I forgot to mention that I use the appropriate center filters for each lens. The problem isn't exposire falloff but color casts (mostly magenta)Is this being caused by the falloff on wide angle lenses corrected by center filters?
Which model CF are you using? Maybe there's a mismatch? Wouldn't you be better off trying to track down the source of the problem rather than trying to correct it in post?I forgot to mention that I use the appropriate center filters for each lens. The problem isn't exposire falloff but color casts (mostly magenta)
I think that's similar to what people are doing with medium format digital, creating LLC for extreme wide angle lenses and applying them in Capture 1. although I think instead of a white card they put a piece of translucent frosted material over the lens. I was hoping that one of those sliders in Adobe Raw for lens corrections could do something similar. On my next roll I'll try shooting a frame like you suggested.The route I would explore is to shoot a reference white card with those lenses to isolate the color cast
.What you are seeing on the lens is the coatings which should not change the light color. So that is not the source of the color you are concerned about. However all lenses have a light falloff as it moves the center line of 1/cos4(angle) and it is more pronounced on very wide angle lenses and that may influence the overall color cast whatever the source.
I use a Scheider IVb for the 72 XL - It compensates properly for the light fall off but I think it may contribute to the color cast.Which model CF are you using?
I'd like to know what is the cause of colour cast in the corners with Film. I understand that in digital, it is due to the cross-talk between the wells and colour filter array. But what is the cause on Film?
I do remember corner vignetting often resulted in a colour saturation change, and that may also cause some colour drift due to the different sensitivity of the 3 layers, but a center filter was used here.
@Sirius Glass the question wasn't about vignetting but about color casts.
To my mind, this means that either:Some of my wide angle lenses ... have magenta color casts most noticeable in the corners
To my mind, this means that either:
1) the transparencies themselves have an overall magenta cast, which is barely visible in the lighter centre parts and most noticeable in the (darker) corners, and which in turn is reflected in the scans; or
2) the transparencies themselves exhibit uneven colours, which in turn is reflected in the scans; or
3) the scans themselves are uneven in their colour.
If #1 is the case, the solution is colour correcting filters on the camera. Alternatively, an overall correction to the colour of the scan may suffice.
If #2 and/or #3 are the case, the OP needs a colour correction that is applied in different amounts at different parts of the image - that is where the LLC tools come into play.
#2. I don't think it's the scanner as it only happens with the super wide lenses. I tried CornerFix but it only works with images created from a Bayer sensor. I'm scanning 617 on a Coolscan 9000 but I have to do it in three sections using offsets (it only scans a maximum of 6x9), but it's the final stitched image that has the color cast in the corners - not the corners of the 3 individual scans. Even when I do it in one pass on an Epson flatbed I have the same problem.2) the transparencies themselves exhibit uneven colours, which in turn is reflected in the scans; or
.1. both center filters for these two lenses are 1.5 stops. So there is differential in F stops and if center filter has a colour cast due to aging or inherent cast, 1.5 stops is just enough to cause visible colour cast.
Correction lens profiles can be created for Lightroom
#2. I don't think it's the scanner as it only happens with the super wide lenses. I tried CornerFix but it only works with images created from a Bayer sensor. I'm scanning 617 on a Coolscan 9000 but I have to do it in three sections using offsets (it only scans a maximum of 6x9), but it's the final stitched image that has the color cast in the corners - not the corners of the 3 individual scans. Even when I do it in one pass on an Epson flatbed I have the same problem.
.
The IVb filter is a whole 2 stops, probably why it's worst
I checked out Lightroom and there is a flat-field correction option that works with any file type. I don't have a correction frame yet but I played around by creating one in photoshop using a circular magenta gradient and it looks like it should work. I'll try to shoot some LCC over the weekend and try it out .
The falloff in wide angle lenses is based on some calculation using the cosine, which frankly I don't understand. However, wouldn't the gradient of the correction have to be able to match the cosine falloff exactly?> Otherwise, you'll never be able to match it and it will be obvious.If you prefer to use gradient. Masking option in Lightroom can be also set to gradient. I find Lightroom way faster to work with images and only get them to Photoshop if I really have to.
