In photography, creating the perfect exposure is a juggling act, balancing aperture, shutter speed and ISO. The trick to balancing the Exposure Triangle is to get all three elements working together so you get the results you want, and not what the camera tells you.
The Exposure Triangle
The Exposure Triangle is a triangle which allows you to visualise the relationships between aperture, shutter speed and ISO. As you decrease ISO you have to increase shutter speed or aperture to compensate. The diagram below helps illustrate the exposure triangle.
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Shutter speed is simply a measure of how long the sensor or film is able to capture light. The shutter speed is measured in seconds, tenths, hundredths or thousandths of a second, expressed as 2" (2 seconds), 1/10 (one-tenth second), 1/100 (one one-hundredths) and 1/1000 (one one-thousandth). The longer the shutter is open the more light is gathered.
Shutter speed is the most important aspect of exposure. If your shutter speed isn't fast enough to give you a sharp image, nothing else will save the image.
As a general rule of thumb for handheld photography, the slowest shutter speed you should use is one over the focal length. So for a 300mm lens the slowest shutter speed you should use is 1/300 sec.
Image stabilisation and tripods can be used to increase the shutter speed, but for handheld photography, you should look to use a wider aperture or increase the ISO.
Aperture is the size of the opening in the lens and is generally used to adjust the amount of light entering through the lens. Aperture is measured in f stops or f numbers and the numbers represent the ratio between the focal length of the lens and the diameter of the aperture. These f numbers are typically written as f/2.8 or f/22. Aperture is often used to control depth of field, creating a pleasing background blur to enhance the subject.
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Smaller f-numbers (wide aperture) lets in more light while reducing the depth of field. Large f-numbers (narrow aperture) let in much less light and increase the depth of field.
ISO is an International Standard for measuring the speed of colour negative film, or how sensitive it is. In digital cameras, it is an equivalent measure of computed amplification of a digital signal. As with its film counterpart, digital ISO increases the sensitivity to light, at the expense of quality. High ISO film suffers from graininess, while digital ISO suffers from noise during the amplification process.
When working in low light you will at some point find yourself using the widest possible aperture and the slowest shutter speed you can to stop action. At this point, your only choice is to increase the ISO to get a balanced exposure, however, you should always use the lowest ISO as possible. That way, you can try to achieve the cleanest images with the best dynamic range and colour depth.
If you there is not enough light and your pictures are blurry because the shutter speed isn't fast enough, and you're already using the widest possible aperture the only thing left to do is to increase the ISO. You could leave this on Auto, but I usually don't as I feel my camera always seems to choose a higher ISO than it needs. Don't forget to put it back down after you're done.
Target ISO values for outdoors will range from ISO 100-400 and indoors ISO 400-3200. Please note that these values will vary slightly depending on the ambient light in the situation and your camera.
Exposure Value (EV)
We call a specific combination of aperture, shutter speed, and ISO an exposure value (EV) and often refer to a change that either doubles or halves the amount of light reaching the sensor (or doubles or halves the sensitivity) as a stop.
A change from f/2.8 to f/4 is one stop and this halves the amount of light entering through the aperture. To compensate we need to increase the shutter speed from say 1/60 sec to 1/30 sec (double the shutter speed) or increase the ISO from 100 to 200 (double the ISO).
Regardless of the subject matter you wish to shoot, understanding exposure is crucial to ensuring that the image you want to create and the image you end up creating are one and the same.