Grayscale imaging with a color digital camera
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Last updated October 22, 2009
In a world gone color-crazy with color TVs and monitors, color printers, color faxes, color braces and now even color bicycle tires and cell phones, black-and-white (B&W) photographs still manage to hold their own. The good ones positively command attention.
What's the enduring charm of B&W? Its inherent power and beauty are largely beyond explanation and certainly beyond justification. But to my mind,
all enter the equation somehow. In any event, an understanding of the last item pays when it comes time to convert color digital images to B&W.
We'd better take a moment to shore up some terminology. Some of these distinctions will come in handy later.
Photographs lacking color have traditionally been called black-and-white (B&W), even though they owe much of their punch to the interplay of gray tones. Strictly speaking, these images are grayscales, but they won't shake the B&W label anytime soon.
Truly black and white (1-bit) images lacking intermediate shades have their uses but are generally of very narrow interest and won't be discussed further.
Technically, images rendered in black, white and shades of gray are grayscale images. In photographic contexts, you'll find the terms "B&W" and "grayscale" used interchangeably, and this article is no exception.
The 256 steps from black to white available in 8-bit grayscale images are smaller than most human eyes can distinguish.
Monochromes can contain black, white and any shade or tint of any one color. Grayscales are monochromes that happen to be based on neutral gray, while sepias are monochromes off a particular brown.
I'll generally reserve "monochrome" for monochromes based on colors other than gray. This seems to be a common usage.
That said, digital B&W recording can come in quite handy for previewing potential B&W scenes in the field—particularly for those of us with limited skill at previsualization. Even though you'll ultimately record in color, you need to be thinking and seeing in B&W along the way for best B&W results.
In-camera B&W recording always starts with color data acquisition in digital cameras using a single CCD with Bayer pattern color interpolation—and that includes all consumer-grade digital cameras, now and for the foreseeable future.
To generate B&W images, such cameras apply a simple grayscale conversion algorithm to the interpolated RGB color data before JPEG compression and file writing take place. Firmware size, processing time and user interface constraints all conspire to limit the sophistication and flexibility of the in-camera conversion. Unfortunately, you often need sophistication and flexibility to get the best B&W output.
Granted, grayscale conversion before compression sounds like a good idea, but in fact, there's little to be gained and much to lose by letting the camera do it. For starters, compression does little damage to the brightness data. By design, the JPEG compression algorithm goes to great lengths to preserve luminance data at the expense of chrominance, just as the brain-eye system prefers.
Taking Charge of the Grayscale Conversion
For many experienced digital B&W photographers like Carl Schofield and Jeff Alu, the flexibility and control gained by recording in color and post-processing for B&W are well worth the extra work and the negligible JPEG losses involved. (The latter could easily be eliminated with uncompressed TIFF or RAW recording if they become a real concern.)
The table below lists several ways to reach B&W from color recording. The last two options are your best bets. The first two really differ only in convenience.
Other Color Recording Advantages
Color recording eliminates the up-front decision to record in B&W mode and the requisite trip to the camera menu. You'll have many more options down the line by retaining a full color version of your B&Ws.
Color recording also eliminates the need to shoot B&Ws with color filters for contrast control, as explained in the channel mixing section below. In post-processing, complex selections and masks are often easier to define in color than in grayscale.
Note that in-camera B&Ws offer no significant storage advantage over color images. In theory, 8-bit grayscale camera output files should be much smaller than their 24-bit color counterparts. In practice, however, both are written in 24-bit RGB format. B&W JPEGs may end up a tad smaller because they're more compressible, but B&W and color output files are generally about the same size.
Below is a pearl-packed B&W how-to interview with dpFWIW contributor Carl Schofield, the obviously talented and broadly experienced photographer behind Digital Grey and Beyond Red, two of my favorite online galleries devoted respectively to B&W and infrared digital photography.
Carl's impressive results speak for themselves. He generally relies on a relatively simple technique—aperture priority, judicious use of exposure correction and occasional spot metering—and a 2.1MP Nikon CoolPix 950. Of course, there's nothing simple about Carl's eye.
Do you prefer shooting with B&W or color recording?
Do you need a narrow-angle external light meter, or can you work effectively off the camera's spot meter? Also, do you use mostly manual exposure, or do you find ±2 stops of exposure correction off a priority mode flexible enough to reach the tonalities you're after in most instances?
