Using IR mark on lens.

[Xmas]

Is it because the index of refraction for glass is all roughly the same?

If you can recall the experiment in school, the prism bends the light, but the red and blue are bent by different angles, hence you can see the spectra of white light.
There are (were) two classes crown and flint which dispersed the light by different amounts, separated the colours by greater/lesser amounts(dispersion) then Leitz discovered anamalous dispersion glasses, as calcite was a pig to manufacture, calcite has anamolous dispersion...
So there are lots of different glasses, some very high refractive, i.e. bending the light by large amount, and variable in dispersion, there are catalogues of different glasses to feed into the computer design lens software.

Noel
P.S. Some one must be an optics engineer...

[keithwms]

Thanks, I know there are different glasses, my question was where the number 250 comes from, so that I can ascertain whether the expression is generally useful. My guess is that it assumes a specific change in the index of refraction as a funciton of wavelength.

Anyway here is my answer...

[Kiron Kid]

My second time out with Kodak HIE will be this morning.I'll be using a Nikon FE with a Tamron SP 28-80mm lens.
Do I focus on my subject ,then turn the lens to the IR mark on the lens barrel?

Thanks

If you're using a non-opaque filter, there is no need to adjust your focus point. Just use a fairly small aperture. If you are using an opaque filter and your lens has no IR markings, focus slightly in front of the subject matter. I routinely meter, focus and expose my Kodak HIE through a deep red #29 filter, and get consistentlyy good results.

Kiron Kid

[AgX]

keithwms,

Sorry for not answering you. I can no longer search for my own postings (and the follow up) here.

Yes, you are right. I based that number 250 on old literature where numbers between 200 and 300 were stated. Obviously it is a matter of refraction related to effective (filtering) wavelenght. Keep in mind that there are specially constructed lenses which do not need adjustments.

[keithwms]

keithwms,

Sorry for not answering you. I can no longer search for my own postings (and the follow up) here.

Yes, you are right. I based that number 250 on old literature where numbers between 200 and 300 were stated. Obviously it is a matter of refraction related to effective (filtering) wavelenght. Keep in mind that there are specially constructed lenses which do not need adjustments.

Thanks. Indeed, I recently started using APO process lenses for IR and have found no need for refocus.

[Ed Sukach]

Trying for simplicity:

Glasses, and other optical materials; Crystalline Quartz, and plastics, have widely different Indices of Refraction.

The working Index of Refraction WILL change, slightly, with the wavelength of light concerned.

Lens systems with many "first surface mirrors" (reflective) will be affected LESS - in general - than those depending on refraction. Less refraction, less concern.

"Apochromatic" lenses are - usually - more "highly corrected" in regard to chromatic abberation. Or so it says. sometimes I wonder. They are certainly NOT "miracle" lenses, completely immune to the laws and effects of optical theory.

Whether or not try to compensate for "focusing shift" with IR film ... It depends a great deal on the character of light involved and the sensitivity of the film receiving that light. With the films whose sensitivity "peak" is closer to that of the visible spectrum (Konica IR, SFX ...) and where focusing is done in a SLR complete with a "mild" IR filter (Wratten #25, or like that ..), I'd forget trying to "compensate". Through experience, I've found that has caused far more out-of-focus-ness than it has corrected.

Kodak HIE, first focusing without a filter, and adding filter attenuating most visible light, should be another ball game entirely. There, the IR Index Marks seem to make sense.