How to: measure cm to blur for sph and cyl (especially for higher myopia)

Disclaimer: this is just based on my experience and my opinions but I see so many posts with people complaining about not being able to measure cm to blur accurately or reproducibly or asking how to do it that I thought I’d make this how to.

  1. Get Varakari’s vision log tool Vision Log Tool for Windows, Test Version [Current: 0.1.3] . It is best used on a large monitor, not a laptop.

  2. Get a nice retractable metal tape measure or a ruler (so it’s rigid and you don’t have to hold both ends)

  3. In the vision log tool create 1 log for yourself if you have plain myopia and if you have cylinder in your prescription make 2 logs - one for your pure spherical blur and one for your astigmatism blur

  4. To measure, in the tool go to the new entry screen. Get your tape measure. Pick ONE part of the GREEN FOCUS text which you will always use. If measuring only spherical blur it doesn’t matter where. If you have astigmatism, use your axis e.g. if your axis is 90 degrees (simple case) you will see horizontal blur either side of vertical lines and no directional blur on horizontal lines. (The axis shows your strongest median’s angle). So pick a line that runs 90 degrees/perpendicular to your axis (or for simple sph case pick any line). For example, my axis is almost vertical so I pick the top of the letter “F” across (the crossbar if you like).

  5. Look closely, with one eye covered, only at the EDGE of your chosen line (might be harder if you are at low myopia far from the screen). Extend the tape measure from next to your temple, running past your eye to the screen. Slowly back away until you see the first hint of blur on the edge of your chosen line (on your strong axis). Look back at the RED “FOCUS” text to see that it is still clear/not blurry. So you are on the border, where the green is a bit blurry but the red is still clear. Holding the tape measure next to your temple, without moving your head trace your finger from the front (not side) of your eye to the tape measure and the ball of your finger will line up with a cm mark.
    THat is your cm to blur for that eye.

  6. Repeat with the other eye while covering the first eye and record these in your log or in your “sph cm to blur” log

  7. For astigmatism blur limit, repeat steps 4 to 6 but picking a line running ALONG your axis, so you see the directional blur on its edges. E.g. I would pick the vertical line forming the GREEN F in FOCUS since my axis is nearly vertical. Record these results in your second log file.

  8. Then, in theory the CYL value is the cm to blur on weak axis converted to diopters minus the cm to blur on strong axis converted to diopters. i.e. 100/cm . Of course it is only accurate if you measure accurately.

  9. The cm to blur when measured directly to your eye yields a contact lens prescription. To estimate your prescription for glasses then, you would also have to convert these numbers using the vertex distance formula to a glasses prescription (slightly higher power, not significant below about -4 D i.e for low myopia). if you’re having trouble extracting the needed information, the formula is Diopters_glasses = Diopters_contacts/(1 - Vertex_distance_in_m * Diopters_contacts). For astigmatism it’s a BIT more complicated, but not much - see the wikipedia page. I’d use Excel or any other spreadsheet so you input the formula only once then can play with the numbers.


Honestly I have hard time because of this and this is why I say that precise measurement (going down even to 0.5 cm) is not possible. At least for me it’s definitely not. For me there is no clear cut where clarity ends and blur starts. Usually I go back and forth a lot before decide that yeah, maybe this point. But then I’m still not sure :slight_smile:


I used to measure using Jake’s calculator and a laptop. It was difficult for me as I don’t know how to close only one eye. Can’t use my hand as I have to manage the tape measure and I didn’t know how to make an eye patch that does not give slight pressure to the eye which affects the measurements. Sometimes, when I hold the tape, it is slightly tilted which makes me wonder if I am even measuring accurately. I’m not even close to be considered to be in low-myopia category but hey, what if this works for me and it’d be harder to measure as I have more distance from the screen?
So last month I made this:

Let’s call the taller one Block A where I place the text and the shorter one Block B. The paper at the bottom is actually a homemade measuring tape and it is glued to Block A so that I don’t have to hold anything and measuring tape is stretched and perfectly horizontal.