How much post-processing do you apply on average, beyond the grayscale conversion from color? Do you rely heavily on tone curves?
Do you follow the common digital wisdom "Expose for the highlights, bring up the shadows in post-processing"? It's diametrically opposite the old film-based zone system dogma "Expose for the shadows, develop for the highlights", of course, but it addresses unpleasant realities in the way current CCDs handle blown-out highlights and takes advantage of uniquely digital opportunities in shadow management.
How do you "blend" 2 different exposures, and what's contrast masking?
If you're going strictly for a B&W, do you shoot differently than you would if you're also potentially interested in a color version?
What compression level did you use for the images posted?
Did you enable or disable in-camera sharpening for the posted shots?
Are you happy with the way your 1600x1200 B&Ws print at 8x10? If not, at what size are you happy?
Thanks for sharing your experience, Carl.
NB: Both PhotoShop and Corel PHOTO-PAINT 9 (PP9) refer to the Lab luminance (L) channel as the "lightness" channel. The instructions below are for PP9.
There may be a slicker way, but this works well. The port to PhotoShop is straightforward.
Acknowledgement: Thanks to Michael Cervantes' Corel PHOTO-PAINT Lab color tutorial.
Relatively speaking, color filters brighten like colors and darken complementaries in the scene. Thus red filters darken blue skies to dramatize clouds and lighten reddish blemishes more than the surrounding skin to smooth out skin tones. Green filters brighten and define foliage. Blue filters brighten clear sky at the expense of just about everything else. For a more detailed description of color filter effects in B&W photography, see the Filterzone Black & White Contrast Control Filters page.
The advent of digital RGB color images brought new B&W opportunities. Mining the red, green and blue channels for good grayscale material came as a natural outgrowth of the B&W color filter tradition. After all, each RGB channel is the equivalent of a B&W taken through a filter of the corresponding color.
Channel mixing (CM) takes this idea to its logical conclusion by blending up grayscales from any desired combination of constituent RGB channels. You can always make grayscales from a single channel if you want, but CM amounts to capturing the scene in B&W three times using red, green and blue color filters and mixing the results to taste in post-processing. The B&W photograph at the top of this section was mixed from the red and green channels.
A big CM bonus is relief from several significant up-front commitments and hassles:
However, CM has its limitations.
Experienced B&W photographer Richard Saylor of RPD has convinced me that CM can't duplicate the effects of certain color filters, including the very popular red #25 and #29. After looking over a spectral response plot for the ubiquitous Sony 3.34MP CCD, Richard wrote,
It would seem reasonable to expect other CM limitations along these lines. To get a feel for what CM can do for you, compare the representative transmission spectra at Heliopan Filters for B&W Photography to the spectral response curves for your camera's CCD.
The obvious answer: The pixel-by-pixel contributions made to the color image by the channel's designated primary color. In a 24-bit true color RGB image, the red, green and blue channel data can be represented as 3 distinct 8-bit grayscale images.
But there's more to it than that.
On average, the green channel receives the most light energy and consequently enjoys the highest signal-to-noise ratio (SNR) among the RGB channels.
This is no accident. The solar power spectrum peaks at green (520 nm). To capture all that green energy, fully half the sensels in most RGB digital cameras are green. Since the retinal long wave cones feeding the the brightness channel in human vision are most sensitive at green through red wavelengths, you'd expect the green and red channels to carry most of the freight in B&W work, and in fact they do.
Luckily, green light suffers only a mild to moderate degree of atmospheric scattering. Green light is much closer to red than to blue in this regard.
The red channel is typically noisier than the green but cleaner than the blue. Of all the visible wavelengths, red is the least susceptible to atmospheric scattering. The red channel thus enjoys exceptional clarity, expecially on hazy days. Red light also contributes heavily to the visual brightness channel—just the visual channel B&W photos play to.
The blue channel tends to be pretty noisy, even in scanned color film photographs. Blue and violet are also the most scattered by the atmosphere. On a hazy day, the difference in clarity between the red and blue channels can be striking.
Luckily, blue light primarily stimulates the retina's short wave cones, which contribute little to the visual brightness channel. Little wonder then that the blue channel tends to take a backseat in B&W work. But don't dismiss it out of hand—it has important things to add now and then. CM allows you to give blue its due without wrecking your final product.
