Fixing Front and Back Focus
We will now discuss how to use your in-camera autofocus adjustment to correct front- and back-focus. First, navigate to the focus adjustment screen (for Pentax DSLRs, it's found at the end of the Custom Function menu).
As of February, 2018, the following Pentax K-mount DSLRs support AF Fine Adjustment:
- K-1 / K-1 II
- K-3 / K-3 II
- K-5 / K-5 II / K-5 IIs
- K10D (firmware 1.30+ required for SDM/DC focusing, adjustments via debug mode)
Below are screenshots showing how to apply focus corrections in the Pentax K-5. Up to 20 distinct lenses are supported! While the camera remembers the settings by lens and not by serial number, this shouldn't be a problem unless you have two copies of the same lens and both have focusing problems. Manual lenses (i.e. lenses that the camera doesn't recognize) will need to use the global adjustment (Apply All) rather than a lens-specific adjustment (Apply One). Certainly older Pentax DSLRs may only support global adjustments.
Once you've accessed this menu, all you need to know is by how much you need to adjust your lens(es).
The adjustment process is quite simple:
- Choose a clear subject that has something in front of and behind it (lens alignment tools work best)
- Take a photo with autofocus enabled
- Check if focus is accurate on your computer monitor at 100% magnification
- If things in front of your subject are sharper than the subject itself, apply a -1 focus correction and start over from step #2
- If things behind your subject are sharper than the subject itself, apply a +1 focus correction and start over from step #2
- If your subject is perfectly in focus, you are done!
While you can certainly adjust your lenses by "eyeballing it", you will be much better off using a chart or lens alignment target, as these will allow you to more accurately determine whether or not your lens has any issues, and you'll clearly see when the alignment is perfect.
For the purpose of this article we took a look at two lens alignment products:
Essentially, each of these units consists of a ruler and a mechanism to make it easy for you to align your camera with the target. Read on to see how we used these to test one of our lenses!