I've written a few posts about camera-scanning film negatives and converting the negatives in Lightroom. I've been beyond thrilled with the results produced by Negative Lab Pro, but one lingering issue I've seen is a lack of uniform color across the frame and particularly in the corners caused by uneven lighting of the negative when scanning. I've had to manually apply fairly strong vignetting correction in the Lens Corrections panel to correct for this issue.
Here's an example showing the manually corrected image with the settings followed by the uncorrected scan after conversion. You can see that because the digitized negative is dimmer near the edges of the frame, the inverted image is brighter in those areas. In addition to being brighter, the colors are thrown off as well. And although the "corrected" image is better, the colors and brightness are still visibly uneven near the edges of the frame.
In the 8.3 release of Lightroom Classic, Adobe added the Flat-Field Correction tool designed to remove shading and color cast from images. The tool corrects for shading using a calibration frame that is shot using the same optical setup as used for photographing negatives. Simply shoot a calibration frame before or after shooting a roll of negatives, then use the Flat-Field Correction tool to automatically detect and analyze the calibration frame and remove the light falloff and color cast from the negative photos. (It turns out this tool has been available as a plugin from Adobe Labs for several years, but I don't recall ever reading about it when researching camera scanning of film negatives.)
With my scanning workflow, I shot my roll of negatives, then removed the negative carrier from the front of the rail, set the camera to manual focus so that the light source was out of focus and adjusted the exposure (I had to reduce the exposure time since the light source by itself is much brighter), then shot the calibration image. Then in Lightroom, I turned on automatic Lens Corrections and set the white balance on the calibration frame, then ran Flat-Frame Correction before making any adjustments to the scanned negatives. The tool runs quickly and exports the corrected images as DNG raw files. The original raw files are removed from the Lightroom catalog but preserved on the drive (there is an option to delete the original raw files after correction.)
So how well does it work? Take a look at the photo below. The top image is the converted raw file without Flat-Field Correction applied. The lower image is the same capture processed after Flat-Field Correction. I also included the calibration frame below the comparison for reference. (Keep in mind that the square 6x6 negative was captured from the center portion of this frame. The shading effect is even more detrimental when capturing 35mm negatives that fill the frame.) The effects of shading are most visible in the lower right side of the image where the greenery is brighter and more yellow. In the shading-corrected image, the colors and brightness are uniform across the photo. It is also apparent that Negative Lab Pro provided a much better overall conversion of the negative when the shading had been corrected.
An additional benefit of using flat-field correction is with stitching of multiple frames to increase the resolution of scanned images. For example, after cropping, the 6x6 photo above captured in a single frame with a 24-megapixel camera has a resolution of only about 12 megapixels. But a medium format 6x6 negative holds a lot more detail that can be captured by moving the camera closer to the negative and taking multiple images. However, when I've attempted this in the past, the shading issue created visible banding across the final stitched image. Flat-Field Correction eliminates this problem allowing for very high resolution stitched negatives. The photo at the top of this post is a stitched composite of 3 images of the medium format negative with a cropped resolution of almost 37 megapixels. I was amazed at the level of detail in the negative when I zoomed in to 100%. Just FYI, I made some strong tonal and color adjustments to this image in Negative Lab Pro, then exported a TIFF copy for some additional work in Lightroom. This image is my personal favorite film photo that I've captured-I don't know why exactly but it's something about the way Portra captured the colors and the overall scene.
Flat-Field Correction provides excellent results, is quick and easy to use, and maintains an all-raw workflow within Lightroom for color-correcting scanned negatives. I will definitely be using this tool as a regular part of my camera-scanning workflow with Negative Lab Pro.