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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G9: Aperture variations with zoom

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The aperture in a camera’s lens is simply the opening through which light passes to strike the sensor or film. Instead of quantifying aperture size directly, for example, speaking of an 8mm opening, we usually speak of the “f-number” or “f-stop”. The f-stop is the relative aperture in comparison to the focal length of the lens; that is, the f-stop is the focal length of the lens divided by the diameter of the aperture. For example, if the focal length is 100mm and the aperture is 25mm diameter then the f-stop is 100 / 25 = 4. Since most people use aperture and f-stop interchangeably, we’d probably say that the aperture was “f4” in that example. For more details, check Wikipedia.

What about f-stops for zoom lenses? If the physical opening remains constant but the focal length changes, then the f-stop changes with the zoom setting. This is the situation with the Canon G9 and most digital cameras having built-in zoom lenses. By tinkering with my G9, I found the zoom points for which the maximum aperture (f-stop) changes and constructed the table below.




The table shows, for example, that the maximum aperture for the wide angle (7.4mm focal length) is f2.8 but near mid-zoom, say 25mm focal length, the maximum aperture is f4. At maximum zoom (44.4mm focal length) the maximum aperture is f4.8.

Notice that, contrary to the simple definition, the maximum f-stop for the G9 is not exactly proportional to the focal length. This apparent contradiction is an indication that the available physical aperture is also changing. If this were not the case, then the f-stop at 44.4mm focal length would be f16.8 instead of f4.8 !

Try this little experiment. Put the G9 (probably any camera) at maximum wide angle, Aperture priority (Av mode) at f8. Oh – you might want to turn off the focus assist light! Point the camera towards yourself and look deep into the lens. Push the shutter button halfway (you don’t have to actually take a picture) and watch carefully for a little bit of movement deep in the assembly. You’ll see a small circular opening become even smaller. Now change the f-stop to f2.8 and try again. You’ll see that the small opening doesn’t change. Now zoom to about mid-telephoto and repeat the tests. Notice that the initial size of the circular opening (the physical aperture) is somewhat larger at mid-telephoto. Zoom all the way in and repeat. Although it becomes more difficult to see deep into the lens, the initial opening becomes even larger at maximum telephoto settings and it changes from a large opening to a smaller one at f8.

Finally, remember that, counter-intuitively, a larger f-number (f-stop) actually means a physically smaller opening (aperture) that lets in less light.

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G9: The Sweet Aperture

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Many devices have a “sweet spot”; that is, a place, operational mode or setting where the device works best. Sports provide some of the best examples: baseball bats, tennis racquets and golf clubs all have a sweet spot (even if I could rarely find it!). Machines are the same way. Long ago I learned that a automobile engine runs best under conditions corresponding to a highway speed of something like 45 miles per hour. Wonder if that is still true? Centrifugal pumps certainly have a sweet spot; it’s even called the Best Efficiency Point (BEP). Generally speaking, when additional information is not available, I assume that the sweet spot of any device is near the middle of its operating range. The end points of an operating range are typically compromises in some way or other.

The G9 aperture can be varied between f2.8 and f8. The sweet spot in this range is somewhere between f4 and f5. After a few tests, I’d say that the sweet aperture of my own G9 is about f4.5.

A simple test was set up as shown here with a ruler on a table in window light. The G9 was on a tripod, in Manual exposure mode, shooting raw + jpg at ISO 80 and set for the 2 second shutter delay to avoid camera shake. The coins were placed along the ruler for reference points. After determining the correct exposure, pictures were taken at various apertures (changing shutter speed to keep correct exposure). The focus was obtained automatically at the penny but I was concerned about the effects of focus so all tests were repeated using focus bracketing; however, this turned out to be unnecessary.

Here is a composite of the crops from the in-camera jpgs. Several things are going on here and all at the same time. The exposures vary a little bit and the outdoor window light also varied somewhat. As expected, depth of field increased as the aperture decreased (f-number increased). Some sort of in-camera sharpening, contrast, etc is being done as well. All these images look reasonable and about the same at first glance. Now click on the image to get the 100% crop view. Take a close look at the scale markings of the f4.5 variation at the penny. The scale graduations are in 1/64 inch and every graduation is visible. Two inches closer to the camera, near the nickel, the f5 version shows more detail and the f4 version shows the least detail. This is the effect of depth of field. The same effect should be seen near the dime at the 8 inch mark – the most detail in the f5 shot. But wait! The f5 shot is worse than the f4.5 shot. Notice that details of the graduations are visible out to about 7-32/64 on the f4.5 shot but only about 16/64 on the f4 and f5 shot. What is going on here?

Remember, I was shooting raw + jpg but taking the easy way out and using the in-camera jpg to draw conclusions. Time to examine the raw files. The raw files led to exactly the same conclusion: f4.5 is the sweet aperture for the G9. The raw files also revealed much more detail than the in-camera jpgs. In this composite (click for an enlargement), the f4.5 crop from raw (middle) reveals detail out to about the 8-8/64 inch mark. But notice that the f8 crop from raw (right) shows even less detail than the f4.5 crop from the in-camera jpg. In fact, the f8 crop from raw is noticably soft along the entire length. These are unexpected results except for the concept of the “sweet spot”.

What is the technical explanation for this sweet spot near f4.5 for the G9? The answer to this question appears to be related to the diffraction limit. I’m certainly no expert in diffraction but here are some handy links for a more detailed explanation and even some calculations.

Wikipedia has a detailed explanation of diffraction.

There is a good explanation of diffraction in digital photography and even a calculator to estimate the limiting aperture at http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm.

In a final comparison, instead of the 100% pixel peeping monitor view, I made 8x10 sized, full frame matte prints on a Canon i9100 printer. Near the 8 inch mark, the 1/64 graduations are not detectable with the unaided eye from any of the variations! Even so, the f4.5 print from raw is obviously preferable to the others.

When the absolute maximum quality image is required, I’ll be tending to shoot my G9 at f4.5 but the larger and smaller apertures are still very useable – and I do drive faster (and slower) than 45 mph!

Note: Knowing I would be writing that raw revealed more details than jpg, and anticipating the grief that would come my way for doing so, I suddenly remembered that when the G9 is set for raw + jpg, the in-camera jpg is compressed at the “Fine” level whereas in pure jpg mode the “Super Fine” compression is available. I repeated the test shots in jpg only, using “Super Fine” compression mode. The results are the same: f4.5 is the sweet spot and raw reveals more detail than jpg; however, Super Fine compression does reveal slightly more detail than Fine compression. I believe that the difference in the raw and jpg shots is not “raw vs jpg” but is instead an indication of the in-camera noise reduction that is done on the in-camera jpg even at ISO 80. The raw file did not have any noise reduction applied. Should noise reduction have been applied to the raw files? Well, you were looking at 100% crops, would you apply noise reduction?