Motion - Part 1

Today’s topic deals with how use your shutter speed to control the appearance of motion in your image.
This posting is primarily for those of you that can control your camera either on manual or shutter priority mode. If you have a fully automatic point and shoot, sorry to say the tips won’t help but read on as you may learn a few things.
If you remember from a few postings ago, we pointed you to a site that discussed the interaction of the aperture and shutter speed in regards to exposure. Here’s a brief review. The aperture controls the amount (quantity) of light that enters through the lens to exposure the film or be read by the digital sensor. The shutter controls the amount of time the light coming through the lens is allowed to hit the film or sensor. The combination of these two items (along with ISO setting), determine the exposure of the image.
Aperture settings are standardized so that when switching from one lens manufacturer to another, the setting is the same in terms of the amount of light that is allowed to pass through the lens. In photography lingo, this measurement is discussed among photographers as an “F Stop”. Not to confuse you anymore, but the people that invented this lingo and measurement system decided that a smaller F stop number meant more light would hit the imaging surface. Shutter speed is measured in terms of seconds. Why, because quite simply the shutter speed is a measurement of how long the film or sensor is exposed to light. For example, a shutter speed of 1/125th means that for 1/125th of a sec, light passing through the lens was allowed to pass through the open shutters to hit the surface of either the film or the sensor. So, to recap, aperture (or f-stop) is “how much” light and shutter speed is “ how long”. Think of it as a part of a receipe…how much heat, for how long.
At this point, you may be asking “why do I care. My camera does this automatically so I can’t set this incorrectly and mess up my pictures.” While this is true, your camera does not set creative control over how you capture the image. It averages everything out to a neutral (18%) grey. What do we mean by this ? Say, for example it’s a cloudy day, and you are taking pictures of your kids at a little league baseball game. You camera, in programmed or auto, captures the images at ISO100, F5.6 and 1/100th sec (part of our receipe for that picture) using a 200mm lens. After the game, you get your images loaded onto the PC, or the pictures back from the processor and what do you see ? Some of the images are in focus, and some are blurry. Looking closer, the images that are blurry are ones where the kids are moving (running, swinging the bat, catching the ball, etc). All the other images where they aren’t moving are OK. Stupid Camera, right?? WRONG.
Because you had the camera in Auto/Program, it was only trying to set the proper exposure and shutter speed to the manufactures recipe for a good image. Again, it can’t interpret that you wanted a picture of the baseball stopped mid air just before little Timmy hit the ball with the bat. Your picture had the bat and parts of Timmy as a blur. Why ? Your shutter speed was too slow. You probably needed to have your shutter speed set at 1/1000s, depending how fast Timmy was swinging the bat, and how fast the ball was pitched. Maybe faster is Timmy is a playing on a World Little League team.
Here’s another way to think of this. Think of a painter. He can select the size of his brush and how much paint to put on the brush. Too big of a brush with too much paint, well, you have a blob and paint running everywhere. The same is true with light. If you have too big of an aperture for too long of time, light runs everywhere and you get the overexposed completely white print that we’ve all gotten at least once by accident.
Too small a brush with not enough paint, you’ll be wondering why you’ve been paying this painter for 6 hours of work to paint a 2” x 3” area that looks like your 4 year old hit it accidentally with his/her water color paints. Again, the same is true with light. Not enough, for too short of a time, there’s no detail and the image is underexposed. So, there is a delicate trade off between speed and exposure.
To stop fast action, such as someone swinging a bat, jumping in mid air, etc one needs to use a very short time of exposure of the image. If you think of the old 8mm movies, each frame captured a very small instant in time. Put together, it formed the movie. In your blurred images from the ball game example above, you would have essentially captured a portion of a movie (2 -3 frames) onto your single image (a singe instant in time). In otherwords, smudging the paint, or light, so that no defined single image was clear.
Here’s a simple rule of thumb that you can use.
1: Want to stop a fast action -> use fast shutter speed (>1/1000 s)
2: Want to show motion -> use a slower shutter speed
Unfortunately, we can’t give you the exact recipe (f-Stop, shutter speed), but your camera can help you. This is where shutter priority and manual control comes in. Ideally, you’d like to use shutter priority as your camera will adjust the aperture for you to get the correct exposure. In the case of manual, you’ll have to take a light reading and do the math and bracket your shots.
Now, there is a problem that crops up that we should briefly discuss. What to do if you can’t get the shutter speed you need because the lighting conditions won’t allow you to.
Let’s go back to our example of the ball game. If you remember, we said it was a cloudy day. From a planning standpoint, that should tell you that you may have some problems with lighting…meaning having enough light to use the quicker shutter speeds to stop action. If you have a SLR (or D-SLR), you may be able to use a faster lens (a F1.8, for example). Why would that help. Remember, the lower the F-Stop, the more light passes through the lens. More light = less time required (in general). In our example, we said the images were taken at 1/100s. For the purpose of this example, let’s say we need 1/800s to stop the motion. To get to that shutter speed, we don’t have the number of F-Stops alone to make that adjustment (trust us, we did the math).
So you don’t have to do the math, here’s a little trick for the digital camera users. Set you shutter speed to 1/800 s, and ensure your camera is on shutter priority. Your camera should be telling you that it’s too dark to exposure the picture properly. Adjust your ISO from 100 to 200 and check to see if your camera says it’s OK. If not, try ISO 400. Just remember, increasing your ISO will increase your noise. We really don’t recommend shooting above ISO 400 because the image can get real noisy. If you can’t get the shot at this point in time, either wait for a sunny day, kick back and enjoy the game, or settle for what you can get. Chances are, your flash won’t have enough power to reach little Timmy out in Center Field, so we wouldn’t recommend the attempt unless you enjoy spending money on underexposed pictures.
A word of warning: You CAN NOT adjust the ISO in this manner if you are using film unless you do it for the entire roll. Also, some films will not respond well to this technique. If you do expose an entire roll of film in this manner, you MUST tell the lab before they develop your film so they can correct the processing time. We don’t recommend film users do this unless it’s a last resort. Again, if you shoot with a film at a different ISO than the film is marked with, you must do so for the entire roll and inform the lab.
For the other end of the extreme where you want a slower speed, say 1/2 s, to show motion (maybe a waterfall), you may need filters for your lenses as there could be too much light (say on a sunny day at the beach). These filters are called neutral density filters and can help you get 1/2 – 2 stops less light in through the lens to allow you to use slower shutter speeds. You may also be able to use a polarizing filter.
There are a couple of techniques that can be used to show motion that require more than just adjusting the shutter speed of the camera. We’ll discuss those later…..






