# Optical Forums > Ophthalmic Optics >  Bifocal PD Measurement

## denschng

Hi, I am new in the forum Board. I am an optometrist in Singapore. I have a question here. I have a round segment bifocal with me, I need to vertify the PD on this spectacle. Am I suppose to take the Optical Centre of the distant and measure the distance? I have heard that the correct way of measurement is actually to half the segment and then measure the distance, the PD measure suppose to be for near PD instead of distant PD. I am confuse here. Please advice on the correct measurement of the PD and the reason behind.

Thank you

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## Darryl Meister

You would measure the Distance PD (for far vision) by spotting the optical center of each distance lens and measuring their separation. You would measure the Near PD (for near vision) by measuring the separation between the leftmost edge of one bifocal segment to the rightmost edge of the other.

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## denschng

Hi, Thanks for the prompt reply. But from what I understood from the Lab, they actually mention that it is impossible to get a accurate PD for the distance. They also mention that the normal practice is just by measuring the near PD which is the separation of the edge of the segment. Is this true? Please advice.

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## Darryl Meister

For mounted lenses, Yes, you can measure the Distance PD. The Near PD (segment separation) is important as well, but both should be verified. An incorrect Distance PD will generally induce prismatic imbalance.

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## shanbaum

> Hi, Thanks for the prompt reply. But from what I understood from the Lab, they actually mention that it is impossible to get a accurate PD for the distance. They also mention that the normal practice is just by measuring the near PD which is the separation of the edge of the segment. 
> 
> Is this true?


No, the assertion that "it is impossible to get a accurate PD for the distance" is not true. 

Nor is it particularly accurate to describe the measurement of the near PD as "the separation of the edge of the segment", which suggests (to me, anyway) the distance from the nasal edge of one segment to the nasal edge of the other. 

The NPD in for segmented multifocals is normally considered to be the distance between the _centers_ of the segments (thus ignoring any prismatic effects at near), which can be more easily measured (binocularly) from the temporal edge of one to the nasal edge of the other.

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## QDO1

You can control both the near and distance PD in a bifocal, you just need to understand the prismatic effect of both the distance and near portions of the lens. You can measure the PD at near and distance too.  One can order a seg with different insets to control both the distance and near PD's.  It is not allways the case that the near centre falls in the geometric centre of a seg - it depends on the underlying distance RX

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## Darryl Meister

> You can control both the near and distance PD in a bifocal, you just need to understand the prismatic effect of both the distance and near portions of the lens.


I think you are referring to what we call the _resultant_ PD at Near, here in the US. This is the PD you would measure _through_ the bifocal segments, which is a consequence of the relative positions of the distance (major portion) and segment optical centers. As you noted, you can play around with prismatic effects in order to achieve the desired resultant PD, though it is customary to use the separation of the actual segments, not the OCs through the segments.

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## QDO1

> You would measure the Near PD (for near vision) by measuring the separation between the leftmost edge of one bifocal segment to the rightmost edge of the other.


 left and left? or left and right?

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## Darryl Meister

> left and left? or left and right?


Oh, Yes, sorry.

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