G9: Exposure and Flash Compensation in Tv Mode

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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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Swiss Army Knives at Work and Play

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Swiss Army Kni...

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By Gordon S. Buck

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A new book is on my coffee table: Swiss Army Knives at Work and Play. Of course, it's my own book, published at Blurb. So far, it is a, uh, very limited edition. The book has been entered in the 2009 version of Blurb's book contest.
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G9: Exposure and Flash Compensation in Av Mode

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The previous post showed the effects of combinations of exposure and flash compensation on a typical indoor scene when the G9 was using its built-in flash and operating in Program mode. This post shows the results of those same combinations when the G9 is operating in Aperture Priority (Av) mode. For these photos, I selected f5.6 to get away from the largest aperture (f2.8) that was often the automated selection in Program mode.

In Av mode, the photographer manually sets the aperture and the G9 selects the shutter speed to give the “correct” exposure (according to the G9). Simple enough, but there are a few peculiarities and precautions. The G9’s choice for shutter speed is based on ambient light; however, similar to Program mode, Av mode will lock into 1/60 second as the slowest shutter speed if Slow Sync is turned OFF in the menu. Because the photographer has already locked the aperture, this means that exposure compensation is actually rendered inactive when the flash is turned on if the G9 is in Av mode with Slow Sync OFF. Figure 3 (continuing the numbering from the previous post) illustrates this limitation.

Figure 3

Although nine photos were taken, there are really only three different photos in Figure 3 because there actually was no exposure compensation – only flash compensation.

With Slow Sync ON, there are nine different photos resulting from this bracketing as shown in Figure 4 below.

Figure 4

Again it is obvious from these two examples that the Exposure Compensation and Flash Compensation values are not intended to summed to provide a basis for comparison. In fact, sometimes Exposure Compensation doesn’t actually do anything!

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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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Neighborhood Fireworks

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In many locations fireworks are permitted in suburban neighborhoods. This shot was taken with my tripod mounted G9, ISO 80, manual exposure set for 2 seconds at f4.5. Off camera flash was triggered by a Cactus wireless trigger to a snooted Nikon SB-28. Shot in RAW mode, converted using ACR, cropped significantly (hey, it was nighttime!)

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Kodachrome R.I.P.

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On my mind, but not exactly a news item today, on June 22, Eastman Kodak announced the end of Kodachrome film. Of course, that meant that I had to have some Kodachrome, shoot it and get it processed while the last remaining Kodachrome processor in the world (Dwayne’s) is still processing it.

Although I haven’t shoot any slide film in about two years, I decided to shoot two rolls of Kodachrome just for old times sake. My old Konica T2 seems to be the right camera for this exercise. I checked out the T2 yesterday and it still works fine although I’m not so sure about the light meter. Seems that many others must have the same idea because I had problems getting the Kodachrome – even Dwayne’s was out of stock. After a day’s delay, B&H replenished their stock and I got my two rolls of Kodachrome (B&H is out again).

I was first introduced to Kodachrome II in 1970. I liked Kodachrome but (as I recall) it was more expensive than Ektachrome, took longer to process (mail order) and was only ISO 25. At the time Kodachrome was special to me, and to this day, Kodachrome is still special. I wish I’d shot more Kodachrome because those old Ektachrome slides are faded whereas the Kodachrome slides are fine.





This scene above is from my last previous use of Kodachrome about four years ago. It really was a reddish sky and Kodachrome exaggerated the effect. No doubt many would consider this picture to be an overdone Photoshop job but the slide looks the same.

So I have something in common with Steve McCurry. He will shoot one of the last rolls of Kodachrome and donate the slides to the George Eastman House. I will shoot two rolls and keep the slides at the Gordon Buck House.

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