# How To: Choose Differentials Lens Power

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:

1. 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.
2. 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.
3. 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.
4. 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

Note:

• 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
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Theoretically you are right, practically for me the scattergun approach worked better: do some cm measures, guess some dioptre number roughly and order 4-5 glasses around that dioptre. And then try each one and determine which works the best for your close-up. Most likely you will be able to use some or all of them as later differentials and / or normalized, so it’s not totally waste of money.

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Or you could combine the approaches ordering a few around the value predicted by the above approach

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Zenni is so dang slow

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@Lajos, I’m not into math, so I’m not going to attempt to check your calculations.

But as far as the procedural aspects…measuring the distance you want to work from, converting to diopters, and subtracting from your distance power…I agree.

For example, say I’m -4 worst case. That’s about right.

And I want to work 80 cm from the computer screen. 1/0.8 = 1.25. This is my differential factor.

Then I subtract that from -4:
-4+1.25= -2.75

That is my differential power…at least a decent starting point. In fact, this is what I will be ordering!

Since I don’t wear contacts, it’s straightforward.

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It’s not about wearing contacts or not! The conversion is because you measure cm to your eye. So if it’s for glasses you add some power due to the vertex distance of the glasses. But at your level it’s not that significant. Though it might add 0.25 d to the power I’ll have to check the calculation when I’m at my desk

But anyway my goal was to post this and get the concept checked not the math

Yeah cos they are made in Hong Kong so the shipping takes ages.

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I put the vertex distance calculation as the last step - it seems to make more sense. In this case it does make 0.25 D difference.
But I think this is the correct way.
For you @FMR it’d imply only increase to -2.87 which is insignificant compared to -2.75

I slashed 1.25 from my recent full prescription. It was a perfect starting point, even if it was not challenging. When I found that I could still read the normal texts on my laptop even after pushing beyond reach, I reduced by another 0.25. It has been 4 weeks and I am once again reaching the stage where it needs reduction.

OK, but this thread is not about gains updates and this method is more accurate than just subtracting a fixed number from your full prescription

@Lajos, I’m not understanding why you’re putting vertex distance into the calculation for glasses. I’ve never heard of that being done.

I think it makes sense, because he is not using a test lens kit and Snellen chart to measure the eyesight. Its just measuring tape from eyes to screen. Hence, the vertex correction.

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yeah I added that part myself because I noticed that it was missing from all the calculations on the blog etc.
well it makes sense to me that if cm to blur is measured exactly to the eye then the diopters you calculate from that would be for a contact lens prescription i.e. which sits on your eye. So if you wanted to use that for a full prescription you’d also have to factor in that glasses would sit e.g. 15mm from your eye…

of course, as arnab hints at, under normal conditions the optometrist would measure using test lenses or phoropter that has some vertex distance e.g. 12mm is common and you would use the formula the other way to get a reduced equivalent contact lens prescription. I’m just applying the same principle but moving the lens the other way. This is also why contact lens prescription is less than an equivalent glasses prescription.

But this is also why I wanted to check that it makes sense to you guys as well. I think it does

either way, below about 4 D it makes little difference. But for the mid and high myopes it could be the difference between perfect and slightly off differentials.

I get what you guys are saying, but still don’t see why you’d need to use vertex measurements when talking about only glasses. Do opticians use vertex measurements when making reading glasses for people? Do optometrists take it into account when prescribing lenses for near use? I’ve never heard of it being done, unless one is switching between glasses and contacts. I know that people wearing very strong lenses (like -10) have to wear them at a certain distance from their eye or things blur, though.

@Lajos, the situation in which it might have a noticeable affect, I would think, would be if someone is stacking plus lenses on top of contact lenses, and they forget to convert the contact lens power to an eyeglass power…and simply purchase a pair of plus lenses with the differential factor needed. Then it might be off a bit. But…

I don’t know…this part is a little beyond where I personally like to go. I would say if the vertex calculations are indicating a stronger lens is needed, you want to go with the weaker one, anyway. Always going with the weakest that works in the situation.

Additionally, no one here can really get lenses in smaller than 0.25 D increments, anyway (except maybe @Bigkittyqueen). So, you just move back a bit further to compensate if your lenses are too strong. I don’t like the idea of using a weaker lens and moving forward as much, since I (personally) find that moving closer tends to increase strain. Also, I think it’s good for high and medium myopes to get used to working further away with clarity. We tend to ordinarily get too close to things as a habit.

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If you are coverting to diopters from cm measurements to eyes it’s a vertex distance of 0. If you’re not on the same page based on this I have no more arguments

@FMR well need to get our provider list back and see which optometrists are willing to prescribe in 1/8 diopter increments. I don’t think the shop I’m getting them from will sell lenses wo a prescription. If you live near Chicago I would recommend paying a visit to Dr Getzell and he has 3 favorite optic shops and I’m getting my lenses from the one closest to me. Also if you got vision problems other than refractive errors be prepared to find out lol

The store superior optical is really friendly to people seeking various forms of vision improvement including myopia control and reduction in that they have a 90 day warranty on their lenses. When my prescription changed after the first visit I only paid the difference for the second set of lenses. They also let me keep the frame and cut new lenses each time I was back with a new prescription

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You can’t really do that with reasonable priced lenses (and by that I don’t mean the zenni priced, but the one you can get in an average optometrist shop for few hundred dollars). Why? Because the reasonable priced lenses have a refractive error of 0.125 dioptre.

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My lenses costed around I think 200 for a pair. It does help that I paid with a separate medical account for these sorts of things and ran the special lenses thru vision insurance.

These calculations in my opinion dont seem to work well. it really depends on the screen size and index of the glasses too.
I have a 12.5 inch screen and I want to work 65cm from the computer (1/0.65 =1.53) i am -5D, so I order -3.5 dfferentials with 1.5 index (-5+1.5)
But with these differentials I can only see my laptop at actually around 48 cm and I have to get close to the screen. not an ergonomically comfortable distance
unfortunately it seems that only trial and error can be the only solution based on the cm measurements.

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-5D is your full prescription or your normalized? The differentials should be calculated from the full prescription.
But of course these calculations are always estimations, the “perfect” glasses depends on a lot of subjective thing (I would not be surprised if the frame would have some effect too).

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