Why is autofocus micro-adjustment usually tested at about 5–50× focal length?
Asked 1/17/2013
6 views
2 answers
0
When calibrating autofocus micro-adjustment, many guides suggest placing the target at roughly 25× the lens focal length, or somewhere in the broader range of 5–50× focal length. Why is that recommended? What problems can happen if you test much closer or much farther away?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
2
Two reasons. The first is that the distance is more-or-less in the middle of the lens's focus range. Too close to nominal infinity (which is very easy to get to with short lenses) and you won't be able to accurately determine the point of focus on the calibration scale, so you want to make sure the target is close enough. If you're too close to the target you may find that focus errors cover the whole visible scale. 5 to 50 times focal length is a pretty wide range to work with.
Second, it really helps if the scale is about the right size in the image. If you're too far away for the focal length, the scale will be too small, and the markers will be difficult to read. If you're too close, you'll either crop out the scale or have to recompose off of the focus target to be able to see the scale, and that movement kind of invalidates the calibration.
It's not about optical science or anything weird like that, it's just a practical way of using the equipment.
Originally by user2719. Source · Licensed CC BY-SA 4.0
user2719
13y ago
0
Generated from our catalog & community — verify before relying on it.
That distance guideline is mainly practical, not magical.
A target placed around 5–50× focal length usually puts you in a useful part of the lens’s focus range: not so close that depth of field is extremely tiny and focus errors spread across the whole test scale, and not so far that you’re effectively near infinity, where judging exact focus becomes harder.
It also helps the test chart appear at a workable size in the frame. If the target is too far away, the scale becomes too small to read clearly. If it’s too close, you may crop out parts of the scale or have to recompose away from the focus target, which can affect the test.
So if you “disobey,” the camera and lens won’t break—you just risk getting a less reliable calibration because the test becomes harder to read and interpret accurately. The suggested range is broad because different focal lengths and setups still work as long as the target is at a sensible distance and large enough to evaluate clearly.
Recommended products
UniqueBot
AI13y ago
Your Answer
Related Questions
Canon 7D Mark II and Sigma 50mm f/1.4 Art back-focusing: can the Sigma USB Dock fix it?
Why doesn’t doubling focal length always exactly double image size at close focus?
Can a 2/3-inch C-mount lens cover a 25×25mm sensor by increasing extension?
Why doesn’t the simple focal-length formula match my measured subject size in a photo?
Why do objects near the corners of a top-down photo look stretched or larger?