Today’s Question: I recently had eye surgery and now I have to wear glasses for the first time. I need to adjust my diopter to compensate for my changes in vision. Is there a prescribed way to accurately adjust the diopter?
Tim’s Quick Answer: There are two basic steps involved in adjusting the diopter on your camera. First, you need to make sure that critical focus has been achieved on a test scene. Then adjust the diopter so that the scene through the viewfinder is also in focus, based on the way you intend to actually use the viewfinder later.
More Detail: The diopter adjustment available on many camera models enables you to essentially adjust the focus within the viewfinder to compensate for your own vision. For example, many photographers who wear corrective eyeglasses prefer not to wear their eyeglasses when using the viewfinder. The diopter makes it possible to apply compensation so that (in this example) you can see a sharp image through the viewfinder when you aren’t wearing your eyeglasses.
When establishing focus before adjusting the diopter setting, I strongly recommend using a scene that will make it as easy as possible to evaluate the focus. For example, I often use the focusing target of the X-Rite ColorChecker Passport Video (http://timgrey.me/checkerpassportvideo) to make it easier to achieve accurate focus initially as well as to evaluate the diopter adjustment setting. You could also use something like a page with crisp printed text. The key is to have something to focus on that makes it easy to evaluate that focus.
You can certainly use autofocus to establish focus on the subject you’re using for this purpose. Personally, however, I prefer to use the Live View display (with the camera mounted on a tripod) to help ensure the best accuracy. Zooming in with the Live View display (not with the lens) enables you to get a close look to evaluate focus, and then to adjust the manual focus setting on the lens as needed.
Once you have the scene focused accurately, you can look through the viewfinder and evaluate the scene. Be sure to use the same approach you intend to use for your actual photography, in terms of whether or not you’ll be wearing eyeglasses, for example. Then adjust the diopter until the viewfinder display is in sharp focus.
Be sure to rotate the diopter dial in both directions to an out of focus position, so you’ll be better able to determine the diopter adjustment setting that will produce the sharpest view possible. In other words, don’t just turn the diopter adjustment until you think the image is in sharp focus through the viewfinder. Instead, continue past the point you believe represents optimal focus to confirm that doing so results in an image that is less sharp. You can then turn the adjustment back to the point of optimal sharpness.
Once you’ve adjusted the diopter setting it is a good idea to make a note of what setting you have used. That way you can periodically check to confirm that the diopter setting hasn’t changed. And, of course, it is worth noting that our vision does tend to change over time, so it is a good idea to periodically perform this adjustment process again to confirm you are getting the sharpest view possible through the viewfinder.