How to Fix Sloppy Redscale Scans from a Minilab with GIMP


We all shoot our films, some of us also process and scan it, but I prefer the convenience of the commercial labs for the purpose. Guess my surprise when I got totally different scans from the same roll! This GIMP tutorial covers the basic technique that I used to rectify the problem on one of my first home-made Redscale rolls. Although a digital computer and a software program is used, I do not consider that a manipulation, but rather "fixing" an error that has been made by another computer program elsewhere. We are going to analyze the histogram and based on some basic knowledge about film and exposure, we are going to get the redscale film to look like it was originally intended.

Here is a quick glimpse at the problem:

In order to correct it, let us start with some theory where it all begins: at the exposure. Redscale is an ordinary colour print film, that is exposed from its back:

Colour dye-forming layers are shown as their complimentary colours (negative) for clarity. Some of the layers of the film structure are omitted for the same purpose.

In a “normal” exposure situation the blue-sensitive yellow layer is exposed first. The yellow filter behind it serves the purpose to remove the blue and UV light form the mixture, that hits the red- and green sensitive layers. The later are also sensitive to blue (as are all silver halides), so this filter is essential for correct colour balance in print film. We are however exposing our film from behind, which means that our red layer gets exposed first, but now it’s behind the antihalation layer, which attenuates the amount of light it sees, and behind that lovely transparent-orange film base that we all love.

Being shot without the blue-absorbing filter, our red-sensitive layer also becomes sensitive to blue, and hence the intense red cast all over the image frame. Our green-sensitive layer is to follow, but now it is behind the red-sensitive one, and gets less of that blue goodness, so the image turns a bit more yellow, but only in areas that have enough green and blue light to excite our green-sensitive halide crystals. Remember that in additive colour theory red light and green light give yellow when mixed:

The blue-sensitive layer is on the very bottom now, but wait, isn’t it behind both the blue-blocking layer and the orange film base? Well, it is, and it is the layer that gets least light from all. The blue light that it gets is so little that it really never shows up as a saturated colour (remember that our red layer really gets excited by both blue and red now), but it shines through in the most over-exposed areas of the frame, where it contributes to the white of the extreme highlights.

So, if we are to build a histogram for the perfect redscale shot, it would include reds everywhere, greens in most of the mildly over-exposed parts, and blue only in the extreme highlights:

Display the histogram by selecting Window→Dockable dialogs→Histogram and then select RGB in the small histogram window.

What if the scanning was sloppy? Well, we could get something like this:

Here all of the colour channels have un-equalized black points (the histogram doesn’t start from the left), which we are going to correct, but without cheating and changing our colour balance.

For the purpose of this tutorial, and in general too, I use GIMP.

It is an amazing peace of software. And it’s free. But don’t get carried away editing your photos, because the magic of the analogue will be lost.

We are going to use the Curves tool (find them in Colours→Curves) and fix each colour channel separately. Let us start with RED:

RED is where Redscale comes from, so we really want to make it shine, as it originally does would if it wasn’t for the sloppy scan. Make your curve (left bottom end) start where all of the exposure does. This is going to be our black point in the red channel. This particular one does not need the right (bright) end to be trimmed.

Let us continue with the GREEN channel. It is the second most predominant colour in our REDSCALE photo and it contributes on the YELLOW areas in the highlights:

Don’t get carried away, green must stay more on the left. Trim the black point like you did with the red, and trim a bit the “white” point too, until the mildly overexposed areas become yellow.

Now, the most absent colour of all: BLUE. It must stay strictly on the left, but its black point must still be trimmed:

Now, if we press OK, our inner redscale will shine:

So, according to our histogram we have a perfect redscale: Blue is on the left, red is on the right, and green in between.

written by adash on 2012-08-01 in #gear #tipster #scanning #gimp #fix #curves #lomography #tipster #histogram #tutorial #redscale #colour #color


  1. adam_g2000
    adam_g2000 ·

    I'm glad you wrote this, many people think editing anything you get back from a lab or your scanner should be where it ends but I personally think this sort of work is fine, because the minute you use a scanner, it's making digital choices and normally poor ones. I was very disappointed by the scans from mine, they seemed low in contrast and too far towards blue. Initially I thought it was my photos and until I decided to do something similar was disheartened. I will give this a thorough read and attempt to do something similar in Photoshop. In fact, another article like this for standard colour images that have scanned poorly would be appreciated!

  2. sixsixty
    sixsixty ·

    Very well written and intelligent article. Please write more.

  3. reminator
    reminator ·

    This is mighty handy, my first roll of redscale is going to the lab soon and I would be dissappointed if my scans came out dull.

  4. astilla
    astilla ·

    good article! :) most of my scanned redscales from the lab come out like the first picture. thanks for the tip!

  5. megzeazez
    megzeazez ·

    Wow, excellent article!

  6. dkersbergen
    dkersbergen ·

    Great article even though sloppy scans are part of Lomography ;)

  7. gatokinetik-o
    gatokinetik-o ·

    Well done... definitively i'll try it!

  8. abbsterocity
    abbsterocity ·

    Must try this!

  9. buckshot
    buckshot ·

    Aha...! Maybe *that's* why I don't like most of the redscale shots I get back from the lab! Will have to try your recommended tweaks... Thanks a lot!

  10. kneehigh85
    kneehigh85 ·

    Oh this is a great article - thanks a lot

  11. reminator
    reminator ·

    I had to re-do almost every scan I made, but it was quite easy when I got the hang of it. It's easy to see if there's a lot of blue and/or green and making it go away is even more easy. Worked trough the pics in no-time really!

  12. blu132
    blu132 ·

    this doesn't belong here, but this is the KDE desktop you are using, isn't it?

  13. jla
    jla ·

    @blu132 Those are not KDE window decorations, I believe that is GNOME 2.

  14. blacksburg25
    blacksburg25 ·

    thank you!

  15. clownshoes
    clownshoes ·

    Great stuff

  16. impaktor
    impaktor ·

    Nice! A big plus for using GIMP.
    @blu132 looks like a GTK based window decoration, like Gnome or XFCE, or no desktop environment at all.

  17. samwise_camus
    samwise_camus ·

    Thank you so much for this article! 1) I learned something about why redscale does what it does. 2) My redscale scans now look awesome, with all the blue noise reduced in the shadows.

  18. herbert-4
    herbert-4 ·

    Thanx!! I use GIMP when I must.

  19. sibu_sen
    sibu_sen ·

    Amazing article!!!

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