Where I had problems with some films was a vignette which was difficult to remove because a colour shift occured.
The problem itself results from the light path from the flash to the film strip. But since this setup has several advantages I wouldn' t like to change it much.
Recently I found out the applying a flat field correction solves my problem. I can select a photo taken without a film strip in Rawtherapee and this works much better than the vignetting correction.
Has anybody else experienced this problem and found a solution for it?
p.1 #4 · Digitizing film negatives and flat field correction
Out of curiosity, explain your light path from the flash to the film being scanned. From your photo, it has to make a 90deg turn, which is expected. How are you making this turn and what diffusion surfaces are between the flash and film?
A right angle prism, with a diffusing optic (milk glass) between the flash and the side of the prism facing the flash might work. The following item might be worth a try for 35mm, but is not large enough for medium format.
p.1 #6 · Digitizing film negatives and flat field correction
The flash light comes from the back, through a diffuser, hits a 45°angled white paper surface for further diffusion, goes through a second diffuser, than comes the film strip holder.
Around the first diffuser there is an LED Ring light focussing help which is not active during exposure.
I use a Godox TT350 flash with radio trigger, the camera is triggered with another radio trigger. I use 1/250s, f/8 so room light is not much of a proplem.
With a phone as light source the vignetting is gone but I loose light and the continuous spectrum of the flash.
The flat field correction is more or less a correction of a vignette with a non-standard shape.
There is a stand alone software for film scanning that uses flat field correction and brought me to this idea.
It helps most with the oldest film, 1980s, early 1990s. Perhaps the colours have faded enough to make them difficult to digitize.
p.1 #7 · Digitizing film negatives and flat field correction
In the optical area that is normally a correction for field curvature, so I must have misunderstood.
In the old days I would take several images focused slightly differently, before I got an anti-Newton glass film holder.
p.1 #8 · Digitizing film negatives and flat field correction
That's right, I know flat field in the context of microscope objectives as a correction of the field curvature.
Ordinary microscope objectives have no flat field, one has to focus throught the specimen all the time anyway. When micro photograpy became more a craft than an art there emerged a need for flat field corrected objectives like the Zeiss Plan series from the late 50s.
In Rawtherapee they use this name for the correction of uneven exposure: https://rawpedia.rawtherapee.com/Flat-Field
In camera lenses a field without curvature is expected so they recycled the name.
p.1 #9 · Digitizing film negatives and flat field correction
I think most folks in the photo groups think of field curvature as something separate from vignetting (unlike the RawPedia link), so there might be some miscommunication here.
-Is the vignetting due to the lens or the light source? (What lens are you using?) The light source can be corrected, but a lens vignetting would need software or maybe closing down the lens to a smaller aperture (if it has an iris).
-If Rawtherapee is working, is the issue the time (added steps) to apply the correction?
p.1 #10 · Digitizing film negatives and flat field correction
I'm correcting for the combination of the enlarger lens at f/8 and the light source. The lens will not contribute much here.
I would be interested it other members are having this promlem and how the cope with it.
Using something else than a surface light source is not uncommon, so sombody will have had the issue.
p.1 #11 · Digitizing film negatives and flat field correction
Sorry, I misunderstood. Ages ago I had a setup with a 5x7 light source and did not have much light falloff. I used a handmade shroud to eliminate room light. But I went to MF film scanners and did not look back.
