Soft image from 70-200 f4 IS

Mike wrote:

A couple of things -

1. Be prepared to shoot using the aperture in the F-8 or so range. This is just a matter of fact with most zoom lenses. Leave the shutter speed and ISO set to automatic. Try it it works..!

No, the 70-200/4L IS is sharp at f/4. We're not talking about a consumer grade zoom here.

Auto ISO is also a bad suggestion for testing, because the additional noise of a higher ISO speed takes the edge off sharpness and also reduces contrast which has an effect on subjective sharpness. Set a low ISO speed and test in good light so the shutter speed will be high enough. Ideally use a tripod unless you are very confident that there is no camera shake, and choose a static subject, not one which is fluttering in the wind for example.

2. I own three "L" lenses. They are ALL soft when mounted to an APS-C (small sensor) camera. BUT...they all three work VERY nicely, sharpness wise, when mounted to my full frame (larger sensor) 6D. Like two different lenses. I bought a 28-300L lens and was very disappointed with the lens as mounted to my...SL1, my 40D and my 50D bodies. But when I bought the 6D...it and my other two "L" lenses magically got remarkably sharp. Still not "prime" lens sharp, but MUCH better than when mounted to the small sensor cameras.

I have several Ls (including as it happens the 70-200/4L IS, but also other primes and zooms), used exclusively on crop cameras and they are perfectly sharp.