Originally posted by Miserere Like Lowell, I don't find 50-135mm that useful, and would prefer 70-200mm on APS-C. And wouldn't it be nice if that lens were designe for APS-C? One of the reasons the current Tamron and Sigma 70-200mm f/2.8 lenses are big and heavy is because they are FF. If they were designed for the smaller sensor of APS-C they could be made smaller and lighter.
Be careful with this assumption.
Although the rear of the lens can be made a little smaller (at least the glass) the barrel can't change size because the barrel diameter is dictated by the lens mount, and the front element can't be made any smaller because for 200mm F2.8 implies a front element diameter of 72mm. Unless the lens barrel has zero thickness, you need a 77mm filter on the front, which is exactlly what the sigma and tamron have.
If you then consider IF and space in the middle of the barrel for the focusing mechanism etc, there is not a lot to save anywhere.
The laws of physics simply don't care about the image circle difference when you look at tele lenses.
If you want a good example of how little can change in lens design for an ASP-C sensor, the DA300 F4 weight is 1070 grams, where as the SMC 300F4 is 1020 grams.
Both have 77mm filters.
Like I said, an ASP-C sensor, except for some very special lenses, makes no difference
AT ALL in weight.