Originally Posted by D-Lucas
The ISO 12233 comparisons show crops from the center, mid-frame, and corner (labeled on the left side). When comparing EF-S to L-lenses, keep in mind L-lenses are EF lenses with a full-frame image circle - the larger the image circle, the more difficult (and more expensive) the design. So, it's harder to design FF wide angle lenses than the EF-S counterparts, so for a similar price point, the EF-S lens will likely outperform the L lens, because the elements do not have to project as large an image circle. To take that to an extreme, look at lenses for medium format cameras - the 'cheap' ones (sort of the MF equivalent of the $100 Canon 50/1.8) start in the $600 range. The cheapest 'general purpose zoom' for MF is over $2K.
Even if Canon releases an EF 17-40 f/4L II, I doubt it will match the IQ of the EF-S 15-85mm when comparing them on a 1.6x crop body.
<div></div>
As I suggested above, there are a few EF-S lenses that deliver L-quality optical performance. At this time, those are the 10-22mm, the 15-85mm, and the 17-55mm. So, it's not too surprising that the EF-S 15-85mm lens beats out the 17-40mm L lens, even though it has a wider range. Often, you get what you pay for, and you're comparing one of the cheapest L lenses with one of the most expensive EF-S lenses...
Right - wide open, the 15-85mm is sharper. But wide open is 2/3-stop faster at wide end, and 2 full stops faster at the long end. Optical design involves trade-offs, and wider aperture vs. sharpness is one of those trade-offs, especially at the wide end. So, if you're shooting at ~50mm with one of those zoom lenses, and you need f/2.8 for a high enough shutter speed to get the shot, you can only do that with one of those lenses...
Also, Bryan's statement about the 15-85mm being 'nearly equal' to the 17-55mm seems to be based on lens performance across all focal lengths and apertures, and if you look at one specific focal length/aperture combination, there may be bigger differences. Here's an example - to me, the results from the two lenses at the selected apertures (on the same body so the comparison is valid) look very similar. One of the two lenses is a Rebel kit lens that costs $100 or less with the package, the other is an L prime costing over $5K, so if you base your comparison on just those crops, it's a 50-fold price difference for the same optical performance.
Bottom line, if you're shooting outdoors or indoors with flash, and not planning on getting a FF camera soon, I think the EF-S 15-85mm would be the best choice for you.
Reply With Quote