When I click on the autofocus button, I get a different value each time I press it. Why does it do this? Isn't there a best focus position?

Synopsis

Autofocus consistency

Solution

When you click on the autofocus button, a mini-optimization takes place. A first derivative of the spot size (or OPD) vs. location is calculated and the defocus is adjusted accordingly. Depending on how aberrated your spot is and how far you are away from the "best" focus, the autofocus mechanism will only get close to the "best" value. Repeated pressing of the autofocus button should get you closer to the "best" value, but there is really no good method to determine what the "best" value is using this approach (there is no way to define sampling or weight the aperture cross-section, ..etc.). If the autofocus option is not working adequately for you, we recommend that you use a "real" optimization. Here are a couple of other items related to autofocus that you should be aware of:

1. Depending on your system aberrations, there theoretically should be only one " best" focus. But in reality, " best" focus is typically not at a single defined point at all, but rather it is a caustic over a focus range. The spot size typically doesn't change much over a small range of focus values near the "best" focus. Any focus value within this focus range can be considered to be as good as any other.
2. Since the autofocus is a mini-optimization, the result is limited by where your starting focus was before you press the autofocus button. It might not always result in EXACTLY the same answer (but it should always result in an answer within a reasonable focus range).