# Optical Forums > General Optics and Eyecare Discussion Forum >  Distance PD vs Near Pd

## Leighlee

Hi,

Ok in the olden days if a distance PD was 67, you would take 3 off, or substract 3 to get a near PD of 64.  Is that still the rule?  Remember, I am a returning optician after a +10 year break.  Thanks all!!:hammer:

----------


## Chris Ryser

> Hi,
> 
> *Ok in the olden days if a distance PD was 67, you would take 3 off, or substract 3 to get a near PD of 64. Is that still the rule? Remember, I am a returning optician after a +10 year break. Thanks all!!*:hammer:


That is still the rule for near PD at normal reading distsnace. However it can change if the Rx is for very clos up or further away. Pure mathematics.

----------


## finefocus

> Hi,
> 
> Ok in the olden days if a distance PD was 67, you would take 3 off, or substract 3 to get a near PD of 64. Is that still the rule? Remember, I am a returning optician after a +10 year break. Thanks all!!:hammer:


People with narrow PD's converge less, wide PD's more, so 55/53, 60/57, 70/66 - PAL's usually allow 5mm inset from MRP to reading - if any Kentucky windage necessary, remember base in is better than base out for reading

----------


## Diane

Each  should be taken independently.  Every person is different.  With a CRP, it's easy enough to simply set the distance and take the correct measurement. If you do it using a penlight and a ruler, simply have them focus at the appropriate near distance.

I've never taught take the distance and subtract 3 mm.

Diane

----------


## obxeyeguy

I also take one using CRP if fitting sv or prog.  If they are wearing flat-tops, I use what finefocus said above.  People do not like it when one bif looks different than the other(more inset).  "My old ones ain't  that way".  Me: "well mr. W, your left eye sits about 2mm farthur out than your right and these are optically perfect"  Mr. W "my old ones ain't that way"  Repeat above about 4 times, then re-do using the above way to get a near PD.

----------


## drk

You can also measure a Distance PD and calculate the Near PD for any reading distance using:
Near PD = [Distance PD mm * (10 * Reading Distance cm)] / [(10 * Reading Distance cm) + 27]
where the Reading Distance is given in centimeters and the PDs are in millimeters.
__________________

----------


## AdmiralKnight

That's also assuming normal convergence though. I usually just take off 3, unless they have an odd distance PD, then I take both.

----------


## HarryChiling

> You can also measure a Distance PD and calculate the Near PD for any reading distance using:
> Near PD = [Distance PD mm * (10 * Reading Distance cm)] / [(10 * Reading Distance cm) + 27]
> where the Reading Distance is given in centimeters and the PDs are in millimeters.
> __________________


LOL, I use a table but I know the formula well.

NPD / WD = DPD / (WD + CTR)

Then you cross multiply to get the form of the equation you have there.

Also the seperate sides of the equation give you the angle the eye deviates to view the object. Here's a graphic example.  Beat that Drk. ;)

----------


## drk

I got....nothing.  :cheers:

----------


## HarryChiling

> I got....nothing. :cheers:


LOL, you had it.  Feel free to download and use the image GPL.

----------

