Were lenses for film designed to focus different colors at different depths, and what does that mean for digital?

Question

I was reading the book Adobe Photoshop CS5 for Photographers: The Ultimate Workshop by Martin Evening & Jeff Schewe (Focal Press. 2011) and read this paragraph about film lenses and DSLRs:

… film lenses were designed to resolve a color image to three separate … film emulsion layers which overlaid each other. Consequently, film lenses were designed to focus the red, green and blue wavelengths at fractionally different distances and at even further distances apart towards the corner edges of the film emulsion area. Since the red, green and blue photosites are all in the same plane of focus on digital sensor, lenses … should now focus the red, green and blue wavelengths to a single plane of focus.

What does this mean in practice when using a film lens on a DSLR? The book does not state any effects. The text above is from a chapter titled "Improving camera capture sharpness", so it presumably has to do something with the image sharpness. Could this also affect color accuracy? How? Anything else? Are the differences "lab only" or should they be seen with the naked eye?

I have a few photos taken with film lenses on a DSLR, but I don't know what/where to look at. The lens' effects to sharpness are hard (impossible) to judge from my shots as most of them are out of focus due to the combined effect of unexperienced manual focusing and the quality of focusing screen & viewfinder of Canon 450D.

Answer 1

It's a lot of nonsense. The goal for lens designers in the film days was the same as it is now -- to approach (or achieve) apochromatic performance. That is, to design a lens that focuses all visible wavelengths of light on a single plane (or at a single point). That's not an easy thing to do.

It is true that some more modern lens designs come a lot closer to this ideal than typical older lenses. That has to do, though, with advances in materials (such as low-dispersion materials that produce reduced "rainbows" on refraction, and anomalous-dispersion materials that produce "backwards" rainbows) and construction, not with a change in design philosophy.

Failing to hit the apo target (something most lenses do, especially at shorter focal lengths/wider angles) results in lateral chromatic aberration (color fringes that you can see in areas of high contrast). As long as they aren't really bad, they can be corrected (often, the camera will do it for you if you are shooting JPEGs). RAW processing programs will often let you apply a lens profile to deal with both chromatic aberration and geometric distortions.

The only real "digital difference" I'm aware of (besides creating lenses specifically for the smaller formats of many digital cameras) is that greater attention is being paid to the antireflection coating at the rear of the lens, since the digital sensor is much more reflective than film, so flare originating behind the lens is a much greater concern.

Answer 2

That statement about lenses actually being designed, on purpose, to focus colours in separate planes sounds dubious. Even in film days, "APO" was a selling point for lenses - "Apochromatic"; APO meaning that all three colours are actually focused in one and the same plane. We can infer from this that achieving this was no mean feat and that it was a desireable feature to have.

Film being slightly three-dimensional may have given lens designers a bit more leeway, I suspect. Digital is more ruthless.

Answer 3

This is unlikely for two reasons:

1. It ignores the case for monochromatic film.

2. It is difficult to get film to lie flat against the backing plate and the slight curvature of the film will in all probability greatly exceed the layer thicknesses.
   See this discussion by Norman Koren (scroll down three to four pages).

Answer 4

Designing a lens to focus on different layers based on wavelength, where the layers themselves are thinner than the mechanical tolerance of the lens/body parts seems a little over-design (if not an overkill) to me.

However, as an engineer, I've seen some funny attempts at overdesigning a project, so it is possible that a design team or two really made that consideration and took the effort to actually optimize the lens for the film structure.

On a side note - if this statement is indeed true, then Foveon sensors may have an edge here...