As everyone on this forum (and elsewhere) probably agrees, one of the biggest issues of the Foveon sensors is horrible (color) IQ for anything shot at over ISO 400. ISO 800-1000 is okay but you do see blotchies when you have any reddish areas where the sensor seems to struggle.
This, of course, has been argued by many that it is most likely due to the sensor architecture that layers the red pixel layer under the green layer all under the top-most blue layer. Which means red light information has to penetrate quite a bit into the sensor to be picked up. Unfortunately that means the signal then has to be amplified BIG TIME which is where all the noise and artifacts come in. And no amount of pp magic has been able to make color images shot at higher ISOs where lower eV means increased amplification is needed to get the levels right.
Enter Sigma's monochrome which I've found to have two neat sections in their palate. Namely a "Color Mixer" and "Film Grain". Film Grain varies the size and randomness of the grain (or noise) to mimic the grain of some of our beloved BW films like Tri-X etc. (and, thus, reduce our objections to the noise).
But as I'm sure most of the real SPP aficionados here know this already I found the Color Mixer is the magic area. Here you can decide how much each color layer will manifest itself in the final image by selecting a point on a color circle the degree of Blue, Green and Red of the sensor you want to comprise the final image. In the samples below you can see the degree of noise present by various combinations and purities of these three layers. What I found most interesting is when you navigate the mixer to make the image comprised of 99% the pixels from the TOPMOST BLUE layer, amazingly, the noise level in the Foveon sensor is suddenly up there with a color version of some of the better single layer sensors renowned for their prowess at higher ISOs (like Fuji's X-Trans, for example).
Keep this in mind when you view these variances: it was shot at ISO 4000!!!
SPP settings: Sharpness: -1.0 and Exposure and Fill to taste (although for some of the lighter images it could be better balanced). The only Noise reduction was Luminance set to the first notch over from the left and Banding also set one notch over from the left! The shots were saved as TIFFS with the only further tweaking done in LR being clarity set at +18 and vibrance set to +17 (something I've discovered I like to provide a little snap for nearly all Foveon images, color OR BW). Then a conversion to 1500 pixel width jpgs for get DPI's gallery. LR's noise reduction wasn't used. And while I have Topaz' DeNoise I didn't need that either. (And it still doesn't help much with color shots over ISO 400 by the way.)
ALL Red (the bottom-most layer where light has to penetrate the most):
All GREEN (just the middle layer)
A mix of RED and Green Only (the bottom TWO layers):
A mix of RED and BLUE (Topmost layer and bottom most layer):
A mix of Blue and Green (the two top-most layers):
Even distribution of Red, Green and Blue (picked at the center of the color circle):
A mix of 17% Red, 25% Green and 58% Blue:
And, finally, 100% Blue layer (The topmost layer where light does not have to penetrate any further):
Remember, this is an image taken at ISO 4000!
This may be one guy's opinion but IMHO while great color IQ with the Merrill Foveon sensor is limited to shots taken at lower ISO's (ISO 100 - 320 for sure) due to the noise created at higher ISO's by the amount of amplification needed in the lower layers (and particularly the red) to bring out an image, as far as BW goes it's a whole different story. And now I see why Sigma came out with the monochrome option in SPP and deftly slipped in the Color Mixer control. Were the Foveon sensor just a single pixel layer like every other sensor it would easily rival the best color images out there at higher ISOs. So despite the difficulty in bringing out clean color images at higher ISOs, FWIW it's clear to me Sigma has figured out some outstanding image processing algorithms as demonstrated by the images in BW.