Conveying motion: Choosing the correct aperture and shutter speed
The primary characteristic that sets still cameras apart from video cameras is the former’s ability to “paint” the moment. Therein lies the opportunity to bring out your creative streak and make reality look surreal. This tutorial addresses the considerations towards exposure time and aperture while capturing life in motion.
When I took up photography in 2001, I could only afford a cheap point-and-shoot camera. My primary gripe with the camera was that it was fully automatic and offered little control over its settings. I could never get it go photograph motion and present it the way that I wanted. Times have changed… I now shoot with a decent quality Canon Digital SLR, and have a decent quality compact point-and-shoot to accompany me.
“Painting motion” is not limited to SLRs alone, and can be achieved with any half-decent point-and-shoot camera too. It is important to know which functions to use to achieve these effects.
To begin with, we need a basic understanding of what bearing a lens aperture setting has to capturing an image. The aperture setting of a lens refers to the opening that the lens has to allow light in. The aperture of a lens is represented by an f-number (often referred to as an f-stop). The larger the f-number, the smaller the opening, and hence the less light that the opening in the lens permits to come in. Conversely, the smaller the f-number, the larger the opening, and amount of light that the opening in the lens permits to come in.
For an aperture setting of f/22, the f-number is “22”.
Another thing to note about the F-number is the depth-of-field or DOF. The concept of DOF is quite simple – it is the depth of the range that will remain in focus. For a larger f-number, the DOF is larger (or deeper), and vice-versa.
“ The depth of field (DOF) is the portion of a scene that appears acceptably sharp in the image.”
The table below summarises what we have covered so far:
|As f-number…||Aperture size||Depth-of-field||Amount of light||Exposure time|
|Increases||Decreases||Increases||Decreases||Must be increased|
|Decreases||Increases||Decreases||Increases||Must be decreased|
The exposure time (sometime – mistakenly – referred to as the shutter-speed) is the other variable that you will deal with. It refers to the amount of time that the lens is left open for light to hit the film (or sensor, as is the case with digital cameras). The exposure time is measured in seconds. The longer the exposure time, the greater the amount of light energy that strikes the sensor (and hence, the brighter your image will be). The shutter speed is the inverse (mathematically, the reciprocal) of the exposure time. Hence, a longer exposure time means a slower shutter speed.
Freezing motion involves thinking backwards. For example, light trails of cars, or rides at carnival will require that you allow for a longer exposure time. This will allow the light patterns to vary and paint the effect of motion on your camera’s sensor. Since your exposure time is going to be long, you will end up having more light energy strike your camera’s sensor. To control this, you have to limit the amount of light coming in to avoid over-exposing your image. Hence, you will need to use a higher f-number (or smaller aperture size).
|If exposure time is desired to be…||Shutter speed is…||f-number must be…|
These are merely the technical basics. However, there is a human element that one needs to consider. Choosing how much time you need to expose your frame to is a bit of trial and error, but as a general rule, you do not need your object to be moving really fast to get a cool effect. Sometimes, the most interesting effects can be achieved by shooting something moving relatively slowly with a moderately long (between 0.5s to 1s) exposure.
Remember to try a few different exposure times and aperture settings and experiment with your results. There is no set formula – and that is what makes long exposure shots so appealing. There is, however, one common factor in long exposure shots. Above all else, it is imperative to keep the camera still while taking the shot. For this reason, you MUST have your camera either mounted securely on a tripod, or placed on an item (such as a wall, or a rock). where it will remain at rest throughout the exposure. If you can’t be bothered carrying a tripod, consider using a small bean bag, a hand towel, or a sock filled with rice to place your camera on top of. Also, to minimise your contact with the camera, consider using a remote control, remote cable release or the timer function. Last, but not least, choose a place where the camera is least likely to be disturbed by wind and vibrations.
Unless you are adept at remaining absolutely motionless for extended periods of time, ensure that you have a tripod and remote cable release to mount your camera on and keep steady when shooting a long exposure shot.
The images above give you an idea of what different settings to your camera can do for you. They were all taken at Melbourne’s Moomba Festival on the evening of Friday March 5th between 7:30pm and 8:30pm on a very overcast evening. I’ve tagged each of them with the camera settings to give you an idea of how each effect was achieved. Remember, learn to use the functions of your camera well – specifically, learn to interpret the histogram, as it helps you get a good understanding of how well exposed your image is.