Masking is basically separate layer just like in Photoshop and isn't destructive.
Quickest workaround here is probably:
1. Acquire LCC image of white background (at multiple f stops if needed).
2. Import in lightroom and add gradient mask
3. Settings or mask itself can be copied to any images you have. If you find any images that need further correction, every single one can be fine tuned for that masked area only.
To adjust gradient to your liking when mouse is hoovered over white area in the center, RGB values will show bellow histogram in develop mode.
They you can adjust gradient to get the same RGB values in corners.
If you're not using RAW dng scanning through Vuescan, I would recommend trying out Vuescan for this. It can be trialed with watermarked output.
dng output makes big difference for editing in Lightroom. Size is also smaller than TIFF.
The falloff in wide angle lenses is based on some calculation using the cosine, which frankly I don't understand. However, wouldn't the gradient of the correction have to be able to match the cosine falloff exactly?> Otherwise, you'll never be able to match it and it will be obvious.
What you are seeing on the lens is the coatings which should not change the light color. So that is not the source of the color you are concerned about. However all lenses have a light falloff as it moves the center line of 1/cos4(angle) and it is more pronounced on very wide angle lenses and that may influence the overall color cast whatever the source.
See post# 7 about the 1/cos4(angle) falloff which explains the greater density in the corners.
#2. I don't think it's the scanner as it only happens with the super wide lenses. I tried CornerFix but it only works with images created from a Bayer sensor. I'm scanning 617 on a Coolscan 9000 but I have to do it in three sections using offsets (it only scans a maximum of 6x9), but it's the final stitched image that has the color cast in the corners - not the corners of the 3 individual scans. Even when I do it in one pass on an Epson flatbed I have the same problem.
.
The IVb filter is a whole 2 stops, probably why it's worst
I checked out Lightroom and there is a flat-field correction option that works with any file type. I don't have a correction frame yet but I played around by creating one in photoshop using a circular magenta gradient and it looks like it should work. I'll try to shoot some LCC over the weekend and try it out .
The falloff in wide angle lenses is based on some calculation using the cosine, which frankly I don't understand. However, wouldn't the gradient of the correction have to be able to match the cosine falloff exactly?> Otherwise, you'll never be able to match it and it will be obvious.
Here's my "reverse-engineering" of Sirius formula:
Light decreases with distance (inversely to its square i.e. 1/distance^2). The same distant object becomes brighter as you bring it closer. Now visualize a triangle with the lens at one tip, "looking" at the opposite side which is the flat focus plane. Obviously the distance to the center of the plane (adjacent) is shorter than to the edge (hypotenuse). Hopefully this brings back middle school trigonometry lessons on how to express the relationship between the two. Then you combine the trigonometry with 1/distance^2. The wider the lens, the greater the distance to edge objects, the less light you get at the edges.
BTW I suspect that in a real world lens the fall off is mostly caused by the engineering trade-offs (image circle vs weight?) by lens designers. The new Voigtlander Classic line shows a lot of vignetting and at the same time they're suspiciously compact, so I am quite unsure if the formula cited by Sirius is actually "visible" in our photos.
Polarizers also add to the problem, because their effect varies according to angle.It is noticeable in photographs as I learned from using a polarizing filter on a 21mm lens.
Polarizers also add to the problem, because their effect varies according to angle.
Which lenses would you avoid using polarizers in 4x5? Does that apply to both BW and chromes?Polarizers also add to the problem, because their effect varies according to angle.
See post# 7 about the 1/cos4(angle) falloff which explains the greater density in the corners.
1/cosine4 is a geometric effect, the circular aperture appears smaller (oval) when you view it from an angle. You see this particularly on pinhole images. It does not affect colour cast.It is an overall caste that you have to deal with.
To my mind, this means that either:
1) the transparencies themselves have an overall magenta cast, which is barely visible in the lighter centre parts and most noticeable in the (darker) corners, and which in turn is reflected in the scans; or
...
These make sense to me, vignetting increases the saturation in the corners, which may accentuate an over all magenta cast....I still think that suggestion from my previous post about colour cast coming from filter is a further possibility to check...
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