Having an Auto Focus Problem … start by checking the Focus Encoder

This menu helps you check if the Focus Encoder in your lens is working properly, without disassembly. 

• If you're having Auto Focus problems, this is a good place to start.

It shows you a signal as you slowly turn the focus ring by hand. If the signal moves smoothly and all the boxes on the screen light up during the test, the encoder is working. If not, there may be a problem; often caused by bent or broken parts inside the lens.

Also, this same signal is important in setting the power level for ETTL flash.

⚠️  The most common cause of encoder failure is bent or broken wiper blades on the focus brush.