Diopters and Degrees

[OPTIDONN]

OK I was reading in PRINCIPLES OF OPHTHALMIC LENSES something about converting diopters to degrees. How is this done and what is the relationship between the two? I am a little confused.

[chip anderson]

Only done in prism deviation. One diopter deviates a point of light 1mm at a distance of one mm. One degree (almost never used in eyewear, and when it is the precriber or optician usually doesn't know the difference or correct terminology) deviates light one degree on the 180 degree protractor scale. This is usually only used in telescopes, astronomical calculations and long distance constructions. Then one may actually have degrees, minites and seconds.


Chip:cheers:

[CME4SPECS]

Only done in prism deviation. One diopter deviates a point of light 1mm at a distance of one mm. One degree (almost never used in eyewear, and when it is the precriber or optician usually doesn't know the difference or correct terminology) deviates light one degree on the 180 degree protractor scale. This is usually only used in telescopes, astronomical calculations and long distance constructions. Then one may actually have degrees, minites and seconds.


Chip:cheers:

I always thought it was 1cm at 1m.

[chip anderson]

Specs, I think you are right. Somewhere around here I have a book that describes the difference and how to calculate the variation from diopters to degrees.


Chip

[Darryl Meister]

A prism diopter can be defined mathematically as:

Prism = 100 * tan Angle

where Angle is the angle of deviation in degrees. Solving for the angle of deviation gives us:

Angle = arctan (Prism / 100)

So, for instance, 1 prism diopter is equal to:

Angle = arctan (1 / 100) = 0.57 degrees

And 1 degree is equal to:

Prism = 100 * tan 1 = 1.75 prism diopters

For relatively small angles, since the tangent of an angle is nearly equal to the angle itself in radians, you can these as a rule-of-thumb:

1 Prism Diopter for every 0.57 Degrees and 1 Degree for every 1.75 Prism Diopters

[OPTIDONN]

Ah! Thanks guys!!:cheers:

[QDO1]

I always thought it was 1cm at 1m.

that is correct. This is a useful measure, as you can multiply it up and explain it to a patient easily - 7 prism dioptres relates to the image moving 7 cm @ 1 M or 70 cm @ 10M, "Imagine a car headlight at 10M the light would be dispalces 70 cm - it would look like 2 cars etc...."

[rolandclaur]

hey optidonn, what is the exact name of that book and who wrote and published it and where can i get my hands on one? Thanks

[QDO1]

hey optidonn, what is the exact name of that book and who wrote and published it and where can i get my hands on one? Thanks

writen by Mo Jalie, ISBN: 0900099208

It is an old but excelent book, and it still stretches me. Nowadays I wish it was updated to cover more modern lenses, but as the title says - its all about learning the PRINCIPLES