you can ignore or disregard a lot of what you read on the internet.
The SA works fantastic with almost no post processing on the M9 if you work in black & white. It's just like any other lens but the falloff is beautiful. Either code it as a 21mm Elmarit or don't code it at all. You'll have to dial in -2 2/3 exposure compensation to have the meter read close.
If you do color work then you'll just have to download cornerfix and create a profile and send your images through that program before going on to your next step in processing. The extra step is not much trouble anyways.
I'm a Linux user so cornerfix hasn't been ported over here yet and what I did was to create two profiles of the lens against a white backdrop and import them into GIMP after RAW conversion. One at close focus, one at infinity. Since the rear element is significantly closer at infinity, the red edge is much more pronounced at far distances.
-Anyways, once you have two well-lit red edge raw files, import them into your computer, open them in photoshop or GIMP or whatever.
-Save the file as a TIFF or if you use Photoshop, maybe a PSD.
-Invert the colors so you have a dark green edged and black image.
-Create a duplicate sized file with a transparent backdrop.
-Copy your SA image into the transparent background as a layer.
-Select the erase tool and create a new brush.
. -Round
. -Diameter almost as much as the short axis of the image
. -Edge hardness at 66-75%, opacity 100% (you want it to fade out nicely)
-Place the eraser in the center and erase the majority of the black. You may need to move it around a bit but "push" the boundaries of the green/blue tint some to make sure you get rid of the black.
-Save again as a file you'll come to often to use as a plugin.
-Overlay the new file on your color SA images and adjust the opacity in color rendering mode to get the edge color just right. The red/magenta edges + the cyan/green/blues you have should replicate the light falloff of the lens by creating a neutral gray.
-Make sure to know which side is which because the edges are all actually subtly different colors so if you turn your camera vertical and load your SA color vignetting layer the other way, you'll get some weird edges that aren't quite corrected.
-Load the close profile for close images and the far one for distant images, of course. The vignetting is very different when up in the very close-focus range. It's almost negligible but still there.
The problem with some plugins like cornerfix or some of the vignetting tools in PS is that they lighten the edges too much which, I personally don't want. I know most folks love all the ways this lens renders, not just its center sharpness and lack of distortion.
It's an amazing piece of glass. One of my very favorite ever.
Phil Forrest