This is for the more technically inclined.
If you use your camera equipment long enough you will eventually have a problem.
One of my lenses was producing slightly soft images. This was a lens that had been used for quite a long time with good results on a different camera body. I decided to do some testing before sending the lens in for repair.
Today's digital cameras have much higher resolution than film. Small mismatches between camera body and lenses that were undetectable in the old film days can now be quite evident. To correct these problems the auto-focus system on most recent camera bodies can be fine tuned to match the characteristics of each lens. This is a great feature, but I had never taken advantage of it. My eyes aren't that great and I have a hard time seeing the very small differences between test shots at various fine tuning settings. I must not be the only person with poor eyesight, recently a couple of companies have released software to help with the analysis.
I borrowed a test target and bought a copy of FocusTune from Michael Tapes Design. It's $40. (Free try-before-you-buy demo available.) That may sound like a lot, but sending a lens to the repair shop to be tested would be several times that cost and could take weeks. What's worse, the problem often doesn't surface on the first trip to the shop. If the Focustune software worked well it could be used any number of times. I figured it was low risk and worth the effort to give it a try.
I won't go into the details here, there are several reviews on the Internet (Go ahead, Google Focustune.) To make an already long story short, you take a number of photos of the target at different fine tuning settings, the software analyzes the photos and tells you which setting is best. The software is not difficult to use, but setting up the camera and target requires care. You'll want to use a tripod and make sure that nothing varies among the photos other than the camera's fine tuning setting.
My results were very good. The report is easy to decipher. I wouldn't hesitate to recommend the software to anyone that is comfortable with the custom settings on their camera. If you're curious about the software, there is a lot of information available at FocusTune.com.