Creating a photoshop filter that works like a centre filter

[Ardpatrick]

I'm a fairly basic photoshop user because I am focussed on analog workflows. I am doing dslr scanning, and I have a consistent unevenness in my light source, which has a negative impact on my final scanning results.

I assembled a 'lightbox' structure with a DIY diffusion chamber underneath, placed on the baseboard of a copy stand. This is used as a platform for Dslr scanning of negative sheets for 'contact sheets'. The light source shining into the side of the chamber is a wireless flash gun, which is reflected upwards inside the chamber to illuminate the glass top from below. Its works quite well as a lightsource, but the results aren't perfect. There is some light fall-off on one side of the glass topped lightbox, and in the corners etc. IF the hottest pixels are in the 240 range, the darkest corner is around 195, with other corners in the 210-220 range. Fall off is somewhat non-symmetrical.

I'm looking for a way to digitally correct / compensate for the unevenness in my diffusion chamber. If I shoot a 'blank' frame of the top of the lightbox at the end of a session, with all negatives removed, as long as camera and lightbox haven't been moved, the 'blank' shot maps the varying intensity of the illuminating light source, which should be usable digitally to invert the fall-off type effect of the light source - hence thee analogy to a Wide-Angle centre-filter. Unfortunately my rather basic knowledge of Photoshop means I've only tried to add this 'blank' frame as a layer above an inverted neg (the top layer isn't inverted as I'm not trying to multiply the unevenness, but to cancel it out). There are a range of blending options between layers, and I'm not sure which would be best. Our is there another approach? Any help appreciated.

 

[Pieter12]

I would first ad an adjustment layer and adjust the image so the side with the fall-off looks good to you. Then I would select the mask for that adjustment layer, and turn off all the channels except the mask. Paste your blank frame scan into the mask. when you look at the image layer and the adjustment layer, theoretically everything should be even.

 

[koraks]

I have a consistent unevenness in my light source, which has a negative impact on my final scanning results.

Frankly, just fix this. Don't try to patch it up in digital post because it'll always remain a marginal proposition and sooner or later you'll realize you're not happy with the outcome, anyway. While at the same time, fixing the uneven illumination is definitely possible, probably not even all that difficult, and will ultimately save you a lot of time.

Having said that, your basic approach of shooting a calibration/correction frame is in itself a usable approach.

There are a range of blending options between layers, and I'm not sure which would be best.

Just try them. For each setting that seems to do more or less what you want, you can change the opacity of the correction layer to moderate the effect until you're happy with it. I think you'll end up using something like 'Soft light' or 'Overlay'.

 

[gary mulder]

Capture One has a tool called LCC (Lens Cast Calibration) This is is frequently used in a digital workflow and camera movements. It is likely that other software has a similar tool.

 

[Ardpatrick]

Thanks all. I’ll get to try this tomorrow at some point,

Koraks - your comment prompts me to again try to correct the uneven lighting, but believe me I’ve already spent hours trying small internal baffles & reflectors etc. It’s not as simple as it seems if perfect evenness is the goal. That’s why I suspect Photoshop might be more accurate ultimately. But I’ll try both,

 

[koraks]

I'm aware of the challenge and how frustrating this can be. I've done the same thing (multiple times) when making LED light sources for my enlarger. It's easier if you increase the distance between the light source and the diffusor screen, and it can also help to place a first diffusor fairly close to the light source and then another one at a greater distance. What works best also depends on the geometry and nature of the light source itself.

 

[George Collier]

I don't know how steep the exposure difference is between the middle of your image to the outside, but I assume the center area is brighter, in positive final scan. I also am guessing that it isn't always exactly the same.
You might try this -
1 enter quickmask
2 select the gradient tool, choosing the round shape at the top of the window
3 you want to darken the outside edges, but leave the center area as is
4 so - (while in QM) set the foreground color - double click the overlapping square to the upper left near the bottom of the tool pallet - to absolute black - in RGB this would be 0R, 0G, 0B in the color picker, which should open when you double click the square
5 click ok to the new color for the square
6 now (still in QM) set the background color - double click the square to the lower right of the one you just changed, near the bottom of the tool pallet - to a gray. The trial and error will be between how light or dark to make the lighter background gray than the foreground black
7 then draw a gradient with the tool from the the brightest place (center?) to just outside one of the corners. It will be a circular gradient steep or soft, depending how much lighter the outside edges of the gradient are.
8 exit QM, and you will have a selection,
9 create a curve adjustment layer (masked by the gradient), while the selection is active and just to test the gradient, when the curve pallet comes up, drag the dark end of the curve range to increase it's darkness. You should see the outside edges become darker as you drag. The solid black at the center of the gradient will keep the center of the image from changing.
10 the lighter the background gray in the gradient, the more effect the curve will have on the edges. Play around with the gradient tool to try this technique. The shorter the gradient is (stopping before you get to the corner) will start the curve correction closer to the center of the image. If you are lucky enough to find a gradient that works for everything, save it in the curve pallet and just load it when you have a new scan - you can always adjust the curve as needed, use the shape of the curve to further adjust the "shape" of the curve on the image.

 

[nmp]

If you have symmetrical unevenness (like one would have in a lens) you can try using the Lens Correction option (Filter>Lens Correction>Custom) where one can add +ve vinette to compensate for the light fall off at corners.

:Niranjan.

 

[_T_]

Set everything up as if you're going to scan a frame of film.
Do not put a strip of film in the system.
Take a picture of the uneven light source by itself.
Invert that photo.
Use the appropriate blending mode to apply it to the photos of your negatives.

You may have to alter your exposure settings to get the shot of the uneven light source.

 

[Kino]

I used to work with a motion picture scanner that used a sequential flash LED RGB lamphouse. It was capable of a "pixel leveling" routine whereby you placed a piece of clear film base in the aperture and the software would expose in RGB and then "level" each pixel for uniform exposure.

Of course, I didn't write the code, but it might point to a solution for someone with proper coding skills...