on a full-frame camera (figures are approximate). Note the "f/4"; people often just convert focal length and then state that a 70-200/2.8 lens is much larger and heavier than a 50-135/2.8 on APS-C, neglecting the fact that a 70-200/2.8's on FF equivalent would be a 47-133/1.8 on APS-C. It is no wonder that a 50-135/2.8, i.e., f/2.8 instead of f/1.8 as the widest f-ratio, can be smaller and lighter. Very little to do with the image circle, but a lot with the maximum aperture size (which also dictates the front element size).
If you are interested in this stuff, I can recommend
this article.
Originally posted by alamo5000 I take it lenses made for 1.5 crop sensors are physically smaller than their counterparts?
As alohadave already wrote, it is difficult to turn a smaller image circle into a size/weight advantage. You gain a little on the size of the rear elements, but the overall size of most lenses is dictated by their light gathering abilities.
Lenses for smaller formats only often appear to be smaller because they often are "slower" (i.e., have less light gathering ability). This reduction in "speed' is not readily apparent because f-ratios of small sensor formats look "fast" before you convert them to their full-frame equivalent values. For instance, the full-frame equivalent of the 8.5/1.9 lens for the Q, is a 47/10. Doesn't look that fast anymore, does it?
Originally posted by alamo5000 Unless your full size sensor is a 32 megapixel camera the original light capture on a 16 MP crop sensor in theory could give you a sharper image than cropping down a 24MP full frame to the same size...
To convert MP between different sensor sizes, you need to multiply/divide with the square of the crop factor. So, in this case, 36MP on FF has the same pixel pitch -- and thus allows cropping without loss of resolution -- as 16MP on APS-C.