Unfortunately, film inversion basically throws off any white balance assumptions that originated from the camera, so another white balance pass must be performed after film inversion, and the exact nature of that depends on aspects of the film itself (such as the exponent ratios).
Order of operations when I use RT’s film negative tool:
- If no lens vignetting profile is available (lensfun doesn’t have the Rokinon 100/2.8), take flatfield reference image under same backlight conditions as capture with no film holder or negative present. This flat should be reusable for all subsequent captures, set that file in RT’s flatfield tool
- White balance on the orange mask. Shouldn’t be necessary with my new code, but something is weird with the color pickers such that using the WB tool and then finetuning with my scale factors changes works better. I’m trying to eliminate the need for the WB tool to be used at all. If there’s no orange mask on the current frame, orange mask from another frame captured under identical backlight works reasonably well, although I’ve found that small changes in exposure (shutters aren’t perfect) can really throw off the shadows. I need a film holder that gives me reliable orange mask per frame…
- If you have some good neutral references, use these to adjust the power ratios. If not, expect some quality time with WebPlotDigitizer, a datasheet for your film, and a lot of fiddling. History for density_plot.py - Entropy512/rgb_led_filmscan · GitHub - I’m working on implementing auto-fitting to make this less painful, but I do have Kodak Gold 200 values. They translate to RT’s filmneg tool settings of:
– Reference exponent: 1.72
– Red ratio: 1.06
– Blue ratio: 0.95 - Use RT’s filmneg tool to do another white balance pass. This MUST be redone any time you touch red/blue ratios!!!
Same settings as last image except for changing the input color profile from Fuji Superia X-Tra 400 to Kodak Gold 200, using model-fitted values for the exponents from a Kodak Gold 200 datasheet, redoing WB on the tree, and then cooling it down a bit (Previous images used RT’s defaults for the exponents):
10-1-3.jpg.out.pp3 (14.6 KB)
Bumping up EC a bit:
10-1-4.jpg.out.pp3 (14.6 KB)
As far as choosing a color profile of the film and not the camera: In the ideal situation, the negative is captured in such a way as to eliminate as many behaviors of the camera’s SSF and the film dye spectral response, leaving only the film’s SSF. RT’s filmneg tool can operate in the input colorspace, e.g. perform inversion before applying the profile. It’s actually BETTER to capture a negative with a backlight that consists of three strong monochromatic spikes (such as a phone’s OLED display), and best to composite three captures each with just red, green, and blue backlight (this is what professional film scanners use, up to and including stuff like Blackmagic’s Cintel. Cintel adds an IR capture for dust/scratch compensation but I’m not full-spectrum-converting my camera.)
See Digitizing film using DSLR and RGB LED lights , see NateWeatherly’s posts along with https://medium.com/@alexi.maschas/color-negative-film-color-spaces-786e1d9903a4 which was linked to from that thread.