John Leathwick
Well-known member
(This post could have easily gone into the image-processing thread, but I have put it here because it tackles an issue that mostly crops up for users of medium format technical cameras.)
Quite a lot of my recent shooting with my Fuji GFX 100s/F-Universalis has been with the three wider lens options I have available to me, i.e., an S-K ApoDigitar 35mm L-88, a Digitar 47mm, and an Apo Componon HR 60mm. In using these lenses, I have become increasingly disenchanted with Adobe’s current implementation of lens cast calibration images (LCC’s) in Lightroom (LR), which they term flat field correction. In LR’s earlier implementation it was possible to highlight a set of images, each with their corresponding LCC, and it would reliably return corrected images converted to .dng format, with the original raw files optionally left in the directory. In the current implementation, I can only rely on it to perform as expected when I select a single image and its LCC; sometimes it works well, but at other times it seems to correct for illumination but not colour casts, even if the colour cast correction option is checked.
This set me thinking about whether I could build a smoother workflow to correct for vignetting and colour casts using Lightroom’s radial masks. In my initial experimentation, I started with my S-K Apo Componon 60mm, a lens with moderate vignetting, particularly when shot wide open. I started by shooting LCCs at a range of different apertures, and after importing these, experimented with settings for inverted radial masks until I found one for each LCC that minimised the width of the histogram and corrected for the blue colour cast in the outer part of the image. I then saved each mask as a named preset, one for each cardinal aperture from F/4 through to F/11. Correcting images taken at any of these apertures was then easily achieved simply by applying the preset after import, with results virtually indistinguishable from those achieved when applying an LCC using LR’s flat-field correction option.
By way of qualification, for some images taken with moderate shift, I needed to adjust the position of the mask to centre it on the estimated centre point of the lens, guided by the evenness of the correction, particularly in the corners. In some cases, where I didn’t want to apply full correction, I found that the slider that allows control of the strength of a mask enabled me to back off from full correction to leave a mild vignette.
I then extended this approach to my Digitar 47mm and ApoDigitar 35 L-88. Finding settings for the latter was the most challenging, requiring compensation not only for exposure and temperature (y-b) but also for tint (m-g). However, with masks saved as presets by aperture, results again very closely matched those achieved with an LCC applied using LR’s flat frame correction option.
Example results below show: an example radial mask; an uncorrected image taken with my 35mm L-88 at F/5.6, the same image corrected with first a radial mask, and then an LCC using LR's flat-field correction.
Based on these results, I then explored the extension of this approach to stitched panoramas, which I'll describe in another entry.
-John
Quite a lot of my recent shooting with my Fuji GFX 100s/F-Universalis has been with the three wider lens options I have available to me, i.e., an S-K ApoDigitar 35mm L-88, a Digitar 47mm, and an Apo Componon HR 60mm. In using these lenses, I have become increasingly disenchanted with Adobe’s current implementation of lens cast calibration images (LCC’s) in Lightroom (LR), which they term flat field correction. In LR’s earlier implementation it was possible to highlight a set of images, each with their corresponding LCC, and it would reliably return corrected images converted to .dng format, with the original raw files optionally left in the directory. In the current implementation, I can only rely on it to perform as expected when I select a single image and its LCC; sometimes it works well, but at other times it seems to correct for illumination but not colour casts, even if the colour cast correction option is checked.
This set me thinking about whether I could build a smoother workflow to correct for vignetting and colour casts using Lightroom’s radial masks. In my initial experimentation, I started with my S-K Apo Componon 60mm, a lens with moderate vignetting, particularly when shot wide open. I started by shooting LCCs at a range of different apertures, and after importing these, experimented with settings for inverted radial masks until I found one for each LCC that minimised the width of the histogram and corrected for the blue colour cast in the outer part of the image. I then saved each mask as a named preset, one for each cardinal aperture from F/4 through to F/11. Correcting images taken at any of these apertures was then easily achieved simply by applying the preset after import, with results virtually indistinguishable from those achieved when applying an LCC using LR’s flat-field correction option.
By way of qualification, for some images taken with moderate shift, I needed to adjust the position of the mask to centre it on the estimated centre point of the lens, guided by the evenness of the correction, particularly in the corners. In some cases, where I didn’t want to apply full correction, I found that the slider that allows control of the strength of a mask enabled me to back off from full correction to leave a mild vignette.
I then extended this approach to my Digitar 47mm and ApoDigitar 35 L-88. Finding settings for the latter was the most challenging, requiring compensation not only for exposure and temperature (y-b) but also for tint (m-g). However, with masks saved as presets by aperture, results again very closely matched those achieved with an LCC applied using LR’s flat frame correction option.
Example results below show: an example radial mask; an uncorrected image taken with my 35mm L-88 at F/5.6, the same image corrected with first a radial mask, and then an LCC using LR's flat-field correction.
Based on these results, I then explored the extension of this approach to stitched panoramas, which I'll describe in another entry.
-John
) who haven't buy an X2D yet, and who would like to buy a XCD28P. I would be more than happy if LR could correct lens colour cast/vignetting of this lens using a workaround. Thank you John for your tricks !