Accurate vignetting calibration

[lelouarn]

Hi !
I am using Lightroom to process images taken with my digital back (IQ160) which is used on a tech cam.

I'm happy with this, except I am missing lens profiles, and particularly vignetting correction. So what I do is to shoot an LCC and I use the Lightroom flat-field plugin. This works fine, but is a bit cumbersome (convert IIQ raw files to DNG, then apply the plugin, get _ff.DNG files, delete the temporary DNGs, stack and then use the _ff files).

I would like to simplify this workflow, and just measure the vignetting from the LCC, figure out how many Lightroom units of vignetting correction needs to be applied and just dial that in LR. Since I always shoot the same lens with the same aperture, I think this should work fine.

Now the question is: how do I determine precisely how much vignetting correction to use ? Of course, I could try to eyeball it: take the LCC, and apply vignetting correction until it looks uniform. But that seems a bit crude. I'd like something like a cut across the (corrected) LCC to see when it really becomes flat.
Any ideas ?
I could also use Photoshop, but my skills are not good enough to know how to do that...
Thanks for your ideas !

[kirkt]

You could try using Fiji or ImageJ to visualize the LCC before and after you apply varying amounts of correction.  In the attached screenshot, I used PS to make a radial gradient on a white background (a faux LCC).  This is my "before" image.  Then I used the ACR filter in PS to apply vignette correction and generated an "after" image.  I saved these as JPEGs and brought them into Fiji ("Fiji is Just ImageJ") and used the "Analyze > 3D Surface Plot" tool to visualize the grayscale surfaces of each.  I suppose a truly flat field will appear perfectly flat in such a 3D surface plot.  You can make your adjustments "by eye" in LR/ACR and then get quantitative feedback by bringing the adjusted image into Fiji and iterating in on an optical solution by hand.

You can also plot a grayscale profile of the image by "slicing" the image with the line tool in Fiji and then using the "Analyze > Plot Profile" tool shown in the second screenshot.

After on the left, before on the right of the screenshot.  You get the idea.

Fiji is cross platform, a well-maintinaed version of ImageJ, the newer version of the good old NIHImage:

http://fiji.sc

kirk

[AFairley]

Trial and error using the white balance eyedropper tool to measure levels at various points on a uniform tone image?

[Wayne Fox]

With that setup, seems the issues are far more than simple vignetting. Correcting lens cast color issues to me is the bigger problem.

Any reason you don’t use capture one?  It’s LCC correction tool is very simple and works very well.

[lelouarn]

Hi,

My problem comes with the HR 32mm lens, which (for the moment) I use mostly unshifted. Even a ~f/8 it has a fair bit of vignetting (color cast is very small in this case, no shift) - I don't have the center filter. This is disturbing when I do nodal point panos (Autopano pro doesn't like vignetting, so I have to take it out before assembling the pano).

I have also done shifted panos, and in that case I have used both C1's LCC routine and LR's LCC plugin. The color cast does become an issue, and there you can't avoid using LCCs "properly". C1 is probably a bit better, but I am much for familiar with LR so that's what I tend to use.

I'll try the eyedropper method. Thanks for the tip about Fiji, didn't know that...

Thanks for your ideas !