In
another thread lytrytyr pointed out the possible problem of focus shift when focusing fast lenses like my A*85mm f1.4. As I never had heard of this problem before I was puzzled. Doing my homework I found that the problem has been described, like
here and
here, but I was still as puzzled. If this was a common problem with fast lenses, why had no one even mentioned it in the lengthy praisals of lenses such as the A*/FA* 85mm f1.4 or the A*135mm f1.8? Not to mention the M/A/F/FA 50mm f1.4 lenses.
Only one thing to do. Print a test chart, pull out the tripod and start testing. I have two (well, three, but the F and FA fifties are basically the same) f1.4 lenses, the A* 85mm and the F 50mm.
Tripod 2 sec mirror lock up. Focused manually at f1.4. No refocusing when stopped down. The words "Focus here" in center of original picture. Pictures converted to jpg and pp-ed with auto contrast in Photoshop, no sharpening. Resized and cropped for forum posting. Here's the result, starting with the 50mm.
F 50mm f1.4@f1.4
F 50mm f1.4@f2
F 50mm f1.4@f2.8
As you can see, there is a bit of focus shift, but the intended focus still sharp due to increased DOF. The tiny focus shift also looks a bit stronger due to a slight back focus to start with. We're talking of fractions of mm's here. I'd say that with this lens, the focus shift problem is measurable but hardly a real life issue.
A*85mm in next post.