How to remove Sigma 50-500 vignetting with Photoshop CS
Sigma 50-500 f4-6.3 EX APO is one of today's most interesting and popular mid-priced lenses. Because of its large zoom range, expectedly many design compromises were made, resulting in some not-so-ideal characteristics. One of more obvious shortcomings is light falloff at the picture edges (vignetting). Fortunately, this drawback can be very easily cured using RAW plug-in v2.2 or latter installed (after being downloaded from Adobe) into Photoshop CS. The process obviously presumes that pics were shot with digital camera in RAW mode. Vignetting is more severe with full size 35 mm (both film and digital) cameras. All the analysis, measuring and tailoring was done using Nikon D70 with it's 24 x 16 mm DX CCD sensor. I presume that results will be very close with other digital cameras having the same sensor size, probably including Canon EOS 300D with it's only marginally smaller sensor. Values shown are best match with my sample of Sigma 50-500 and that could be little off with other samples. Light values across the whole pic are tailored within 1% of the average light value, which is much bellow visual threshold.
HOW TO DO IT
Start your Adobe Photoshop CS with previously installed RAW plug-in (remove Nikon's plug-in from the PS Plug-in folder as it takes over PS's RAW handling). Open the picture shot with Sigma 50-500. Check EXIF file for f-stop number and focal length. Find appropriate values in the table bellow. Put them under "Lens" menu. First value should be adjusted with "Vignetting Amount" slider, and second with "Vignetting Midpoint". For intermediate stops and/or focal lengths put some intermediate values! Make other corrections in RAW plug-in if needed and proceed with picture processing in Photoshop. That's it!
This correction will noticeable improve your Bigma pics, especially when shot wide open.