Sunday, December 28, 2008

Canon G10 Exposure Compensation Matrix

I shot a few images with the G10 at a Christmas Eve / Birthday celebration with family, and decided that I needed to spend some time experimenting with the G10's exposure compensation capabilities.

Let me be dead clear here ... this is a back light test for fill flash. This is not a "soften shadows in full sun" test for fill flash. I'll have to figure out how to do that one another time.
In this experiment, I shoot the G10 at a Scooby Doo doll for my subject, with no front lighting at all. The background is an industrial-looking corner of my basement that is lit overhead by a compact fluorescent bulb equivalent to approximately 100 watts. Being a modern bulb, it has a color temperature similar to that of a tungsten bulb. Which allowed me to gel the flash with CTO Orange (a small piece taped over the flash) and thus balance the foreground and background light.

I further balanced color temperature by setting WB in ACR5 on the letters on the Glad box on which Scooby is leaning. The first surprise was how much the color balance changes as the flash compensation changed. From very cool to very warm ... which means that this flash follows a color curve from warm at the start to cool at the end (I think) ... squelching early gives a warm color to the subject's light, which when balanced in ACR cools the whole scene. The color balance is perfect from 0ev onwards, but -1 and -2 cast warm light.

I created a 5x5 matrix of these images with the rows representing changes in ambient exposure and the columns representing changes in flash exposure. The G10 responded very well to this and gave me accurate exposures in every cell, judging by the changes from left to right and top to bottom. The only unpredictable occurence was the previously discussed color temperature change. (Double click to see the full image, click back to return.)

It is fairly obvious from looking at the matrix that the 5 most interesting images are the top half of the middle column and the left half of the middle row along with the center cell. I.e. there are five possible best exposures here.

The classic advice for shooting people in a room with fill flash is to drop ambient by 1 or 2 stops and then shoot TTL with flash. So let's look first at these to potential images.

Ambient -2 Flash 0

Ambient -1 Flash 0

Interestingly, the light output is almost perfectly balanced at -2/0 which can be seen by the lack of any shadows on the box on the foreground. The ambient casts an identical light to the fill ... and there is no change in color temperature either, indicating a pretty close match to the CTO Orange gel on the flash.

On the other hand, this also means that the ambient cannot drown out the flash shadows cast onto the background, and they too have equal intensity to ambient shadows. This can by seen on the wall on the right, where the ambient shadows to the right of the 2x4 studs comes from the fluorescent bulb, while the shadows from the working table and some of the junk on it hit the insulation with equal intensity. So this exposure might not be appropriate in this case. We really don't want unnatural shadows (i.e. flash shadows.)

Things get a bit better in the -1/0 shot, where the flash shadows have lost one stop of intensity (i.e. are 1/2 as bright.) They are visible, but becoming irrelevant. There is a slight ambient shadow now on the box in the foreground, which is actually quite accurate since the bulb is plainly visible. So this exposure is better in all ways than the previous one.

Now what about the 0/0 shot?

Ambient 0, Flash 0

Ambient light is now dominating. Flash shadows are almost gone (but not quite.) This is the best exposure yet.

The last two exposures that could perhaps eclipse this one would be the -1 and -2 flash exposures at 0 ambient. Here they are:

Ambient 0 Flash -1

Ambient 0 Flash -2

Well, both exhibit an unpleasant color shift, which could of course be warmed somewhat in ACR. But that would warm the subject as well, which might be inaccurate ... i.e. the flash no longer balances well with the ambient when the flash is gel'ed. It may, however, balance better without the gel, since it is coming up overly warm.

Anyway, there are no flash shadows in 0/-2 and barely perceptible flash shadows in 0/-1 where ambient light falls. The 0/-1 image has a nicely exposed subject, so I think I would prefer it every time over the 0/-2 image. The question is whether it looks better than the 0/0 image?

Well ... the overall colors and balance of shadows looks best to me in 0/0 ... but you may choose differently. My conclusion is that the G10 can be shot without gels when compensating the flash, but should have gels on when compensating only the ambient. For shooting at a party with lots of lamps about the place, I'd shoot with a gel on and set to 0,0 and tungsten WB or 0/-1 without gel and also set to tungsten. You might need to rebalance in ACR or equivalent, but if you like warm images then the jpegs will be fine from the cam.

My other conclusion is that the G10 mercifully responds perfectly to ambient compensation in matrix metering mode, so there is little need to play with other metering modes in most shooting situations.

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