When I posted the image of McKenzie
Exploring the Beach in the weekly assignment, I mentioned that I had set the exposure mode to shutter priority. Michele asked me in an email to explain why, and since I thought the answer would be useful to other folks as well, I chose to answer on the website instead of via email.
Exploring the BeachFill Flash, Shutter Priority, 1/250 sec, f18, ISO 200
I should start by saying that my most often used exposure mode is Aperture Priority. Since I don't photograph fast moving objects all that often, I find that it doesn't really matter (in normal daylight) if my shutter speed ranges anywhere between 1/400th of a second to 1/6400th of a second. If I'm photographing a landscape or scenic, the viewer will typically be unable to tell the difference between 1/400th of a second or 1/6400th of a second in the final print,
all else being equal.
However, not all else remains equal. If I change my shutter speed, then my aperture will need to change (assuming I keep the same ISO) in order to provide the same amount of light to the film or sensor. If I change my shutter speed from 1/400th of a second to 1/1600th of a second, the viewer probably won't be able to perceive the change in shutter speed; however, they
will perceive a significant difference in the depth of field when I change from f11 (at 1/400th sec) to f5.6 (at 1/1600th of a second). It is because of this, the viewers ability to perceive a change in aperture, or said another way, my ability to change the viewers perception through the use of a different aperture, that I choose to consciously control the aperture and let the shutter speed "fall where it may." So my "normal" exposure mode is aperture priority. I change to shutter priority mode only when shutter speed becomes a "limiting factor" in designing the image.
One case where shutter speed
might be a limiting factor is when the light levels are low, and the required shutter speed might be too slow to stop subject or camera movement. In the "old days" I would have switched to shutter priority and let the aperture "fall where it may." Nowadays I switch into "auto ISO" mode, which will automatically bump up the ISO if my shutter speed drops below a threshold I set in the camera. So, in this case, shutter speed is no longer the "limiting factor" it used to be, and I still use aperture priority mode.
Where shutter speed
is still a limiting factor is when using flash. A little background information is necessary in order to understand why. The "flash sync speed" of your camera is defined as the fastest shutter speed you can use and still allow a full-power flash burst to cover the entire frame. If you were able to force the camera to set a faster shutter speed than the "sync speed," then the shutter would obscure part of the frame during the flash exposure, and your light would be uneven across the frame. This might not make any sense until you understand that a modern focal plane shutter operates by opening the first "curtain" and closing the second "curtain." At slower shutter speeds, the first curtain opens, the camera waits the predetermined amount of time, and the second curtain closes the opening. At faster shutter speeds, the second curtain begins closing before the first curtain is completely open, so there is no time when both curtains are completely open, and instead, the shutter is essentially a "slit" moving across the frame and providing the proper amount of exposure to each portion of the frame. The "sync speed" of your camera is essentially the fastest shutter speed that can be used while still allowing both shutter curtains to remain fully open during a full-power flash burst. Pro cameras with more robust shutter mechanisms historically have faster sync speeds than "consumer" cameras.
Most cameras will automatically limit the shutter speed when using flash in order to ensure the entire frame is properly exposed. A modern exception to this rule is FP sync, which fires the flash in rapid pulses that evenly expose portions of the frame as the "slit" moves across the film/sensor plane. (Pretty amazing technology.) The tradeoff is that these quick pulses can provide only a small percentage of the power of a "normal" flash burst, thus significantly limiting the range of your flash. When you are using fill flash outdoors, the underpowered FP sync mode is rarely sufficient to provide enough "fill" light to open up the deep shadows.
