I had problems checking my exact eyesight with certainty and always thought about an easy setup how to do it. (With Jake’s easy method with printing out the ruler and hold it to the eye I could not do it with certainty, I also tried the app with camera but was not really satisfied.)
So I figured out today this method and wanted to share with you. You need a real plastic ruler and something, where you can stick it in, I used the doors of my cupboard, they work perfectly with holding the ruler.
Then you can define which part of your face can touch the ruler (I prefer my nose), while checking your focus distance. It should be the softest touch you can do, no pressure at all. It is important the ruler can’t move while checking and the focus paper should be completely flat.
I use a 30cm ruler, because I have high myopia. Maybe this method won’t work if you have low myopia, because you would need a long ruler which would be too heavy to stick in something.
Of course you need to add the distance from your eyes to your nose (in my example) and maybe leftout areas in the ruler (from edge to 0 cm start) to the checked ruler distance in order to get the right cm distance.
Now since the eye doctor checks only the 0.25 steps, and nothing smaller than that, you should only compare these 0.25 step distances with the ruler and nothing smaller than that (because it is probably too small to be sure). So for my eyesight between -4.5 and -5.5, every step is basically nearly 1 cm.
I found out I have astigmatism, I see horizontal lines worse than vertical lines so I think it is useless to look at mixed structures like letters etc, it makes more sense to look at lines.
So my best method finding the edge of blur now is starting at a high distance and checking until I see the barcode without any double vision blur. I check my eyesight with vertical lines, because these are also the ones I see on Snellen chart. Horizontal lines I see worse because of astigmatism. This discovery is very important for me.
So my advice is check first, if you have astigmatism with the 2 test pictures at the bottom (if you have you will see some lines unclear), afterwards you can decide which lines you choose for checking the edge of blur distance (choose the lines you see without astigmatism clear).
I added 2 additional distance holders (position 1 and 2) to the ruler in 1 cm steps, so I can simple move my nose to them without moving the ruler for every centimeter, highly recommended.