I am having an interesting issue with a couple of my lenses of late, primarily my 100-400 and 70-200 f/4. I am shooting with a 40D. I have just started to see these issues, and I have been extremely happy with both lenses and body for the past year and a half, this just seems to have started showing up.
Over the last couple of weeks, I just haven't been happy with the sharpness of the 100-400. I went ahead and did some tests today with all of my lenses, and the 100-400 and 70-200 have resulted in some questions.
All of my tests were done with the camera/grip on my Bogen 3021PRO tripod using a remote control to release the shutter. I decided to shoot at text, since I figured that would help me see the focus best. The shots were done indoors, to avoid any wind. What is interesting is that I get better sharpness when I crank the ISO. I attached 2 100 % crops from the test shots. The first shot is ISO 100, 400mm, f/7.1 @ 1/6 seconds. The second is ISO 1600, 400mm, f/7.1 @ 1/100. The IS was OFF for both shots.
The ISO1600 is obviously sharper. I saw the same results when I tested with the 70-200, although not as severe. On my 28-75 and 12-24 the ISO 100 was equally, if not sharper then the ISO 1600.
Any ideas on what could be the issue? Should I try a different test? And is it normal for issues like this to show up after prolonged use? I normally wouldn't worry so much, but I am seeing focus issues in my real world shots.
At first I thought it may be the focus system in the body, but my shorter lenses are not showing this, so I am not sure where to problem is even occurring...
Thanks for any help!