How do 35mm film scanners deal with the film base color?

Question

I want to build a jig to scan 35mm film using my digital camera (something similar to https://www.thingiverse.com/thing:2789457 or http://www.frogymandias.org/3-d/index.html).

I have seen several similar scanners on the internet and also as far as I understand this is the same principle that enlargers use for getting image from film to photographic paper, so I think this method should work.

But there has to be something that I'm missing. All these devices send light through the film and that means through the film base. All the films that I've seen have brown transparent base or gray opaque one.

So how come all these can work? I would expect to either have a brownish cast over the resulting image or not see anything at all. Where is the error in my reasoning?

Answer 1

Real film scanners scan color negatives by increasing the exposure time of the blue and green channels (relative to the red channel). This is an analog operation, similar to using color filters in the dark room to filter out the orange and print on paper. There is no clipping due to this analog shift.

Digital cameras cannot do those exposures, and must use post-processing applications to adjust out the orange base (inverts to deep blue). This is an extreme shifting operation, and digital clips at the 255 and 0 ends. Many users may consider it good enough, but experienced people know it just ain't the same.

See my site at https://www.scantips.com/colornegs.html

The digital camera does easily copy positive slides and negative black and white film, but the orange mask of color negatives is a substantial issue.

Answer 2

@ cube --- As to your question -- How optical color darkroom practices deal with the Evans Integral Orange Mask?

The C-41 color negative film process utilizes dyes incorporated in the film during manufacture. There are three, cyan (blue + green), magenta (red + blue) and yellow. The dyes in the film are incomplete. During the developing process, a black & white image forms each emulsion layers. The developer also contains a single missing dye ingredient that will unite with the incomplete dye. This action occurs as the black & white image is forming. This action causes the dyes to blossom in proportion to the black & white image. The process then removes the black & white image. The result is a color negative image consisting of dyes.

The predecessor Kodachrome process required three discreet developing solutions. Think about how difficult it is to make cyan, magenta and yellow dyes that are all missing one single ingredient. Some compromises must be made as to the colors of dyes. As it is, the cyan dye is far from perfect, the magenta dye is fair, and the yellow dye is quite decent.

Prints made from these negatives were yielding substandard results. This problem was solved by Ralph Evans of the Eastman Kodak Co. He tested and found cyan and magenta dyes what in their un-blossomed state have a tint. This tint wanes as the dyes blossom. The orange you see is not a uniform tint. It is strong in areas that were not deeply exposed in the camera and weak where exposure is hefty. In other words, residual cyan and magenta dye form two positive images superimposed atop the three negative dye images. The residual tints appear orange. This is the Integral Mask; it is a countermeasure for the inequalities of the cyan and magenta dye that must be used.

Color negative films are intended to be used to make color prints by optically projecting the color negative’s image onto light sensitive color paper. These papers have three emulsion layers, each has a different paper speed (ISO if you will). Thus the printing paper is tailor made to cancel a color cast imposed by the orange mask.

Think about the complexity, of the predecessor, the Kodachrome process. This process usually had on duty a chemist and quality control engineer. To simplify a single developer step was needed; Thus the C-41 and the E-6 process both use this same scheme of a single missing ingredient in a single developing solution. No color film yet made produces a faithful image. Nevertheless, the E-6 and the C-41process makes excellent images.

Answer 3

A software application is used to control a (film) scanner - whether it is the manufacturer's supplied software or a third-party application like VueScan or SilverFast. When you select in the software settings that the film type being scanned is colour negative film, then the software "neutralises" the cast from the orange mask. Some scan software allows you to choose a specific film type, as the orange mask will be slightly different from emulsion to emulsion.

If you are trying to replicate this process more manually, there is (or at least there was) a Photoshop plugin called ColorNeg that would take care of this for you. I did some reading on it a few years ago, but never actually tried it out. This page explains the procedure. In a nutshell, the steps are to remove the orange mask first, and then invert the image. ColorNeg takes into account the exact film type (and hence orange mask and film colour characteristics) in question.

There's a little bit more reading about it here:
http://dpanswers.com/content/rev_colorneg.php
http://benneh.net/techshit/vuescan-colorperfect-a-guide/
and the (short) book 'The Illustrated Guide to Film Scanning' also describes the technique.