by Lee Varis
Most photographers who print their work encounter this sooner or later. It often rears its ugly head in clear sky gradients – the dreaded banding, posterized tonal gradients that break into discrete bands that destroy the smooth appearance of the sky. This is a "digital" artifact that is mostly blamed on 8 bit files! The fact is that banding in a print can often result even with high-bit depth files during the conversion to the printer profile for output. The problem is hard to predict or pre visualize and this can result in wasting expensive paper to discover that you have to "fix" something in the file.
The following image demonstrates the nature of the problem.
It looks smooth doesn't it... but, if we look at the individual channels maybe we can spot the problem...
The green and blue channels don't really show anything but here in the red channel we can just barely see that there might be an issue. It's subtle though so we can't really be certain that there will be a problem. The issue of banding in skies, or any smooth gradient for that matter, has been around as long as digital imaging has existed and there have been numerous attempts to solve the problem. Back in the day, when real high-end imaging was only possible using Scitex and Quantel Paintbox systems the solution was more or less the same as it is today - one has to add noise in some fashion or another to break up the bands. Outputting a file only to discover that there were bands was quite expensive so many shops resorted to adding noise as a standard procedure before outputing anything. However, adding noise often resulted in a gritty appearance and if it wasn't necessary it wasn't desireable.
One of the original Quantel Paintbox engineers, Ed Manning, invented a technique to pre visualize the bands and old timers like myself will still refer to this as "Ed's Curves" – Now its mostly referred to as "solar curves." This technique is still useful as part of our strategy to eliminate bands. Begin by duplicating the background to a new layer...
To setup "Eds Curves," make a new Curves Adjustment Layer at the top of the layer stack and, once you are in the Adjustments Panel, place multiple points on the Curve...
Now, pull the points up and down so that you end up with an extreme sine wave sort of thing like this:
The result puts all the tone transitions on a mostly vertical segment of the curve so we have a lot of contrast between tones – we also have a fairly psychedelic image...
Despite the rainbow color the image shows very obvious sharp ridges running through the sky. We can leave this temporary Curves adjustment on to help visualize just how much noise we need to eliminate the ridges. Select the duplicate layer and run the noise filter: Filter->Noise-> Add Noise...
The idea is to use enough noise to completely hide or obscure the ridges. This is the traditional approach that most prepress professionals use. The problem with this approach is that often quite a bit of noise is necessary and it can lend the image a harsh look...
Sometimes this will not look as bad in a print but there is a better approach. Instead of using the standard noise filter, use: Filter->Brush Strokes-> Spatter...
The large filter dialog allows you to select multiple artistic filters intended for creating painterly effects.
For our purposes, we want to have a high "Spray Radius" and a Smoothness setting of "1"
This filter is much more effective in smoothing out bands in a gradient than simply adding noise. The only trick is in masking off the dark "spatter" of the non-sky elements at the horizon. For that we can turn to the Blending options dialog...
Setup the "Blend If" sliders for the Blue channel as shown above - the idea is to blend through the dark, non-sky tones to reveal the "un-spattered" image in the Background. Sometimes you can get away with only using the slider in the top layer – here I've used both to get a cleaner image. Often you'll have to do a little bit of masking for final cleanup – add a layer mask to the "Spatter" layer and mask out the dark speckles with black.
The final result is smooth with less obvious noise...
Compare this with the original and with the noise version! Spatter breaks up the bands with diffusion instead of adding light and dark noise so there is no grittiness and no bands. At this point you can throw away the Curves Adjustment layer and print with full confidence that you have vanquished the dreaded bands forever!
Remember "Ed's Curves" and use them whenever you have the slightest suspicion that banding may be present and you can clearly visualize the 'bands" before they bite you in the butt...
Learn more incredible digital imaging techniques from Lee Varis at the California Photo Festival, October 12-16, 2011! Lee will be teaching all 5 days during the festival, discussing topics like The Digital Zone System, Mastering Exposure, High Speed Camera Techniques and more!
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