What is IR?

[Groupcaptainbonzo]

IR focuses at a slightly different possition to visible. As such lenses used to include a small red dot. or letter Rslightly to the right of the focus line.
I have not seen this mark on many modern lenses. I ASSUME (First rule of life is to assume NOTHING) that with the newer technologies available and apo lenses . Maybe these are no longer necessary. It is only a slight difference and with a wider (24 to 35mm ) Lens and a small aperture (f22 or so). it wasn't needed any way.

 

[Hertz van Rental]

Lens manufacturers, like anyone else, are always looking to cut costs. Putting an IR focus mark on a lens would entail some measuring and re-calibration to find out where the mark should go. By not doing this, and not etching the mark, the manufacturer will make a very small saving per lens - but one which will add up to a lot of money saved over an entire production run.
The manufacturer will justify doing this by claiming that not many people shoot infra-red any more.
That's progress for you.
To be accurate (my middle name) different wavelengths of light refract at slightly different angles in glass. If uncorrected you get chromatic aberration - the inability of the lens to bring all wavelengths to the same point of focus.
Objects look like they have little rainbows around them.
Lenses are designed to cancel this effect out. But they are only corrected for visible light. The IR mark is used to compensate for the chromatic aberration the lens suffers from in the IR region. You must also remember that B&W IR film is almost always exposed with a 'black' filter over the lens so you can't see to focus anyway.
The actual mismatch in focus between visible and IR light is about 0.3%. This is critical for 'pure' IR film like B&W, but as the colour film still uses a high proportion of visible light it is not really noticeable. A little unsharpness is also generally less noticeable in colour film.

(And you can believe me or not as you choose. I don't actually care as it doesn't affect me or the accuracy of my answers.)

 

[Weaving Wax]

Wow...this is all confusing. I'm staying away from IR!