The table below gives an idea of what to expect from typical Sony CCD R, G and B channels and their closest color filter equivalents.
Table Reference: Among others, the Kodak Professional PhotoGuide, 6th ed., Silver Pixel Press, and Heliopan Filters for B&W Photography. Filter numbers are for Kodak Wratten gels or their equivalents.
Reading through the effects described on the Filterzone Black and White Contrast Control Filters page will give you a much more detailed idea of the effects various channel combinations will yield.
Practical CM immediately raises the issue of channel weighting. Your best bet is a WYSIWYG (what you see is what you get) editing method allowing immediate feedback on weighting choices. But with so many possibilities, an awareness of the channel attributes we've been discussing can profitably guide your efforts.
CM Implementations in PhotoShop and PHOTO-PAINT
For some time now, Adobe PhotoShop has included a Channel Mixer, a very handy and flexible CM implementation nicely documented in Michael Reichmann's PhotoShop Channel Mixer tutorial. Starting with RGB color camera output, the Channel Mixer allows you to weight the contribution each constituent color channel makes to your final grayscale image in a single convenient dialog.
Corel PHOTO-PAINT 9 (PP9) and earlier versions require manual CM, but all the pieces are there. In the next two sections, you'll find two different PP9 CM recipes, including
These recipes include a final sharpening step to emphasize the need to do that last.
I understand that PHOTO-PAINT 10 introduces a feature similar to the PhotoShop Channel Mixer. If you're more familiar with PhotoShop, the table below will help you translate pertinent terms from PP9 to PhotoShop jargon.
This simple but powerful PP9 method goes beyond the PhotoShop Channel Mixer, thanks to the Range checkboxes in the PP9 Color Balance dialog. This method provides the most direct and immediate WYSIWYG control of the mixing process.
That's pretty simple, even if PP9 doesn't have a dedicated channel mixer.
Acknowledgement: Thanks to Peter Clifton of the newsgroup corel.graphic_apps.photo-paint9 for this very slick technique.
Recombining split color channels as objects against a solid background allows you to adjust their contributions via their opacities. You can then apply the full power of PP9 to each channel.
As Gherry Bender of RPD notes, the sky's the limit from Step 6 on:
Advanced channel manipulations like these would take place in Steps 6-7 above.
Acknowledgement: Thanks to Gherry Bender of RPD and Michael Cervantes of the newsgroup corel.graphic_apps.photo-paint9 for this powerful PP9 technique.
Hanging around on RPD can make digital camera image quality seem like a high-wire act without a net—a frighteningly delicate balance of warring forces ready to take over at a moment's notice with disastrous results. To stay out of trouble, every detail of the process must be optimized to the Nth degree.
Then a highly talented but refreshingly relaxed photographer like Carl Schofield comes along and manages to take undeniable photographs with a reasoned but by no means compulsive approach to technique and a good but imperfect camera rapidly headed for obsolescence by popular standards.
I find that both liberating and motivating.
Carl's photographs certainly get my camera off the hook: My C-2020Z and his Nikon 950 are in the same league. And his aperture-based technique is familiar territory, so technical knowledge isn't the hold-up.
For me, the missing ingredient is what Ansel Adams called previsualization—the ability to visualize the desired photograph from the scene at hand. Previsualization is especially challenging when B&W is the goal. Previewing the scene with my digital camera set temporarily to B&W recording helps sometimes, but it's still very much a skill to be learned or blessed with. I'm counting on the former.
BTW, for tips on post-processing for color output, see the dpFWIW article Post-processing outdoor digital color photographs.
(See also the home page links.)
Digital B&W Galleries
Jerome Nevins'—beautiful B&W work from a Nikon CoolPix 990.
Black & White ArtZone—a rich photographic resource by Eliadis Elias "dedicated to black and white photography" and featuring many worthwhile galleries, tutorials and more. The Filterzone is particularly informative.
Digipub Resource Central—John Peters has collected and organized an impressive array of resources devoted to B&W photography, digital and otherwise.
Creating Grayscale From Color—an online tutorial directed at RGB scanner output but readily applicable to digital camera output. The discussion of RGB channel content is particularly good.
The Complete Color Manager—Linocolor's superb online treatise dealing with everything you've ever wanted to know about color, with important lessons for the B&W photographer, particularly with regard to the Lab color space.
Unless explicitly attributed to another contributor, all content on this site © Jeremy McCreary