Infrared Focusing Equation. Can't understand it! :(

[Ryan Patrick O'Hara]

I was reading up on infrared photography (celluloid based).

From a segment from "Professional Cinematography" (1964) by Charles G. Clarke, he writes:

Infra Red Film/ This film is sensitive to infra-red light in addition to the visible spectrum, and when used with a red filter, produces sensational over-corrected exterior scenes. Particularly useful for day-for-night scenes. Great care must be exercised in its use, for it renders costumes and make-ups in an unnatural manner. A focus adjustment is necessary.

The infra red rays do not focus at the same plane as the visible rays, and it is therefore impossible because they are invisible, to focus on the ground glass by eye. The rule here is to focus forward of the actual distance, 1/4 of 1% of the focal length of the lens.

Therefore, with a 50mm lens, if we wanted something at infinity to be in focus, we would set the lens focus at 50 feet... or

Actual Distance/Infra Red Focus
50'/25'
25'/20'
15'/12'
12'/10'
10'/9'
8'/7'
7'/6.25'
6'/5.5'
5'/4.75'

Other lenses would be focused proportionately according to the rule above.

I've run this a couple of times, and can't quite seem to translate the written instructions into a dependable equation.

If I were to take it literal, I'd figure the following would be the formula... but it's clearly not.

(f x 1% x 1/4) = ir and then do (D-ir) for the new InfraRed Focus
f= focal length
ir= focus forward adjustment
D= actual distance

Anyone know what our good friend Charles G. Clarke is talking about?


-Ryan

 

[j1clark@ucsd.edu]

I was reading up on infrared photography (celluloid based).

From a segment from "Professional Cinematography" (1964) by Charles G. Clarke, he writes:
I've run this a couple of times, and can't quite seem to translate the written instructions into a dependable equation.

If I were to take it literal, I'd figure the following would be the formula... but it's clearly not.

(f x 1% x 1/4) = ir and then do (D-ir) for the new InfraRed Focus
f= focal length
ir= focus forward adjustment
D= actual distance

Anyone know what our good friend Charles G. Clarke is talking about?


-Ryan

Well... yes and no... in the olden days lenses with actual distance markings on the barrel some manufacturers would have a 'red' mark to indicate the adjustment for infrared. Absent that, I don't know exactly how one would do this, but the basic idea is that 'infrared ray lines' converge further 'back' from the lens than visible light, and so one must move the lens to adjust. If you don't have accurate distance markings, I don't know how you would do this... other than... stopping down which will also 'correct' for convergence problems for both visible and infrared light.

The correction is 'minimal', about .25% of the focal lenght of the lens. I believe they are referring to the actual distance from the lens optical center to the film/sensitive surface, so, while the nominal mm label on the lens is for infinity focus, as you get closer, the actual distance from the lens center to the film plan changes, and so, that value is what is 'corrected'.

As a note, most electronic sensor cameras have an IR filter, and so, one needs to remove that and either replace it with visible light blocking filter, or 'understand' the image results... as a note note... since infrared is not a standard human visible 'color'... how the IR interacts with the RGB sensors may result in some 'whackness' on the color representation. Best to go monochromatic if one is doing pure IR
imagery... (or false color...)

Here's an article with pictorial diagrams of the ray traces for visible and infrared.

http://www.ernitec.com/News/CCTV in focus/IR-corrected lenses.aspx

 

[Ryan Patrick O'Hara]

Well, it's a bit frustrating to me because he basically hints at a real formula, yet I can't decipher it from his writing.

There are infra-red dedicated Red1's I've heard about at Red HQ.