chemical focus

[pinholeblender]

Just had an "aha" moment while explaining the red focus dot on a lens to a student. The red focus dot on this lens refers to the focus adjustment that needs to take place when focusing infrared film. Since infrared is not what our eyes see one needs to adjust the focus. So I assume if I used paper in this camera I could adjust the normal focus mark a similar distance on the other side since we see and focus predominately in the green spectrum. RGB, green is normal, red to one side, blue to the other. AHA

[DaveBell]

It's worth a try, but the lens *should* be achromatic in the visible range. The IR dot is there because it is definitely not so for near infrared.

[scheimfluger_77]

I read somewhere once that the actual focus differential between visible light and near infrared is 0.5%. That's 0.25mm in a 50mm lens, meaning that the change in the circle of confusion at the plane of focus is likely compensated with depth of field in most cases.

[pinholeblender]

Right of course, chemical focus refers to images made with meniscus lenses but a SLR lens would be a corrected compound lens. So it wasn't an aha moment after all, darn. But perhaps the red focus mark acts like a chemical focus mark in that IR is not visible to our eyes. Does this mean IR does not focus through an achromatic to the same plane as RGB. If so then I guess the red dot works in a way like the chemical focus adjustment.

[Ned.Lewis]

I think you are right about the red dot being a similar concept. If I had some HIE, I could actually try using the red dot on some of my lenses, I never have!

The one time I thought I saw a chemical focus problem with an old uncoated lens, it turned out I was making a silly mistake instead...

When making calculations for pinholes for paper negatives, I like to use a smaller wavelength, like 500 or 475nm to account for the blue sensitivity of paper, and I've had better luck with a smaller "user constant" around 1.56. I've wondered if this is really because I make most photos outdoors and there is more UV and should be using even a shorter wavelength in my calculation. Add to that, with even smaller pinholes, I don't see diffraction effects kick in as much as the math looks like they should. It has made me skeptical that diffraction is as big a "player" in softness, but could it just be that UV is forming a lot of the image?

[Jim Jones]

Many years ago I experimented with varying the user constants for IR, panchromatic, and blue sensitive film, and found that correcting the user constant according to theory did slightly improve image sharpness. If a pinhole is too small, diffraction certainly does degrade image sharpness. However, contrary to logic and to some "experts", a pinhole aperture selected for optimum sharpness can resolve line pairs on standard resolution charts with smaller pair spacing than the pinhole diameter. At that critical pinhole diameter, we have what might be called constructive diffraction as opposed to the destructive diffraction of smaller pinholes.

[Taraxacum]

Even if you lens is achromatic you'll still need the red - in theory. All the apchromatic does is make the visible wavelengths of light focus at the same point. As your IR light is of a longer wavelength and outside of the visible spectrum the achromatic coatingss etc, have minimal effect of IR light, hence the red dot being there (usually set for wavelengths around the 800nm region iirc).

As Scheimfluger pointed out though, usually the DoF with be enough to correct for the difference in focusing. Also it will depend on what IR filter you are using/wavelengths of IR light your recording. With the range of IR film now made being nonsensitive to the longer wavelengths of Kodaks old films (HIE/HSI) I cant see any reason to use the 'red mark'. I have done a lot of work with IR film over the last 25+ years and have never really seen a difference in focusing if you stop down to around f5.6/f8, even with something like a 87c filter with kodaks film. Though if I were to use a long telephoto lens I'd be a bit more cautious perhaps.

As we all know, theory is great BUT the real issue is practice.