Converting Color Film Negative to Positive Using Photoshop by Removing the Orange Cast
by Jeffrey Sward
 
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Orange Cast in Color Negatives
 
All color film negatives contain an orange cast throughout the negative. The orange cast is clearly visible in unexposed areas, such as the around the edges or around the sprocket holes. The reason an orange cast was added to color film negatives is because of imperfections in the cmy dyes. A technical discussion of the orange cast can be found in this article about color negative orange cast. Because the entire film stock contains the orange cast, the lightest part of a color film negative (and hence the darkest part of the scene) can be no lighter than the color of the base orange cast. This equating of the base orange to the lightest negative part is important theory when removing the cast using the Type IIa method below.
 
Converting Digital Scan of Color Negative Into Digital Positive
 
Photoshop contains a simple command for converting negatives to positives and vice versa, namely image => adjustments => invert. Invert works perfectly with black and white negatives. However, when starting with a digital original of a color film negative, the orange cast in the color negative will be converted into a purple cast in the positive if invert is used by itself. Therefore the simplest way to convert color film negative scan to positive is to first remove the orange cast. After the orange cast is removed, then image => adjustments => invert works correctly.
 
Type I: Original Made by Film Scanner
 

If the digital image of a color film negative is made with a film scanner, then most of this article is irrelevant. All film scanning software comes with color negative options. Look for color negative options in the scan software. Typically a variety of color negative film types are presented in the film scanner software, such as Kodak Gold 100, Fuji NPS, etc. Ideally, match the film type of your color negative to the list of films in the scanner software. If you do not know the type of film of the color negative, check the edges of the negative. Often the manufacturer shows the film type on the edge. Do not panic if your film is not listed in the film scanner menu or you cannot determine the negative type. If the match is unknown, experiment and select the combination which works the best.

After scanning a color negative with a film scanner using the color negative software setting, the resulting file will be a normal positive image. If the output of the film scanner is not a positive image, check the software settings and try again.

When doing any film scanning, either of negatives or positives (slides), the objective of the scan is to avoid clipping in all of the color channels, not making the scan look good. Clipping can be checked by examining the histogram of the scan. Most scanning software allows a preview of the histogram before the final scan. Otherwise, the histogram should be checked in photoshop. A good histogram will show a "bell shaped curve" in each of the color channels, with none of the histogram bumping either the left or right edge. Bumping the left or right edge indicates clipping, which is equivalent to a resulting value of either 0 or 255. If the histogram shows clipping, then repeat the scan after adjusting exposure, contrast, etc.

Once a film scan is made which contains full and complete data is each channel, then a regular workflow is applied to the image in order to correct and optimize the tones. Refer to this author's discussion of photoshop workflow.

 
Type II: Original Made by Digital Camera (or Scanner Could Not Convert Negative)
 
Another method for making a digital file of a color film original is with a digital SLR camera, macro lens, film stage, and light source. There are various commercial gadgets which will facilitate photographing a slides or negatives with an SLR camera. An example of a film-to-digital-SLR-camera gadget can be found at Specialty Photographic. Or, it is possible to construct your own setup with a copy stand, macro lens, enlarger film carriers as the stage, an electronic flash, a diffuser, and a box to hold everything. Since the result here is a picture of the color negative, the orange cast will remain and must be removed at conversion to positive time.
 
Correcting Type II, Method A: Conversion by Photoshop Only
 

As noted above in the background discussion, the lightest part of the negative (and hence the darkest part of the scene) can be no lighter than the color of the base orange cast. Therefore, if the orange cast can be converted to pure white in the negative file (black in positive), then the remainder of the colors by definition will be converted correctly since nothing can be lighter than the base orange.

Note: per this author's discussion of photoshop workflow, the "save as" method is recommended in general and is reflected in the steps below.

The following process works well. Note: every type of color negative film has a slightly different orange cast, and the orange cast will vary from scan to scan. Therefore, the following steps need to be taken individually for each image.

  1. When making the original image with the digital camera, be sure to include an unexposed part of the negative near the edge or sprocket.
  2. If the original image is a raw file:
    1. Run standard corrections in the raw file, such as optimizing the exposure and avoiding clipping in the color channels.
    2. Export the raw file into a psd file, suffixed with something like "orig_neg".
  3. If the original is a jpeg or tiff, save it as a psd file, suffixed with something like "orig_neg".
  4. At this point we have a psd file suffixed with something like "orig_neg" containing the film negative image.
  5. Open the "orig_neg" psd file.
  6. "Save as" the opened file with a suffix something like "orig_pos". This "orig_pos" file will contain the converted positive image.
  7. Use either image => adjustments => levels or image => adjustments => curves. Each of these commands will show a window containing three eyedropper tools. The eyedropper tools are, left to right, black point, gray point, and white point. Select the white point eyedropper tool. With the with eyedropper tool selected, sample an unexposed (pure orange) section of the negative. This action should turn the orange to white.
  8. If you are the persnickety type, while still in levels or curves, check each to the color channels individually and back off the selected point slightly to avoid losing shadow detail. Backing off is accomplished by moving either the right slider in levels or the selection point in curves very slightly to the right. The amount to move the slider should be obvious by examining the histogram.
  9. Commit the changes in either levels or curves by clicking "ok".
  10. Use image => adjustments => invert. Presto, a reasonably rendered positive should now appear. At this point the objective is to have good data in each color channel by avoiding clipping in all of the color channels, not making the scan look good.
  11. Save
  12. The "orig_pos" is now the reference original for applying the normal workflow. Refer to this author's discussion of photoshop workflow.
 
Correcting Type II, Method B: Conversion by Photoshop Plug-In or Commercial Software
 

At least two software products will convert the color film negative digital file with orange cast to positive. Neither of these has been tested by the author, so inclusion here is information, not an endorsement.

A photoshop color negative plug-in (ColorNeg) is available at c-f systems.

VueScan is stand alone software which runs scanners and will reportedly also "scan" a digital original in tiff or jpeg format. VueScan is available from Hamrick

 
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