Tuesday, July 7, 2009

G9: Exposure and Flash Compensation

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Although I’m trying to learn more about flash photography (see many postings!), strangely enough I don’t use the built-in flash of my G9 very often. As a result, I don’t know much about using the Exposure Compensation and Flash Compensation for the G9. Following some discussions on DPReview, I undertook a few simple tests to learn more about flash with my G9. As others have discovered, Canon flash does not necessary act as you might first expect.

First I set up a simple indoor scene with a little shaded window light and overhead tungsten bulb. I placed my G9 on a tripod for repeatability. The G9 was set for ISO 400, Program mode and the Slow Synchronization for flash was turned OFF. The G9 was set for the widest angle (7.4mm focal length) which means that the maximum aperture is f2.8. According to the G9, the correct exposure in the room without flash was 1/20 second at f2.8. I turned the flash ON. With Slow Sync OFF and flash ON, the G9 shutter speed will not go below 1/60 second. In fact, the shutter speed appears to lock on 1/60 second. Then I took a bunch of pictures, all in JPEG with auto white balance, ‘bracketing’ at Exposure Compensation of -2/+2 and Flash Compensation of -2/+2. Those pictures are combined below in Figure 1.




In Figure 1, the Exposure Compensation and exposure values are -2, 0, and +2 from left to right; the Flash Compensation is -2, 0 and +2 from top to bottom. The top row is without flash. With one glance, it appears that, under these conditions, Exposure Compensation did nothing with the flash was turned ON because the flash pictures do not vary from left to right. The difference in the pictures is due to Flash Compensation.

Also illustrated by Figure 1, is that there is no point in summing the Exposure Compensation and Flash Compensation – the sum is meaningless. For example, the picture at Exposure Compensation of -2 and Flash Compensation of +2 has a compensation sum of 0 but obviously not the same exposure as the picture taken at 0 Exposure Compensation and 0 Flash Compensation. Therefore, even though it is tempting to think in terms like “I knocked the exposure down by -1 so I’ll increase the flash by +1” the results may not be as you expect!

Another fact revealed by Figure 1 is that the flash does not automatically compensate for intention variations in exposure. As shown in the FC 0 row, when Exposure Compensation is used the flash does not make up the necessary difference.

Again, Figure 1 is comprised of pictures using Slow Sync OFF which is probably the way that most users have their cameras configured. But what if Slow Sync was ON?




Figure 2 shows the results using Slow Sync ON. Each row of pictures was taken at the same exposure and that exposure was determined by the G9 metering system based on the Exposure Compensation that I dialed in. Notice that the shutter speed has been freed from the 1/60 second although was at 1/60 for Exposure Compensation of -2; however shutter speed is a bit slow for Exposure Compensations of 0 and +2.

The first conclusion from examining Figure 2 is that every picture is different (although some are nearly the same). Again, there is no point in ‘summing’ the compensations.

As shown in Figure 2, the exposure settings determined by the internal G9 logic are not affected by the Flash Compensation settings. Likewise the flash does not automatically provide more of less light to account for Exposure Compensation.

My conclusion for this typical setting and G9 configuration is that Exposure Compensation and Flash Compensation are independent, manual adjustments to the internally determined ‘correct’ exposure and flash power.

But what about Av mode? (later)
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1 comment:

MV said...

Hi,

It's difficult to see figure 1. Can you do something so that it pops up and is larger so that it's easier to see?

Thanks for the great tips!