My steps in measuring my CMs:
1.) Align the blocks. Put Block B close to my face wherein one eye is impeded to see the text on Block A and one eye can see the text on Block A.
2.) Move my head back and forth and decide where my blur horizon is.
3.) Put Block B right into my face where it touches my forehead and one side of my cheek.
4.) Check where the edge of Block B lay in the tape measure. Record the results and go for the other eye.

It looks crazy troublesome but I do feel I can trust my measurements more using my contraption. :joy: :joy: :joy:

P.S. The blocks look a bit tilted because I placed it on an uneven board. All of my flat surfaces are occupied at the time I wanted to take the photo. :joy: :joy: :joy:

Well I can only speak from my experience in the 18cm range and there it is quite clear where the blur begins. But you can also use the green /red difference to find the right range and the long term average. So just measure what you feel on a given day and in the long run any errors will average out so you can still see long term trends

Well I think it is also not good to just close one eye as it also causes a slight squint in the other eye. I think the best way is to put a hand or a patch over one eye and you CAN do it with a rigid tape measure as I say above because you only need to hold one end. Of course it gets floppier as it gets extended more at higher cm… Then you need a better tape measure. But yeah above 50cm or so it’ll be too floppy and slope down hence why this method is better up till there but not really for low myopes

I measure this way, but with an astigmatism dial for each eye aligned to its axis and printed on paper (much higher quality than a typical screen)

I use a hard 40cm ruler, put it above my ear and against the screen, holding the paper on the screen.
I get close to the screen until i see clearly, then go back until i notice blur, stop there and put my finger on the ruler and check the cm results.

Works great for me, 1 minute each morning is all you need…


yeah this also sounds like a good way but you’re only measuring on one axis, so you might not see improvement if you’re looking at a line aligned to your axis you’re measuring the worst case only. So you might not notice improvement in pure sph

I’m measuring both the axis and axis +90 degrees for each eye.

Have you tried with paper and a book? personally its difficult for me to measure my cm since I can only see 20 cm to the screen, so its frustrating. But a crisp text on white paper helps. It takes me about 45 minutes trying to decipher where the blur starts and I get frustrated and i can tell my eyes start to get tense :rofl: which makes the edge of blur harder to figure out and i end up giving up.
Also depending on your myopia-ness, if your a high myope, you’d have to wear differentials to see the screen to measure the edge of blur using jakes focus calculator. glasses tend to make it harder to find out the blur, naked eyes work better.

I dont understand your contraption and how it works?
does anyone know the rx of a diopter using cm? For example I have older glasses and I dont know the prescription but from my 12.5 inch screen I can back up to 54 cm until blur starts.

you can see 18 cm until blur? how do you distinguish that? Ive seen the other forums and todd beker video on how to see the edge of blur, but honestly i cant tell at what point it gets blurry. Is it a very slight blur where for the letters lose their crispness when you back up 1-2 mm? or, when you back up 1-3 mm further and the text or letter, (for example) the “i” in crispness becomes fuzzy and the dot of the ‘I’ loses its roundness? but at that point af doesnt work at all and its impossible to clear the blur

I think it’s easier to see the blur at low distance. Like i wrote above, I use an HD monitor, large monitor and close up the edge of the letters is sharp. Run your eye up and down or along the chosen line, no ghosting, you see one line as the edge of the letter. Back away and you begin to see the line has some slight smudges ie blur. I don’t know how people measure far away, close is easier.
I guess the higher the resolution and the size of the image the easier

Yes this is what I’m doing exactly except that instead of a book, I view the text which is pasted on that block thing. The other block is just to block one eye so I can test each eye separately.
I admit it’s also hard for me to find the exact edge of blur, but after measuring a lot of times, I kind of developed a standard for myself what it looks like. I guess at this point I’m focusing on current measurements in relation to previous ones. I’m focusing to see if there are loss/improvements instead of being too strict if I have indeed found the edge of blur.
Sorry for my confusing post! I was just trying to do my own thing as a hard tape measure and screen just doesn’t work for me.