I guess this or at least similar information is floating around on the blog and/or facebook group somewhere but I haven’t found it and kind of figured it out based on common sense
Anyway I thought it’d be useful since this gets asked a lot. Sorry if it’s already in the forum somewhere…
so anyway, how to set up your lens power - IGNORING astigmatism, so this is only for simple spherical lenses:
- Get some reliable cm measurements to blur limit e.g. ideally at your place of work or where you do most close-up. Let’s say you get 20 cm.
- Measure, with a comfortable posture what is the distance you would like to work at to be at the edge of blur. Let’s say you get 60 cm.
- So, your eye’s lens puts you at 1/0.2 = -5 Diopters. You want to get to 0.6 m = 1/0.6 = -1.67 Diopters. How do you get there? You need to add 5 - 1.67 = 3.33 Diopters negative.
- But the cm to blur were measured to your eye i.e. providing a contact lens prescription. You want a glasses prescription, so let’s assume your glasses put your lens at 15mm = 0.015 m from your eye (may be less), so you use the vertex distance formula at https://en.wikipedia.org/wiki/Vertex_distance to convert to glasses. So -3.33/(1- -0.015*-3.33) = -3.51.
i.e. your differentials (again, ignoring astigmatism) should be -3.5 D
- The formula is: Diopters_glasses = Diopters_contacts/(1 - Vertex_distance_in_m * Diopters_contacts).
(if you use signs then use signs everywhere including negative sign on the distance for moving away from the eye and negative diopters - sanity check: glasses power is higher than equivalent contact lens power - that’s how you know you got the signs correct. )
- If you have astigmatism you can do your cm measurement to edge of Blur to estimate your sph by looking at lines 90 degrees from your cyl axis since this direction should show no astigmatic blur unless you have some complex irregular astigmatism or something
- It is crucial, well it was for me anyway to try to get exactly the same distance to blur on both eyes with the lenses else they cannot be at the edge of Blur at the same time so one eye ends up straining