Thursday, June 22, 2006

Reducing Purple Fringing and Sensor Blooms

A problem endemic to most digital cameras, even DSLRs, is the risk of "sensor bloom" in high contrast exposures with light that is too dynamically strong for the CCD. Unlike lens flares, which occur with direct light and your lens, sensor bloom can happen even with diffused and indirect skylight if it's overexposed and your CCD's individual pixel sensors get flooded and spill the light. If it were film, you would have had a slightly more elegant burn out of all the detail, but with your sensor you might get the dreaded "purple fringe" artifacts, too. But don't go blaming the technology entirely. Right now it's a risk you can easily learn to manage like a pro.

Purple fringing in high contrast areas of photos, particularly where light silouettes an object, shouldn't be confused with chromatic aberration that you can blame on your lenses. Even a brilliant lens like the Pentax FA 77 Limited will allow sensor bloom and purple fringing. Unfortunately, it's a problem with the limited range of sensitivity in most entry level consumer and mid-range professional CCD technologies. When "sensor blooming" spills light from overexposed pixels onto neighboring pixels, the microlenses that help direct the photons over the sensitive diodes for each active pixel sensor also make a refraction error with the color filter array, causing saturated purple (the typical wavelength of the spilled light) along the high contrast border. Even if the right detail and color are ultimately preserved in the light area, a ghostly purple halo can cover the darker area in the contrast zone.

This is a pixel resolution detail of the some of the worst sensor bloom and purple fringing possible, with overexposed skylight overwhelming the detail for the CCD's color filter array. You can tell the difference from CA by the purple hue, as opposed to blue or magenta.

Needless to day, it's annoying and you'll likely end up cleaning it in post by painting a mask of de-saturated color over the fringe. Is there a way to be more careful the first time around? The same skills that avoid sensor blooming can help you reduce the chance of purple fringing.

1. Use "Bright Portion" and "Histogram" in your review mode to recognize high contrast, detailed silhouetting with bright mid-day sunlight or direct light sources that create whie zones in your picture. If you review an image on your Pentax *ist or digital K's LCD with "Bright Portion" enabled, and you see the fine detail is surrounded with hotspots, you're risking purple fringing. Same goes if your histogram shows your levels are clipped at the right side (aka. the bright portion).

2. Stop down your aperture a little. If you're wide open, typically you're going to be letting in that much more light and have more contrast than if you stopped down a bit. Reducing the contrast by closing to about F/8 or F/11 can make the difference that will avoid purple fringing in most situations.

3. Use a short focal length. If you focus closer and blur out the background with bokeh, you'll diffuse the light and its contrasts. Similarly, going wide is going to limit the surface area on the CCD you can potentially burn out with high contrast areas, much less than if you're zooming telephoto into a hotspot. Better yet, the concentration of light is more diffuse and the microlenses will cope better.

4. Underexpose. Sure, it's practically bracketing, and it will require you to bring the exposure back up in post, where you'll probably incur some grain when you lighten up the dark levels, but at least your file has detail and isn't burned out with a purple fringe.

5. Keep the camera's processing from making it worse. If you're shooting in a JPEG mode, reduce the contrast setting a bit. Use Natural Color mode, not Vivid Color (used in the scene modes). When a DSLR like the *ist DL or K100D processes your data, you don't want it accentuating the problem by increasing the contrast and then over-saturating the purple fringe, too.

6. Shoot Raw. If you're shooting RAW, you're more likely to be able to refine the contrast and saturation to minimize the effects of sensor bloom and purple fringing, allowing you to rescue a shot without too much work in post.

Aahhh, that's a little better. Alerted by Bright Portion in review, stopped down to F/14 with a corresponding slower shutter speed to maintain the same metering, with no EV compensation.

Links: Chromatic Aberration & Purple Fringing in DP Review Gloassary


Anonymous said...

Had the CCD of my *ist D replaced and just tried it with a newly arrived 2nd-hand lens. Awful bloom very visible when zoomed in on the LCD. Tried another lens and also my DS, but will do the job properly with a tripod tomorrow morning before contacting Pentax about it.

OK-1K said...

CMOS sensors, like those in Canon DSLRs, are somewhat more immune to blooming because they have controlling gates for the light spill. Until Pentax tries another sensor like the CMOS or Foveon, blooming will always happen to some extent in harsh, direct light.

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