Wednesday, June 28, 2006

Pentax Digital White Balance Controls

In color film and video photography, white balance usually means gels and/or color filters to compensate temperatures, or a GretagMacBeth color chart test, to harmonize light sources with film stock. And while it's still appropriate in many cases to gel mixed light sources, DSLRS can do most of the filtering in camera to make an image warm, neutral, or cool just by adjusting the camera's white balance controls. When most color filters would require an additional stop of light added for exposure, the WB controls in the camera can save you the pain. On top of this, because the color space of JPEGs are inherently more limited than RAW, it becomes more important to choose the most appropriate captured spectrum from the start to avoid possible destructive loss from major shifts later, unless you're shooting RAW.

Manual White Balance kept me from losing the bear's proper color.

Granted, the "AWB" auto-white balance in the Pentax DSLR is pretty outstanding, but there are times when you might want to calibrate it yourself. For example, if you have a mix of light sources with varying temperatures, or a particular light is outside of the typical range of daylight, tungsten or fluorescent temperatures that the Pentax AWB checks (4000 to 8000K), you might find a color cast to your pictures that you'll have to clean up later. Or you may want a particular color cast (warm or cool) that the camera wouldn't automatically choose. However, your camera will let you calibrate quite easily if you take a moment to adjust the settings. Here are some ideas to make a change to the proper white balance as quickly as possible if the situation calls for doing it yourself.

1. Choose a temperature. The easiest way to adjust the color cast is to coordinate the scene's temperature with AWB or a preset. (If you're looking to match the exact scene temperature, jump ahead to the next section, Manual White Balance.) To get a warmer image, choose a WB preset temperature higher than the light source. To get a cooler palette, choose a lower temperature than the light source. Trying to guess the temperature means learning some average light wavelength values (here's a list of average temperatures, below), or you can shop around for a color light meter, like a Minolta Color Meter, which can read temperatures from 1600K to 40,000K.

Chart image from the Pentax *ist DL manual.

If you can tell the light source's temperature or estimate pretty closely, go to Fn>left command button and scroll through a list of alternatives to AWB: Daylight 5200K, Shade 8000K, Cloudy 6000K, Fluorescent (Daylight Colors 6500K, Daylight Neutral White 5000K, White Light 4200K), Tungsten 2850K, Flash 5400K, or White Balance Manual. For the *ist D, you'll chose the WB on the Mode dial and adjust the Av and Tv dials accordingly.

Left image: WB 4200K; Right image: WB 4800K

For example, to make a sunset appear warmer and more golden than it is, you'd ascertain that the available light is probably about 2800K to 4500K. Accordingly, you would set your WB to a higher value, such as Fluorescent Daylight, which is 6500K. The resulting shot will be much more saturated in warm hues.

Left image: WB 4800K; Right image: WB 6500K

2. Manual White Balance Measurement. If you're trying to neutralize a color cast precisely or need to choose a WB temperature that's not one of the presets, you'll have to manually make the adjustment. Some tools that come in handy are a gray card (it's not just for the zone system or exposure, it's great for WB), or an ExpoDisc (the ExpoCap is pretty cool, too).

First, if you have an *ist DS or DL, adjust the calibration settings to the most appropriate way for you to choose a portion of your shot to evaluate the right white balance. Under Custom Settings>Man. WB Measurement, you can choose either "Entire Screen" or "Spot Metering Area".

If you choose "Entire Screen", the camera will find the average temperature. This is the mode you'll want if you're going to be using a gray card that will fill most of the frame for your manual white balance adjustment calibration shot. And it's definitely the mode you want if you're using an ExpoDisc filter, a handy tool which diffuses the available light over the lens into an averaged temperature perfect for the WB measurement.

If you choose "Spot Metering Area" you'll be able to choose one of perhaps many different light sources to be the main arbiter of the WB. This would be the case if you have an area of tungsten light surrounded by daylight and you wish the WB be set for the tungsten light. Or if your pocket gray card isn't proportionally the majority of the viewfinder space.

Next, you'll go to Fn>White Balance (left arrow)>Adjust, or WB Mode>Adjust on the *ist D. The camera will prompt you to take a test shot that it will calibrate to as neutral a value as it can muster on its own. This is where you take a shot of your gray card or ExpoDisc or the spot that is the temperature you're looking to neutralize. It's best not to shoot anything too dark, or with high contrast or glare, so be careful. If you're using a gray card, this test shot doesn't need to be perfectly exposed to 12% or 18% gray; it's just adjusting the temperature of the color cast on the gray. But if you're going to use a gray card and are a zone fanatic, you could use this as the opportunity to kill two birds with one stone. The *ist D will let you save up to 3 custom settings.

Alternatively to choosing a preset with a warmer or cooler balance, you can cheat with a bit of colored construction paper or colored filter to trick the MWB into choosing a value lower or higher than the available light. Keep in mind that unless the paper is a light pastel, you're going to end up pretty wide off the mark.

Presently, there's no direct command to fine-tune the WB manually with +/- incremental adjustments, but because this is a feature of high-end pro DSLRs at Canon and Nikon, hopefully it will find its way into Pentax technology with future cameras (fingers crossed!).

Links: Wikipedia Color Temperature, Olympus Color Temperature Chart, Luminous-Landscape Photo Color Theory, Robin Myers Digital Gray Card, WhiBal Reference Gray Cards, ExpoDisc, Ron Bigalow's White Balance Article, Ken Rockwell on White Balance (Nikon, but a good read), CambridgeColour: Understanding White Balance, "If it's called White Balance, why do people use Gray Cards?", GretagMacbeth ColorChecker Charts, Kodak Gray Card Plus, Lastolite EZ Balance Collapsible Gray Card, Strobist on lighting gels

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