Sunday, July 26, 2009

G9: Exposure and Flash Compensation in Tv Mode

The previous posts showed the effects of combinations of exposure and flash compensation on a typical indoor scene with the G9 using its built-in flash and operating in Program (P) mode and in Aperture Priority (Av) mode. This post is similar but is about shutter speed (Tv) mode and flash. I selected a shutter speed of 1/30 second simply because so many other photos were taken at 1/60 second. The setting and lighting was similar to the previous posts; however, these pictures were taken on a different day.

In Tv mode, the photographer manually sets the shutter speed and the G9 selects the aperture to give the “correct” exposure (according to the G9). Simple enough, but there are a few peculiarities and precautions. In Tv mode, because the shutter speed is selected by the photographer, there is no “Slow Sync” and the G9’s choice for aperture is based on ambient light. However, in low light situations, if the flash is turned on and the aperture is at maximum then the ambient exposure is essentially “locked”. This means that exposure compensation is actually rendered inactive when the flash is turned on if the G9 is in Tv mode in low light situations. Figure 5 (continuing the numbering from the previous post) illustrates this limitation at the 0 and +2 exposure compensation settings.

A peculiarity is that, with the G9 in Tv mode, if the flash is not turned on, and if “Safety Shift” is turned on (menu selection) then the shutter speed selection might be changed if the aperture cannot be further increased. This is shown in Figure 5 by the middle picture in the top row (no flash, 0 exposure compensation). The shutter speed that I selected was actually 1/30 second but the G9 wanted an aperture larger than f2.8. Since that larger aperture was not available, the G9 safety shifted to 1/20 second. (OK, so I forgot Safety Shift was turned on! I usually shoot in Av mode and find Safety Shift useful in that mode.)

Figure 5

Because shutter speed and aperture were the same for both 0 and +2 exposure compensation, those pictures using flash are also the same. Now, take a close look at the picture based on FC=0 and EC = -2. That picture is only slightly darker than the FC=0 and EC=0 picture. It almost looks like the flash is compensating for negative exposure compensation but I’ve been saying that flash compensation and exposure compensation are independent. What’s going on here?

Because electronic flash (yes, there are other technologies) has such a short duration – about 1/1000 second give or take – the light detected by the sensor/film is relatively independent of shutter speed. It is the aperture that governs exposure from flash. For this series of photos, the shutter speed was set at 1/30 second. To achieve exposure compensation, the G9 changed the aperture. For example, an exposure compensation of -2 was obtained by changing the aperture from f2.8 to f4.5 (and shutter speed from 1/20 Safety Shifted to 1/30). The G9, realizing that the aperture was f4.5 and not f2.8, fired the flash more powerfully. I have to admit, begrudgingly, that the G9 somewhat “compensated” for exposure compensation in this case; however, I really maintain that the G9 flash power was actually based on aperture and not exposure as the combination of shutter speed and aperture. In any case, certainly FC=0 and EC= -2 (sum = -2) does not produce the same result as FC= -2 and EC=0 (sum = -2).

After these experiments with exposure compensation and flash compensation in P, Av and Tv modes, it is easier to understand how G9ers (and G10ers – probably all Powershot users) can become confused about flash photography. For starters, there are definite limits on available shutter speeds, apertures and even flash power. There is a Safety Shift in both Tv and Av modes but Safety Shift does not work when the flash is turned on. For P and Av modes, there is a Slow Sync option. Flash exposure is largely based on aperture and not shutter speed. Flash and exposure compensations are truly adjustments based on the normal logic of the G9. Finally, even though somewhat counter intuitive, flash compensation and exposure compensation are independent.

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