Before we get too bogged down in technical terms, let me tell you a simple analogy that I learned years ago, and which really helped to clarify in my mind how aperture diameter and shutter speed combine to give exposure.
Imagine you are using a hosepipe to fill a bucket to the brim. When the bucket is full that corresponds to a perfectly exposed photo.
Two things affect how much water we get in our bucket - the diameter of the hosepipe and how long we run the tap for. There are many combinations which will fill our bucket - from a narrow pipe left running for a long time to a wide pipe running for a short time, and everything in between.
A camera works in exactly the same way, with many combinations of aperture diameter ("hosepipe width") and shutter speed ("water running time") adding up to the same thing - a perfectly exposed photo ("full bucket").
Try to understand
Aperture and shutter speed are bound together like an old married couple. For the same scene, if you open the aperture, you increase the shutter speed, and if you close down the aperture, you decrease the shutter speed. Think of a bucket of water with a hole in the bottom. If you have a large hole in the bottom of the bucket (large aperture), water will drain out quickly (fast shutter speed). Conversely, for the same amount of water, if you have a small hole in the bottom of the bucket (small aperture), the water will drain out slowly (slow shutter speed).
Remember how I mentioned stops earlier? Well one aperture stop is equivalent to one shutter speed stop, therefore if you open up your aperture one stop brighter (say from f/8 to f/5.6), and increased your shutter speed to one stop darker (like 1/250 to 1/500), then you would get the same exposure. So f/8 @ 1/250 is equivalent to f/5.6 at 1/500. This is the theory of equivalent exposures.
Summary: Larger aperture, faster shutter speed; smaller aperture, slower shutter speed. Increase aperture by one stop and decrease shutter speed by one stop = equivalent exposure (and vice versa).
Choosing the Right Aperture and Shutter Speed
As we have seen, in any given situation there are many combinations of aperture diameter and shutter speed that will give us a well-exposed photo - so which should we choose? Well, that depends entirely upon the effect you are trying to capture, but the following points will help you make the right decision:
- A wider aperture decreases the depth of field (amount of the scene which is in focus). While this is fine (and often desirable) for portraits and close-up photos, it usually doesn't work so well for landscapes.
- As shutter speed gets slower, motion blur becomes more apparent. This can either be blur from a moving object, or camera shake if you are hand-holding your camera.
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