Wednesday, April 2, 2008

Sky Too Bright?

Bright sky and dark foreground are not a happy combination for photographers unless both can be captured. This difficulty is not unique to digital photography. Although sometimes the available dynamic range of the digital camera (or film) is inadequate, often the light meter is simply fooled. The three photos above illustrate this problem. All photos were taken within a short time (but the clouds were moving quickly) with my G9 mounted on a tripod, evaluative metering, Av mode at f5.6, ISO 80 and shooting raw + jpg. The photos above were downsized from the in-camera jpg and sharpened for web viewing (PK Sharpener ) but are otherwise unprocessed.

For the bright sky problem, the hoped-for-solution is the “correct” exposure. Think about a bright sky scene from the point of view of the exposure meter. If there is only a little sky, then the exposure is based on the ground – 1/160 sec in Photo #1 above. If the composition is mostly sky, the exposure meter thinks there is a lot of available light – 1/640 sec in Photo #2. As the camera operator, uh, photographer, you might manually set an in-between exposure value. Some metering systems attempt to do this automatically; Photo #3 was taken at 1/320 sec as determined automatically by the G9. So Photo #1 has good ground but no sky detail; Photo #2 has a beautiful sky but dark ground; Photo #3 is sort of a blah in-between.

While taking photos 1, 2 and 3 above, the live histogram of the G9 indicated that Photo 1 was overexposed –a little more than a full f-stop. Photo 2 was slightly underexposed – about a half stop. Photo 3 is slightly overexposed – perhaps a half stop. So the “best” exposure would have been about f5.6 and 1/400 sec manual exposure (not very far off Sunny 16 for the G9) or by dialing in a -2/3 exposure compensation for Av mode. I didn’t get a shot at f5.6 and 1/400; however, I was shooting with +/- 1 auto exposure bracketing so I’m pretty sure of those relative effects.

I was interested in comparing the in-camera jpgs for normal, graduated neutral density filter and a polarizing filter; these photos are shown below (Photo 3 is repeated). Again, all photos were taken in Av mode using evaluative metering without exposure compensation.

In Photo #4, the 50/50 split, graduated neutral density filter (Tiffen ND 0.6) performed as expected but the upper clouds were darkened excessively. At 1/250 sec, it is slightly underexposed – perhaps 1/3 stop. Photo #5 shows the effects of a polarizing filter (Tiffen Circular). The filter was rotated for maximum sky detail. Unlike my G3, the effects of rotating the polarizing filter could be seen in the G9 LCD display. At 1/160 sec, the polarized exposure is just about right according to the histogram.

Nest is a comparison of raw and jpg; shooting in raw + jpg mode allowed a direct comparison. The third set of photos (below) compares Photo 3 (the in-camera jpg from the first and second sets) to the processed raw version, Photo 6. Similarly, Photo 7 is the raw version of Photo 5.

Both raw files (6 and 7) were processed in Adobe Camera Raw using the same settings. Surprisingly, no exposure adjustment was needed -- even though I’d previously noted that Photo 3 was slightly overexposed. The “Recovery” adjustment compensated for this highlight recovery. The ACR settings for Photo 6 and 7 were:

WB: Daylight
Exposure adjustment: 0
Recovery: 15
Fill: 10
Black: 8
Brightness: 40
Contrast: 0
Clarity: 40
Vibrance: 8
Saturation: 3
A “Medium Contrast Curve”
Sharpening: 80
Radius: 0.8
Detail: 30
Masking: 0
Luminance Noise Reduction: 0
Color Noise Reduction: 25

All other ACR settings were at zero. After opening in ACR, “Smart Sharpening” at 100 was applied in Photoshop CS3. No other post processing was applied.

In retrospect, I should have applied a bit more saturation to Photos 6 and 7 just for purposes of comparison to Photo 3 because Canon’s in-camera jpgs are a bit saturated (some say over-saturated).

Lessons learned? Well, I’ll continue to shoot in raw and will be using my polarizing filter more often.

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