So, that requires that we fall back to the old fashioned "sync" modes in order to provide more flash power on our subjects, and this limits the flash sync speed to about 1/250th of a second for most cameras. If I set my aperture to f11, but the ambient light levels are bright enough to require a shutter speed of 1/500th of a second for a "proper" exposure, the camera will still sense the flash and automatically limit my shutter speed to 1/250th sec and my image will be overexposed. On the other hand, if I choose an aperture smaller than I need to, I'm unnecessarily reducing the flash exposure on my subject, and potentially won't be able to get enough "fill" light on the subject to open up the shadows. The "optimum" setting for fill flash in bright daylight is to set your camera to shutter priority and select your camera's maximum sync speed. This will ensure the camera chooses the largest aperture suitable for the ambient light exposure, and give you the best chance of getting enough light from the flash on the subject. (You can determine your camera's maximum sync speed by consulting the manual, or by selecting the maximum "sync speed" available in the flash settings of your camera's menu.)
In a normal exposure involving only ambient light, changing the aperture affects how much light hits the sensor at any given instant in time, while changing the shutter speed affects how much light hits the sensor over time. If you "stop down" your aperture to reduce the amount of light hitting the sensor at any one instant by half of the previous amount, then you will need to keep the shutter open for twice as long in order to compensate and provide the same level of exposure on the film or sensor. If you use a faster shutter speed, you will need to open the aperture in order to compensate. In ambient light, changing aperture or changing shutter speed have equal effect on the final exposure.
In order to really understand how fill flash works, it is important to understand why changing your aperture affects your flash exposure differently than changing your shutter speed. Changing the aperture will affect the flash exposure on the subject (by reducing or increasing the amount of light reflected off the subject that hits the film/sensor plane at any moment in time), but changing the shutter speed (up to the sync speed limits) will not! The reason flash exposures are different than ambient exposures is because the flash pulse lasts for only a very short duration. Let's say for simplicity sake that your flash duration is 1/250th of a second. If your shutter is set to 1/250th of a second, the exposure on your subject will be equal to 1/250th of a second of ambient light
plus 1/250th of a second of light from the flash. If you use a longer shutter speed of 1/125th of a second, the ambient exposure will be doubled (shutter is open twice as long) but the flash exposure on the subject will remain the same (since your flash pulse only lasts for 1/250th of a second of the entire 1/25th of a second time the shutter is open). No matter how much longer you leave the shutter open, the flash exposure won't change, because the flash isn't firing during that additional time.
So let's look at how this knowledge has a practical benefit in helping me "balance" fill flash with ambient light. Let's say I'm photographing a subject that is 8 feet away and the "proper" ambient exposure for my scene is equivalent to f22 and 1/60th of a second. However, my subject's face is in deep shadow and I need to use fill flash. Because my flash head "zooms" to provide adequate coverage across the frame, the flash range is shorter when I'm using a wide angle lens. With an 18mm lens, ISO 200 and an aperture of f22, my flash range is only 4.1 feet (according to the distance readout on my flash). Therefore my flash will not have enough power to provide adequate fill light on the subject (8 feet away). I can adjust the balance of fill flash and ambient by changing my aperture to f11 and 1/250th of a second. Because I "opened up" my aperture by two stops, but compensated by setting a faster (shorter) shutter speed by two stops (or two "exposure values" or EV), the ambient exposure will remain the same. However, shortening the shutter speed from 1/60th to 1/250th of a second has no effect on the flash exposure, since the flash pulse is only lasting for 1/250th of a second, while opening up the aperture increases the amount of light from the flash that is reaching the sensor during that 1/250 second time period. The net effect is that the ambient exposure remains the same, but I've increased the
potential flash exposure by 2 EV, and therefore increased my flash range to 8.3 feet and will be able to adequately fill in the shadows on my subject.
I can use this knowledge to "balance" the ratio of fill flash and ambient light on my subject. Changing shutter speed (up to the sync speed) has no effect on the amount of light from the flash that is reflected from the subject. So, if ambient light is too strong, I can increase the shutter speed to reduce the ambient light (prevent overexposure of the background) without affecting the amount of flash exposure on my subject. Because smaller apertures will severely limit the range of my flash, I typically want to use the fastest shutter speed possible (up to my sync speed of 1/250th) in order to allow for (relatively) larger apertures, and provide the flash range needed for adequate fill light on my subject.
Simple yes?
If any of this doesn't make sense, please ask questions and allow me the opportunity to clarify my explanation.
Keith
