Originally posted by WMBP But the lenses? This Nikon 50 f/1.4 doesn't look any bigger than my Pentax 50 f/1.4:
It's not. But it's also not relevant. I assume you are using Nikon as an example FF system here, with Pentax the APS-C system. Fine. In which case, it's not the Nikon 50/1.4 you need to compare against the Pentax 50/1.4. It's a hypothetical 75/2 - because you need a 75mm lens on FF to give the same FOV as 50mm on APS-C. A 75/2 on FF would perform identically to a 50/1.4 on APS-C - same FOV, same DOF wide open, etc. And you'd get the same noise performance, because on the Pentax system you'd be shooting at half the ISO of the Nikon system. That's one of the main points of the "equivalence" article - that 50/1.4 on APS_C is the equivalent in all ways that matter to 75/2 on FF.
In order to get *better* performance on the Nikon, you'd need a 75/1.4. And that would indeed be a much larger lens than a 50/1.4, all else equal (ie, no fair considering the possibility of a "pancake" design for the 75 unless you also consider it for the the 50).
Of course, at these focal lengths, size isn't a huge issue. But get into longer telephoto focal lengths, and it's a much bigger concern.
Quote: No. These are *not* equivalent lenses when comparing FF to APS-C. A 70-200/2.8 is a larger diameter lens than a 50-135/2.8. As I said, you'd need a 50-135/2 on APS-C to yield the same diameter - and hence the same performance on APS-C - as the 70-200/2.8.
Now, since these are constant aperture zooms, they use some of the "optical trickery" I referred to earlier so their *actual* diameter is less than the effective diameter. Plus the specs give the diameter of the body of the lens, not of the glass in it, which adds a relatively constant fudge factor. That's why youire not seeing as big a difference between the 50/-135 and 70-200 as you might otherwise expect. But look at them in person - it's actually a very significant difference.
Quote: I'm pretty sure I'd get better results from the Nikon lens mounted on a D3 than I get with the Pentax lens.
Right, because of exactly what I've been pointing out over and over and over: it's a larger diameter lens. If someone were to make a 50-135/2, that would be the same diameter, and would yield the same performance as the 70-200/2.8.
Quote: It seems to me that the difference in the SIZE of the lenses is less of an issue if you tend to shoot things that are closer....
Well, let's say it's less of an issue at shorter focal lengths. Distance to subject isn't what matters. After all, landscapes are often shot with wide angle lenses focused close to infinity.