An optical problem

[Daniel Cunningham]

I am an instructor at NAIT in Edmonton.

Some of my students have noticed that if you use the following formula to calculate the power of a lens, it is often different than what you get when you neutralize the lens in the lensmeter?

Does anyone know why?

Fnm = (n" - 1)Flm
0.53

I hope you can read the formula. It was difficult to write as this site does not have equation writer. The 0.53 is supposed to be under the part of the formula on the right hand side of the equal sign. See page 312 of System for Ophthalmic Dispensing, third edition, by Brooks and Boorish, if you can not read the formula.

Thanks

Daniel Cunningham

[Barry Santini]

assumed index and vertex power

Barry

[HarryChiling]

A lensometer measures the power in the materia index since the bulb and light through the lensometer uses the reference wavelength of that used in the US and CA. The formula you have their looks like the formula to convert the power into the tool power when surfaceing lenses. This is unnecessary in the lensometer, however if you were using a lenclock to clock the surfaces and add them then this formula could be useful. A little rearrangeing and you get:

(1.53 - 1) / F_lm = (n" - 1) / F_nm

The 1.53 would be the tool index, and the n" would be the material index the powers F_lm would be the tool power and F_nm would be the power in the material. Daniel any other questions just e-mail me direct.

[Daniel Cunningham]

Harry:

How do I email you direct?

Daniel

[wmcdonald]

Daniel,
Glad to see you here. The affect Barry was describing is the why. Harry is absolutely correct in his post.......as he always is on optical topics. You will find this often a valuable site for help.

Warren