Formula for coming up with intermediate power for a specific distance?

[KrystleClear]

One of our competitors nearby recently closed for good. We are now getting a lot of their clientele. That's a whole can of worms for various reasons (the last man standing there was not formally trained and was telling his patients/customers a lot of incorrect information or getting hostile with them).

Anyway, apparently for computer progressives and computer bifocals he would tailor the intermediate power for the exact focal distance the patients wanted. He'd have them take a yardstick and measure the distance from themselves to the computer screen.

I typically just use the traditions half of the near add to determine my intermediate powers unless the prescribing doctor specifies otherwise. I am not seeing a formula for this in my Optical Formulas Tutorial workbook. Say someone wants a flattop bifocal with intermediate on top and near on bottom for organ playing and they sit 35 inches from where their sheet music is. What formula or method should I be using for this? (I don't have trial frames and they probably won't allow me to order a set due to cost). Normally I will just have someone try on different pairs of OTC readers to try and get an idea for what the intermediate add should be when the near add isn't an easily half-ed amount like +2.75, but that isn't really ideal.

[drk]

You are right and he is way overkilling it.

First of all, nobody sits at the same distance.
Secondly, who wants to send the patient home for a measurement! Is this DIY? (Worst case scenario put the patient in front of your office computer and measure it yourself.)
Thirdly, do a little chart for yourself.

Distance/add
18" /+2.187
20" /+1.97
22" /+1.80
24' / +1.64
26" / +1.51
28" / +1.40
30" / +1.31
32" / +1.23
34"/ +1.16

What do you notice?

[KrystleClear]

You are right and he is way overkilling it.

First of all, nobody sits at the same distance.
Secondly, who wants to send the patient home for a measurement! Is this DIY? (Worst case scenario put the patient in front of your office computer and measure it yourself.)
Thirdly, do a little chart for yourself.

Distance/add
18" /+2.187
20" /+1.97
22" /+1.80
24' / +1.64
26" / +1.51
28" / +1.40
30" / +1.31
32" / +1.23
34"/ +1.16

What do you notice?

Thanks, drk! I suppose I could always start by lowering the add a little bit for the intermediate if the patient feels they need a longer intermediate distance. I just don't want to be fiddling with lens powers and end up having to do remakes because I fiddled too much or too little.

The patient that brought this up to me was telling me he sits at the kitchen table with a book and also an iPad. He has the ipad at arms length. He will read a little and then glance up at the iPad. They were shocked that I didn't ask for the exact inches between him and the ipad, and explained what the other place (the one that closed) would do. First of all, an iPad can be picked up and moved, unless he nailed it to the table... So to me it seems silly to overcomplicate it when both the person and the iPad are not permanently affixed.

[Uilleann]

Remember also that as you decrease add, their depth of field will also increase. It's more important to be precise, the closer you get to the end of the nose. That middle range will have a fair amount of wiggle room naturally for almost all patients.

[CME4SPECS]

Before the computer progressives that we have today, CRT trifocals had an intermediate 66% of the reading add.

[drk]

Here's what I notice:

1. Using for the add the reciprocal of the distance to the computer (the "exact way") gives what will be too strong of add powers, compared to real-world experience, as CME said.

2. This can be "chunked"
18-22" = +2.00
24-28' = +1.50
30-34" = +1.25

So all this fuss over a quarter diopter or so.

My take: +1.00 is good. +0.50 if they want to be able to walk around. Done and done.

[AndyOptom]

I find it more helpful to think in terms of intermediate addition in 0.25 DS increments first, then their corresponding distances. As drk has noted, it gets the job done :)

In terms of how much overplus is being added over the distance Rx (and correspondingly deducted from the near Rx):

+0.25 = 4 meters (~ 13 feet)
+0.50 = 2 meters (~ 6.5 feet)
+0.75 = 1.5 meters (~ 5.0 feet)
+1.00 = 1.0 meters (~ 3.3 feet)
+1.25 = 0.80 meters (~ 3.0 feet)
+1.50 = 0.67 meters (~ 2.2 feet)

Frankly, I rarely go beyond that, and if I do, it's a matter of simple measurements and arithmetic... my most commonly dispensed intermediate adds are between +0.50 to +1.25.

If you wanted the actual formulae:

Intermediate Add = 1 / Working distance in meters

Or, if you're used to measurements in feet/inches

Intermediate Add = 3.3 / Working distance in feet
Intermediate Add = (3.3 x 12) / Working distance in inches

1 meter (or 100 centimeters) ~ 3.3 feet
1 centimeter ~ 0.40 inches

[drk]

Very nice.

[Prentice Pro 9000]

You are right and he is way overkilling it.

First of all, nobody sits at the same distance.
Secondly, who wants to send the patient home for a measurement! Is this DIY? (Worst case scenario put the patient in front of your office computer and measure it yourself.)
Thirdly, do a little chart for yourself.

Distance/add
18" /+2.187
20" /+1.97
22" /+1.80
24' / +1.64
26" / +1.51
28" / +1.40
30" / +1.31
32" / +1.23
34"/ +1.16

What do you notice?

This assumes a +2.50 add correct? So If it's like a +2.25 add then shave off a quarter from all those powers you listed?

[drk]

That's not a prescribing table, it's just me showing the relationship between distance and diopters and how nitpicky the whole enterprise would be.

For me, no one needs intermediate correction until they're about age 50/+2.00. Then you Rx either +1.00 for the computer or +0.50 and let them tilt a little for the "room-distance task lens"/"indoor progressive".