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.