B&W conversion: artifacts when darkening blue skies

So I'm using the latest ACR for CS6 and when I get heavy artifacts/banding in large, even areas when converting images to black and white. This is especially the case when I want to make a blue sky black, or very dark gray. As I pull the blue luminosity slider down, it starts out fine but at some point sort of completely breaks down.

For clarity, this is on raw files, 16-bit editing, and I haven't really noticed the issue anywhere else.

Is there anything I can do to prevent the banding?

Do you mean you're converting to black-and-white in ACR?

Yes. My one experience with LR (LR4) was rather disastrous, as it was slow as hell on my (new) computer, so I'm sticking with ACR/CS6. I like doing the B&W in ACR since it saves me from adding a step in PS. Why, would a B&W layer in PS avoid these artifacts?

Posting up a photo of before and after your changes (and another copy of the after with the issues you're seeing circled in red or something) will help people give better guidance and advice. Also a list of steps you take to arrive there would be useful as well. Don't be scared of posting extra detail if you're looking for help, including how you decide on your ISO, aperture, and shutter speed can help others understand what might be going on.

My (very blind) guess is that you might be seeing natural texture within the data that was recorded, and possibly offset by the quality of your monitor. In full colour the image could be masking the issue, but whatever method you use to convert may be degrading whatever effect masks the texture.
This could also be complicated by the display you're using, depending on how it actually renders monotone shades. I know one of my monitors (Some old cheap LG thing) will handle greyscale gradients worse than blue/green gradients, which I suspect is due to poor handling of its red channel. What can show as a smooth gradient on my main monitor will then display as 4-5 clear bands on the cheaper one.

I think you will find any artifacts you see in the RAW conversion will be there in the RGB one. I do all my B&W conversions in LR as there is an extra channel in the mixer over the PS channel mixer tool. The tools in LR are identical to those in ACR, so it matters not which one you use.

There is one thing I will add though. If you already have a good colour version of the image in RAW. Where you have had to make significant changes to the blue channel, and in some cases the the red, then it can actually be better to convert the image to a 16 bit RGB file, then load that up in LR/ACR to do the conversion. I think this works because when working on a version of the RAW file you are dealing with the original image data, and thanks to the WB multipliers the red then the blue channels are far more susceptible to noise which always seems to show up as you try to pull those channels down. For a monochrome conversion I actually think you would sometimes do better using a Uni-Whibal setting, so that all of the incoming channels share the same WB multiplier of 1. After all you are not really looking for a "good" colour WB when doing a monochrome conversion and using a channel mixer tool. I guess you should be able to find the appropriate WB slider values for UniWhibal by setting it up in camera and just saving the in camera settings as a new WB preset. So you shouldn't have to shoot the image using it. You only have to play with the WB sliders in a mono conversion in LR/ACR to see the affect it has on the image.

I think I shall go away and have a play with that. I have a couple of UniWhiBal landscape shots that I can use for the experiment.

Alan

I have found in lightroom similar experience and the artefacts can also be seen in colour images - in my situation it was adjusting sharpness, noise and exposure settings too much that seemed to enhance/emphasise the texture that was in the original file but not easy to see until the settings were adjusted. Are you adjusting any other settings before the B&W conversion ? Maybe could try reducing Contrast or Clarity and Noise before conversion.

Please post a sample image, preferably a link to download a raw file that is giving you the issues that you are describing. Most of the time, when you shoot outdoors in natural daylight, the red channel is typically underexposed by several stops compared to the green and blue channels. This is especially true of the sky areas of your image. When you make adjustments in a channel mixer or similar BW conversion step, and you try to boost the red component, you see the artifacts of the underexposure rear their ugly head. Moreover, the contrast of BW will make these artifacts more apparent.

One solution, for daylight shooting, is to shoot with a Magenta CC filter on the camera (a real filter in front of the lens, not some computer applied filter in post). Magenta CC30 or so will withhold green, the channel that typically saturates first, and permit better exposure of the red channel (or the blue to a lesser extent). This is sort of like selectively ETTR'ing the red channel, or at least giving it a fighting chance to have better overall data in it. If you really are only interested in the BW version of the image you are making, then you can WB this data any way you want, or not even worry about WB after the fact. At least with this approach, the exposure in all three channels will be closer to each other in EV, hopefully making for less noise in the notoriously noisy red channel, the opposite of sky blue and cyan. One must consider that filtration withholds light, so the appropriate exposure compensation will be necessary - this may affect shutter speed, aperture or ISO to compensate. The Lee CC30M filter, for example, requires +2/3 stop exposure compensation. Here is a table of all of the Lee CC filters and their exposure compensation requirements:

http://www.bhphotovide​o.com …M30STD_4_x_4_Ma​genta.html

Read this very informative article on the subject - it is a little dated, but useful nonetheless. If you use Raw Digger to inspect your raw image data, you can shoot with and without the filter and compare the histograms, as the linked article does, to convince yourself that the red channel exposure benefits from such filtration.

http://www.libraw.org …a-filters-on-digicam.html

kirk

Just do your normal raw editing, then do the BW conversion in Photoshop using Channel Mixer. Choose the channel which gives you the sky that you want.