Today’s Question: I just wanted to point out a tip. The easiest way to adjust the diopter is to just focus on the LED read out in the viewfinder. When the numbers look sharp the diopter is in focus!
Tim’s Quick Answer: This is, of course, a great tip regarding the diopter adjustment. Thank you to Lewis Kemper (among other photographers) for pointing this out.
More Detail: My approach of setting focus with the camera and then adjusting the diopter based on that is mostly an old habit based on my preference to make sure that I am always making a diopter adjustment based on a view of the image, rather than the text that also appears in the viewfinder.
This habit is mostly owing to the fact that I’m a bit of a “control freak”. While setting the diopter based on the text in the viewfinder works perfectly well, personally I feel more comfortable setting the diopter based on the focus within the image I’ll be reviewing when adjusting focus. Of course, if the viewfinder image appears in focus, so too will the text that appears within the viewfinder.
While I do adjust the diopter in the camera based on my vision, in some respects this is more about the convenience of having an accurate view through the viewfinder and at least for me not especially critical for my photography. In actual practice I will either trust the autofocus in the camera to achieve a good result, or I will focus manually. When focusing manually, I will almost always employ the Live View display rather than the viewfinder display, because this approach makes it much easier to ensure critical focus for the scene.
So, while I still use a test subject that I know I have set good focus for to make a diopter adjustment, there is really no reason you should not use the text within the viewfinder for this purpose instead. And of course using that text can be much faster and easier than the approach I use when making a diopter adjustment.