Originally posted by rogerstg I was under the impression that the same image can be achieved by simply cropping the ff image to the same FOV as the crop camera - no?
No. 15 MP instead of 24 MP. You can create the same image on APS-c by using a shorter lens, and the difference will be equally obscure, between the FF and APS-c image. Bottom line, both can do what the other does for 95% of the images you're likely to capture.
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There's just one good reason for buying the DFA 28-105. It's modern glass and comes under th umbrella of "lenses designed to take advantage fo the resolution of the K-1." That being said, it also takes advantage of the higher centre sharpness of a K-3 or K-p (or K-70).
Apart from the 70-200 and possibly the 150-450, there are no other candidates.
The new modern primes are coming but they are coming slowly.
In my experience, no other lens is going to give you the same IQ. You can talk about fast primes, but there are no modern fast primes for Pentax apart from the soon to be released 50 1.4.
Modern glass for modern sensors completely redefines a new set of standards in both design and manufacturing. Older film primes were designed and manufactured to a lower standard. That's the ugly truth.
SO what are your other choices. Around my house, my wife and I both use this lens at every opportunity on both FF and APS-c. But there's no way to explain why. It's images are just more compelling than other lenses. They are cleaner, better micro-contrast, brilliant resolution. Rendering that's to die for, but even with all that, you still might not like it. So if you're hoping for some sure fire reason to buy the lens, their isn't one.
The biggest reason would be you have a K-1 and you want to make the most of it. But there are lots of other reasons that wold be just as good.
When kenspo first suggested this I ripped into him pretty good. Now that I have a lens designed to the new standards, I have to agree with him. My older glass simply isn't comparable.
When my prime loving wife starts lugging a zoom everywhere, you know something's up.