To get a clear photo of a moving subject, you need to make exposure decisions based on several factors. It’s not all that hard to do (unless you’re hindered by low light levels). Let’s look at each factor involved.
Subject speed: Let’s tackle the most obvious factor first, the speed of the subject. The faster it’s moving, the higher the shutter speed needed to stop it. If the subject is crossing in front of you, you’ll need a faster shutter speed than if it is moving obliquely or directly toward or away from you. Indeed, you set your shutter speed one step faster for an obliquely moving subject than for one coming right at you. For a subject crossing directly across your field of view will need two speeds faster.
Distance: Next, how far away is the subject is from you and how long a lens you are using? Both items affect the angular speed of the subject just as much as the angle at which the subject is moving. The angular speed (the speed at which the subject crosses your field of view) is the “real” speed you need to be concerned with.
Blur: How much image blur you can tolerate. Sometimes a bit of blur helps purvey the feeling of motion. Other times maximum sharpness rules!
Case study: The Moon is a speed demon!
Let’s take an extreme case to see how these factors can interact: the Moon. It’s whipping along at over a half mile per second, directly across and in front of you–the worst-case scenario. Right? However, the moon is about 2,160 miles in diameter, so during a one-second exposure the Moon would only move about 3/100th percent of its diameter. Since the Moon is some 239,000 miles away, you wouldn’t see this movement at all unless you were photographing it through a telescope.
Besides, you won’t need a one-second exposure on the Moon. It is a full daylight object, and would normally be photographed at about one or two stops more than a “sunny 16” setting to make it look brighter like we normally perceive the Moon rather than an 18 percent gray. (A “sunny 16” setting means that a sunlit scene taken at f/16 would call for a shutter speed of 1/Film ISO second.)I shot the Moon through an eight-inch, 2000mm telescope. The exposure was about 1/4 second on ISO 100 film. Since such long lenses magnify any camera vibration–including SLR mirror slap (motion resulting from an SLR’s mirror lifting up and returning to position during exposure), which would blur the image far more than subject motion. To prevent this, I used the astrophotographer’s “hat trick.” Literally. I blocked the business end of the telescope with a dark hat and opened the shutter on “B” with a locking cable release. After the scope stopped shaking, I whisked the hat aside for an instant, covered the telescope once more and closed the shutter. I will admit that the telescope’s clock drive was also running and counteracted most of the Moon’s motion as well. A motionless telescope would do pretty much as well.
Meanwhile, back on Earth…
Now let’s look at more down-to-earth action. Assuming you want maximum sharpness, use the fastest shutter speed you can. For specifics on photographing sports, I’ll defer to fellow columnist Jack Howard. Stopping action usually calls for a faster film or ISO setting on your digital camera, particularly if the lighting isn’t very good. You may need to compromise and use a somewhat slower shutter to at least get the shot. But you can turn this into an advantage; keep in mind that a bit of long-exposure blur may better motion more effectively than short-exposure “frozen” action.A case in point is waterfalls. More often than not waterfalls look more pleasing when deliberately blurred than when photographed at fast shutter speeds. Compare the following shots:
You can see the difference in these two photos of the Bald River Falls near the Cherohala Skyway in Tennessee. The top photo was taken at about 1/10th second and the lower one at about 1/200th second. Both photos were taken on Ilford Delta 400 film with a Hasselblad 500CM. See how the water flow is captured more evocatively in the longer exposure?
Stop it in a flash
How about stopping really fast action? You’ll need an electronic flash that can be set on manual and as short as 1/16th Power. This setting will bring the exposure time to about 1/40,000 to 1/60,000th second. Don’t have a manually adjustable flash? For a few cents you can rig the familiar Vivitar 283 to give you a super-short exposure. (Warning to digital camera users: use only flashes recommended for your camera. Some older type flashes intended for film cameras can damage your camera.)
As you can see in this photo, I have unplugged the flash’s auto sensor. Plug a piece of 22-gage wire between the left pair of pins. Plug a 100 ohm 1/4- or 1/2-watt resistor between the right pair of pins. These resistors are available at Radio Shack stores at about five for 99 cents; you can also find them online. Secure the connections by wrapping some masking or electrical tape around the flash. Note: there are no dangerous voltages on these pins if the flash is manufactured anywhere but Japan.You’ll know the connections are good if your flash fires and recycles almost instantaneously when you press the test button.
Here’s a photo I took some years ago using the modified 283 shown above for a review of the “Dale Beam” trigger system (discontinued; sometimes available in the Adorama Used department under Flash Accessories). The Dale Beam uses a beam of infrared radiation bounced off a reflector to create an invisible “fence.” (Other such triggering devices are available today. Just Google “Infrared camera triggers.”) When an opaque object interrupts the beam, any flash unit connected to the device will fire.
In this case, the rubber dart broke the beam and was captured in mid-flight. The blur you see in the dart is mostly focus error. With the flash’s power cut this far, I had to use a wide open lens and favor the boy as he took up most of the scene. The ISO 320 guide number was about 15!
Also, as you can see from the dart’s skewed angle, its trajectory wasn’t very predictable–especially since we were working in total darkness. I locked the shutter open, said “fire” and hoped for the best. This setup won’t stop bullets. They move entirely too fast. Moving at 1000 or more feet per second, they would just be a blur unless you use a microsecond or sub-microsecond flash like the EG&G Microflash. This is best left to pros, who are paid to mess with such things.
