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Basics of Flash Photography

Nikon Auto White Balance - Flash Color Information Communication

All flash tubes vary color temperature with power level in the tube. Speedlight color changes with power level, more red at full power, and more blue at low power. Because, the flash pulse starts out intense and blue hot, and then trails off slowly to cooler red temperatures. At high power, we see it all, and it averages out white (near Daylight). But at low power, a speedlight simply chops off the trailing red part, leaving the blue part. Studio lights vary color with power level too, all flashes do, but almost all mono lights instead use lower voltage for lower power, and shift oppositely, becoming more red at low power.

The White Balance named Flash is a constant value, a generic correction, regardless of the actual color of the flash. I've always thought that when it was obvious that it was flash or daylight, whatever, why wouldn't I just say the obvious, instead of letting the camera automation try to guess at it? But even that is never precise, we never know exactly what the light is, so the easiest way is to just fix it later in RAW. But another option is that for system speedlights on the hot shoe, Auto White Balance becomes something very different than normal. The Nikon Flash Color Information Communication kicks in (a feature of CLS), and reports to the camera (via hot shoe pins) the probable color temperature corresponding to the actual flash power level used. Canon has this too. This is not a measurement, instead the speedlight firmware has a chart of what the temperature ought to be corresponding to each power level. If the camera is in Auto WB mode (allowing WB to be changed), this can be used, and Auto WB becomes quite different than regular Auto WB, and with hot shoe flash, simply uses this reported temperature. This is in contrast to Flash White Balance, which is always the specified constant no matter what the power level was (The camera cannot change white balance if we have specified Flash WB).


Each row of pictures below is at a different flash power level, from full power at f/32 to 1/128 power at f/2.8. All else was the same, I just spun the aperture wheel for TTL. No compensation or adjustment of exposures (except the distance was adjusted to match the one first full power f/32 picture). Nikon D300 and SB-800 speedlight on hot shoe (for temperature communication in Auto WB). Center Weighted TTL metering, and zero flash compensation. ISO 200, 1/125 second shutter, direct hot shoe flash at 6.5 feet, with 60mm lens.

These were shot as JPG (just for this, so that Adobe Raw software does absolutely nothing at its "As Shot" default). All are also cropped very substantially, and resampled much smaller, but uncorrected - except the one last column - the adjacent TTL Auto WB - was also shown White Card corrected (then the color dropper tool reports this white has equal RGB components, to be neutral color). Of course, every other frame is supposedly correct too, but only the last column was actually corrected. The amount of correction that was reported applied by Adobe Camera Raw software to do this is also shown (this is a relative number for JPG, temperature degrees K is unknown since JPG WB was already done in the camera). A more negative temperature number here is moving towards more blue (less red), less negative is away from blue. Maximum scale is + or - 100.

The white card shown is the Porta Brace White Balance Card. 5x7 inches, plastic, durable, washable, inexpensive, accurate, and it is all we need. You click the card in the test image to tell the White Balance tool "This spot is white - Make it be white". Presto! Whatever color cast there is removed from the entire image area. The same color adjustment will also work in all similar pictures in the same lighting situation (if you have software to apply it simultaneously).

Frankly, I always shoot Raw instead of worrying with it in the camera, but after paying more attention to this, if for JPG, and if white balance is not going to be corrected, I think I prefer Flash White Balance to Auto. My flash is seven years old, but it still seems fine to me. Nikon recommends letting them check flashes every couple of years, about the flash capacitor and flash tube I imagine. Rationalizing, I have two SB-800, and they always seem identical. And the truth is, no matter what you do about White Balance, it will need a little tweaking anyway.


Power   Manual, Flash WBManual, Auto WBTTL BL, Flash WBTTL BL, Auto WBTTL, Flash WBTTL, Auto WBWhite Card CorrectedACR Correction

Full
f/32

Temp -16
Tint -10

1/2
f/22

Temp -11
Tint 0

1/4
f/16

Temp -13
Tint -5

1/8
f/11

Temp -9
Tint -5

1/16
f/8

Temp -5
Tint +1

1/32
f/5.6

Temp -5
Tint 0

1/64
f/4

Temp -4
Tint -3

1/128
f/2.8

Temp -3
Tint +1

Full
f/32
again

Temp -16
Tint -10


Copyright © 2012 by Wayne Fulton - All rights are reserved.

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