Ideally with a viewfinder, you want your eye to be relaxed and focused for distance.
So with BOTH eyes open, pull up the chart.
Do this in a darker environment as it will be easier to keep both eyes open.
Try to find a way to do this in a space that isn't your lounge room and lets you look off a little into the distance.
With your un-dominant eye (the one not looking through the EVF) try to focus on the distance.
Then while doing that, turn the diopter to adjust the focus on the evf. You should find a spot that you can peripherally see the image sharpen in your dominant eye that's looking through the EVF.
The idea here is to have your eyes relaxed and focused in the distance and to then adjust your diopter in the evf to suit that state.
It's a good idea to mark this off as it's easy to accidentally adjust.
Now you will find over the course of a day, even when its set correctly, your eyes will fatigue and this setting can change.
If you're finding that you're getting tired, you should also try to practice operating with both eyes open. Many people close their non-dominant eye when looking through a viewfinder.
By doing this you're not overexercising one eye while closing one off all together. At the end of the day, it's like you've given one eye a work out and one the day off, and normally they're used to working out together, so you can get headaches and blurred vision as your brain tries to make sense of one exhausted eye and one that's been doing nothing.
The other plus of being able to operate with both eyes open is you can watch what's happening outside the frame. You'll find it's easier to walk around in hand held, or watch for a que that happens offscreen. You can also give your boom swinger the evil eye when they're coming into shot or signal you're about to change something. It's amazing what you communicate with one eye and eyebrow
It's easier to have them both open in darker environments. In bright environs this can be more difficult because of the difference between what's inside the EVF and the thermonuclear outside world. I sometimes open and close during a take when it's super bright. You want to keep your non-dominant eye exercised.
Some people wear an eye patch too which helps.
With regards to eyepiece chamois, they are great. A lot of the time the rubber can cause a kind of seal when you're eye is stuck there a lot and it then heats up and fogs the optics inside the EVF and you get this sweaty eye feeling which isn't very comfortable.
Importantly, it also helps to greatly reduce the chances of Conjunctivitis (AKA Pink eye) a highly communicable eye disease...
https://en.wikipedia.org/wiki/ConjunctivitisSo....
Focus on the distance, try to match both eyes, set the diopter and adjust a little as you need to through the day. Try to operate with both eyes open and you're half way there...
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