I would guess that most of us don't use the camera's autofocus for shooting coins. Perhaps we use focus-stacking while tethered, or use our lenses in full manual mode.
But, in the event that you use AF for macro shots (which I do when shooting nature hand-held), you might like a tool for calibrating AF for your lenses.
This method assumes that your camera provides for AF micro-adjustment, allowing you to dial-in different AF values for each lens. The basic concept of AF calibration is pretty simple: you focus your lens fully open, centered on a vertical target, shoot a picture, and compare this result against the DoF on an angled ruler. Below shows the tool I constructed, with a vertical target and an angled ruler. The critical detail of this system would be the "0" tick of the ruler being on
the same exact axis that passes through both the center of the target as well as the lens axis, set perpendicular to that target.
My autofocus calibration tool, constructed from foamcore and laser prints of my own design.
When setting up for AF calibration, a couple things are critical for accuracy:
- Calibrating the center point axis of the target to the axis of the "0" on the ruler. This is very critical in accurately determining how far the lens front-focuses or back-focuses on a subject.
- Getting the lens axis of the camera perpendicular to a center point on the target. In other words, the sensor plane should be parallel to the target plane in both the x and y axes and perpendicular to z (the lens axis). While some rather complicated methods are used to get the target perpendicular to the lens axis, including multiple boresights, there is a simpler solution using the sensor plane as the reference. Put simply, if you can achieve equal angles in both the x and y axes between the target plane and sensor plane, your target will be both parallel to the sensor plane and perpendicular to the lens axis:
Applying this principle to achieve proper alignment requires having enough data points on your target for
adequate comparison against the sensor plane. In other words, your photo of the target grid is analyzed as a projection onto the sensor plane. In the target below, I have included grids, axes centers, and other points to help position the lens
approximately* perpendicular to the target plane. To this end, it helps to have a grid on your dSLR's viewfinder and/or use guides and various reference points in an image editing program.
*Note: as with any mechanical calibration that requires human input, your results will be based on the level of precision you can bring to the tools and measurement. As you can see above, the target grid lines are parallel to analogous lines on the sensor (shown here as blue guide lines). Comparing the photo to grid lines as shown above will help you determine if you need to correct the angle of the camera along the x- or y- axes (or both). Obviously, this part of calibration
takes some patience and skill to get everything aligned well.
Once you get your camera aligned, the next step is to take a picture wide open, allowing you to best compare focus calibration using a narrow DoF. In the picture below, I have adjusted the AF fine-tuning so that the focal plane is approximately at "0" or the axis which passes through the center of the target. This takes some time adjusting and "pixel peeping" to pick a value that brings the "0" axis into relative focus, compared to adjacent marks.
How AF fine-tune works on the Nikon D810:
The result of AF fine-tuning--the sharpest lines will be clustered around the "0" axis:
That's about as concise as I can get on setup and use for my AF calibration tool. If anyone would like to build their own, PM me and I'll send you the files.
Posted 08/24/2014 3:47 pm
