DSLR Guide 3 - Aperture and Shutter Speed
Before you start reading, please find your camera’s manual and have it handy - like it or not, you will have to refer to it!
Before we go any further, I’m going to explain the two concepts which will have the biggest impact on your photos - aperture and shutter speed. Getting a correctly exposed photo (ie. not too bright, not too dark) depends on getting the right balance between aperture and shutter speed, but you can also use them to create more interesting pictures.
This is a really important part of the DSLR guide as it forms the basis for lots of camera settings and creative techniques. It’s also a complex topic which I have tried to simplify at this stage. If there is anything in this article which you don’t understand or you think needs to be explained more clearly, please e-mail me. I’d really welcome your feedback.
Aperture
The aperture is the opening in the lens which allows light to pass through. This opening, like the pupil in your eye, can be expanded or contracted and the wider it opens the more light can pass through. On your camera, the aperture settings are shown as ‘f-stops’ - for example f4, f5.6, f8 etc. The smaller the number, the wider the aperture; the higher the number, the narrower the aperture.
Note for those who really want to know: If that seems backward to you, what the aperture numbers actually mean is ‘the focal length divided by x’. Therefore using an aperture setting of f/2 on a 50mm lens would give you a physical aperture of 25mm; using f/8 would make the physical aperture 6.25mm.
Aperture is directly connected to another important concept of photography - depth of field. In essence, this term refers to how much of the image (from foreground to background) is in focus. When you use a wide aperture, you create a shallow depth of field; when you use a narrow aperture, you create a great depth of field. Shallow depth of field is a great way of drawing the viewer’s eye to the part of the photograph that’s in focus and eliminating background clutter. On the other hand, if you’re taking a picture of a landscape, you probably want to convey the whole of the scene, so you would use a narrow aperture to keep everything in focus from front to back.
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Project: The best way to see the effect of different apertures is to set your DSLR to the Aperture Priority setting and take a series of shots of a static object placed a few metres away from the background. Focus on the foreground object and start with the widest aperture available to you and decrease the aperture by a couple of f-stops for each new shot. At narrower apertures, you may need to support the camera on a tripod to prevent camera shake. Download these photos to your computer so you can view them on a larger screen and you should be able to see how the changes you made to the aperture affected how much of the image was in focus. If you have several different lenses, repeat the project with each of them as different focal lengths will have different depths of field.
Shutter Speed
The shutter speed is the amount of time taken to expose the image. If there is a lot of light available (either because it’s sunny or because you have chosen a wide aperture), you won’t need to expose the image for very long. The less light you are working with, the longer it will need to be exposed for. The shutter speed is displayed as fractions of a second, eg. 125 (1/125th of a second), or as full seconds, eg. 3”.
In creative terms, shutter speed can be used to convey a sense of movement, or to freeze motion. By choosing a slower shutter speed, you can record the movement of a fast-flowing stream so that it appears to become a blur. By choosing a fast shutter speed, you can freeze a moving subject.
Project: Unlike aperture, it is difficult to show the effect of different shutter speeds in one sitting, because the shutter speeds available depend to a certain extent on the amount of ambient light. Instead, try photographing a fast-moving subject like a car during the daytime using a fast shutter speed, then take your camera and a tripod out at night and shoot the same subject with a slow shutter speed (eg. 1” or more). The result is that during the daytime shots, the car will look like it’s not moving, while in the night shots the camera records the car’s entire movement across the frame, creating a ‘trail’ of light.
How Shutter Speeds and Apertures work together
To get the correct exposure, you need to select a balanced combination of shutter speed and aperture so that the right amount of light hits the sensor, but there are several combinations which will do just that. You can take away from one as long as you add to the other - a smaller aperture requires a longer shutter speed and vice versa. 1/50sec at f3.5 is the same exposure as 1/2sec at f18 - the numbers are different, but they allow exactly the same amount of light to fall on the sensor.
Fortunately, digital SLRs have a couple of modes which allow you to decide what you want to control without having to do the sums. The ‘priority’ modes allow you to choose either the aperture or shutter speed while the camera works out the other. For example, if you want to take a portrait with a shallow depth of field, you could set the aperture to f5.6 and the camera will work out which shutter speed you need in order to correctly expose the image. Likewise, if you wanted to take a picture with a slow shutter speed, the camera would work out the aperture for you. I’ll talk more about the Aperture Priority and Shutter Priority modes in DSLR Guide #5.
Some Notes on Shutter Speed and Aperture
Just as a slow shutter speed can show the movement of your subject, it can also show any movement you made as you took the photo, which can be a problem if you’re hand-holding the camera. In order to avoid this, you should keep an eye on the shutter speed to ensure it does not fall below the focal length of your lens. Eg. if you’re using a 300mm lens, you need to use a shutter speed of 1/300sec or faster; a 200mm lens needs a shutter speed of 1/200sec or faster and so on. Increase the shutter speed or widen the aperture as necessary to compensate or use a tripod to keep the camera steady.
When you’re using a wide aperture, and therefore a shallow depth of field, you have to be very careful to ensure that your subject is in focus. Make sure that you focus directly on the area of your subject you want to be in sharp focus - if you’re photographing a human or animal subject, you should focus on the eye nearest to you.
You will find that the effect of depth of field varies between different lenses. Wide angle lenses have a greater depth of field across the board, while telephoto lenses have a shallower depth of field. The leaf example above was taken with a 105mm lens, which is why, even at the minimum aperture, the background is not 100% in focus.
If you have any comments on this series, or would like to suggest topics for me to cover in the future, please e-mail me using the link below.
Saturday, 20 May 2006
Techniques, tips and tricks to help you take better pictures.
