I shot some comparisons as promised and the results are quite interesting. For this post, I only compared the lenses at 28mm and 70mm and used unedited raw files.
Sharpness
The short version is that both lenses are great. In the center, at 28mm, the 24-70mm is sharper by a hair at F2.8 compared to the 28-105mm at F3.5, but the difference is negligible. In the corner, the lenses are about the same, though the 28-105mm exhibits more fringing.
At 70mm, again the two lenses are neck-and-neck in the center, even wide-open. Out in the corner, though, the 28-105mm actually pulls a little bit ahead of the 24-70mm, which is softer and shows more fringing wide-open. Given the 1.5 stop difference, though, this is to be expected. When the 24-70mm is stopped down, it catches up to the 28-105mm.
Stopped down, I'd say that both lenses are sharp "corner to corner". So, in practice, the difference between sharpness between the two is negligible.
Vignetting
Both lenses vignette significantly when wide-open at the wide end. At 70mm, the 28-105mm vignettes noticeably more, however, as it has visible falloff near the center of the frame when wide-open. Things clear up nicely after being stopped down to F4/5.6.
For wide-open shots, some vignetting remains visible even after running in-camera corrections. All in all, the 24-70mm is the winner in this category, but you can still see dark corners in the viewfinder with it. The vignetting just isn't as bad at the tele end.
Field of View
This is another area of practical significance. The 24-70mm gives you a field of view that's 8 degrees wider than the 28-105mm. On the other hand, the 28-105mm gives you plenty more reach.
Size and Weight
The 24-70mm is almost twice as heavy as the 28-105mm, and its barrel is some 2cm wider in diameter. I didn't find the larger size to be an issue, but you can certainly feel the difference in weight. Both lenses handle well, though I prefer the tactile feel/distance scale of the 24-70mm's focusing ring.
Conclusion
Both area great lenses! At the end of the day I think it boils down to features and size. The 24-70mm is fast and heavy, the 28-105mm is slow and light, but the lenses' overall optical performance is otherwise similar.
The 28-105mm certainly exceeds expectations given its
$499 price tag. While it looks similar to the APS-C 18-135mm, it's considerably sharper.
Adam
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