Musings on Photography

Black and White

Posted in digital printing, equipment, hardware by Paul Butzi on July 17, 2007

In this post on superstition and the color accuracy of laptop screens, Rosie Perera commented

With all this talk about color accuracy, it seems poetic justice that the photo you posted today was black & white.

Rosie’s razzing me, here, but her comment reminds me of an interesting point which I think is often either misunderstood or else overlooked.

We tend to think of black and white photos as just that – black in the dark bits, white in the light bits, and a mixture of black and white in varying proportions for the bits in between.  It’s a reasonable thought – when we shoot with black and white film, the film essentially records luminance, and all of the color information (what would fall in the a and b channels in Lab space) is just thrown away.  At that point, we really have produced a black and white photo – the color gamut of the image is essentially nothing more than a straight line between Dmin and Dmax – no volume to the gamut at all.

The problem occurs when we want to turn this black and white photo into a print, either one that we can hold in our hands, or one that is displayed on the screen.  When we do this, we’re taking this image that has an incredibly narrow gamut (just a line, remember?) and we’re going to translate the image into the color space of that display medium.  And when we go to do this, we get some rather big and unpleasant surprises.

The first surprise is that, in the color space of the display device (the screen, or the printer, etc.) our ability to represent colors that lie on the L axis of the LAB color space is not as good as we naively expected.  We’re up against two problems, here.  The first is that the color accuracy of the device is not perfect, so when we specify ‘give me a perfectly neutral 18% grey’, what we get instead is the best job the device can do at a perfectly neutral 18% grey, which only in the rarest of cases is going to be exactly a perfectly neutral 18% grey.  It’ll be just barely off the axis.  Sad, but true.

The second, and closely related suprise is this: the human visual system is incredibly, fantastically good at detecting very slight variations from neutral.  You can take a perfectly neutral grey, add an homeopathically small quantity of blue to it, show it to another human, and that human will tell you that the grey is slightly cold.  The upshot is that when we take our conceptually perfectly neutral black and white image, and we print it on our inkjet printer (or display it on our screen) what we are expecting to see is a range of perfectly neutral greys ranging from Dmax to Dmin.  What we see instead is a range of color, ranging from the darkest the display device can produce up to the ‘base’ color, and in between we have a range of colors which are tantalizingly, frustratingly, achingly close to neutral but which our amazing vision tells us are ‘blue’ or ‘magenta’ or ‘yellow’ or ‘green’.  Needless to say, this is not the desired effect.

The upshot is that one of the most demanding tests of color accuracy is to use a device to display a perfectly neutral black and white image.  The human visual system will ruthlessly expose every inaccuracy, every spot on the image where some shade of grey is not perfectly neutral.  An image like the one above, with the greyscale stretched out considerably, is a particularly difficult challenge.

You can spend quite a lot of time getting your device tuned up to the point where it passes the ‘very long perfectly neutral gradient doesn’t show any color shifts’ test.  It takes a really good profile, a device with no drift over time, and attention to all the color management settings. 

The ultimate irony is that you didn’t really want a perfectly neutral ‘black and white’ print anyway, because they look peculiarly dead and lifeless.  Perhaps in another post I’ll tackle why.

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