Soft copy of a lens...??? (Pixel Peeping 101)

When researching lenses over the past couple years I have read a lot of reviews from great websites such as Fred Miranda, The Digital Picture and Photozone. Often, when discussing the Canon 24-70 f/2.8L or 70-200 f/2.8L IS, people mention that they got a "soft" copy of a lens. After dealing with an autofocus (AF) calibration issue on my Canon 70-200 f/2.8 IS I thought I'd share my experiences and offer a technique for checking your AF to determine if you have a problem.  I don't think that there are "soft" copies of lenses floating around, except perhaps in rare cases where there are manufacturing defects.  I think that there are sometimes AF accuracy issues for bodies and lenses, and this is easily rectified by sending gear back to Canon for a calibration.

The following image is a shot of my dog with the 70-200 and can be seen at 100% crop by clicking the image to go to my Flickr gallery (all shots here are downsized slightly by Flickr for posting). After the second trip to Canon I am finally happy with the AF performance. The detail in the eyes is excellent at f/2.8, and the AF is consistently giving good results.

Many times I have seen people post questions in a forum asking about lens sharpness and calibration.  Many, many responses blame user error and seem skeptical that these problems even exist since they may never have had a gear issue themselves in many years of photography.  I have no doubt that technical problems arise.  Under certain conditions the gear has to work very hard, and this is where my problems arose... shooting close to minimum focus distance and maximum aperture (f/2.8) for portraits I was not happy with the detail, particularly in the eyes.  I didn't need to do lens tests to be dissatisfied with my lens, especially for the price.

I sent my lens to Canon for a calibration shortly after I purchased it.  It definitely worked better after the calibration but I was still not satisfied with it.  In fact I got so fed up that I was going to sell it - so I decided to do one last test.  I reasoned that if IQ was improving faster by stopping down than IQ was degrading by bumping ISO, then I might just as well shoot at high ISO with an f/4 lens, since the lens was so "soft" at f/2.8.  What I learned was that the "softness" at f/2.8 as not the optics, but in fact the focus.  Of course as you stop down the lens the DOF increases, thereby masking an AF error.

To conduct the test I used the tripod, remote shutter release and mirror lockup.  The center AF point was used on my Canon 20D, which is a high-precision AF point on a f/2.8 lens.  I set up a test target with good detail (UPC) which is shown below.  The full frame is shown at left, and gives you an idea how demanding this test is when pixel peeping at 100%.  I also found that shooting like this there is some slight variation shot-to-shot in the AF lock, which unfortunately means some photos are less sharp than others, but I don't think there's anything that can be done about this.

I tooks photos using AF and compared these with photos taken using AF, but with small manual tweaks to the focus after the AF lock was established.  What I found was that the lens is excellent at f/2.8, when it's in focus - I was able to improve the focus by manually making small adjustments.  It should not be possible to manually improve upon the auto focus.  The shot below shows a manual tweak on the left, and pure AF on the right.  The lens looks pretty darn sharp to me at f/2.8, when the focus is accurate.

So based on this test I decided to send the lens back to Canon for a second AF calibration.  It came back shooting much better, although under tough conditions still has some shot-to-shot variation as I mentioned previously.  I have chosen not to send my camera body in for calibration since the AF is now good enough for me to be happy.  Unfortunately for me I bought the lens used so had to pay for the first calibration at about $165 Cdn, but the second calibration was done under warranty.  Incidentally when I tested my 70-200, I realized that my 50mm f/1.4 had an AF calibration issue also.  It was calibrated by Canon under warranty.

The paperwork from Canon that accompanied the 50mm upon return did recommend that if the issue continued I should send my camera body to Canon for calibration also.  I think that ideally the lens and body should be sent in together for Canon to calibrate them each independently, and then together.

Hope that some people find this useful information.  And if anybody has a soft copy of a lens that they just want to get rid of, you can send it my way... ;)  Any questions or comments, please drop me a line.