I got a new 50mm 1.8 prime and decided the check back focus on it. I am glad I did. It looked fine until I blew up a portion of the first pics and saw how soft it was. I did AF micro adjustment on it and WOW! Stellar difference! I have a 5DMKII and the results were so stunning I did all three of my full frame lenses. I use a chart that helps you decide whether the front or back focus is off and it worked great. After a few test shots all three lenses all resulted in far crisper more clean photos. I'll never put another lens on any of my cameras without setting the AF micro adjustments.