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Comparison of Canon midrange zooms
24/28 mm - 35 mm - 50 mm - 70 mm - 100/105 mm - Summary
Well, it is hard to sum up this many images in a short conclusion. The quality of the lenses vary depending on the focal length and obviously the aperture.
In my eyes the EF 24-105/4.0 L IS USM does have the best overall quality, as one would expect. At 50 and 100 mm this lens is close to the performance of the prime lenses.
However, the EF 28-135/3.5-5.6 IS USM is surprisingly close on its heels at many settings at only half the cost. It should be noted though that at the 105 mm setting this lens is only f/5.6 wide open - a full stop less than the EF 24-105/4.0 L IS USM.
The EF 28-105/3.5-4.5 USM shows mediocre performance at most settings, except perhaps 70 mm. This latter lens suffers from low contrast wide open.
Some additional tests