Of the 3 factors that influence the exposure of an image, aperture seems to be the one that many newcomers have trouble understanding. Terms like “f-stop”and “reciprocal” make the concept of aperture seem more complicated than it is.
If that wasn't enough lenses are described using their focal length and an aperture setting, such as “50mm f/1.8” or a range of aperture settings in the case of many zoom lenses. Lenses are also referred to as “fast” or “slow” according to their apertures. All in all, it seems like an awful lot to have to learn.
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In reality, understanding aperture isn't a matter of rocket science. In fact, it comes down to just a few simple concepts. What's more, it's such a powerful creative control that it's very much worth learning.
The first thing to know is that aperture refers to the size of an opening that allows light to pass from your lens to the camera shutter. In all but very simple lenses, the size of opening can be adjusted by rotating the overlapping leaves of an “iris” or “diaphragm”, either electronically or with an external ring.
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The next thing to understand is the f-numbers that represent the size of the opening. In the most basic terms, the “f” represents the focal length of the lens and the f-number is the denominator (the bottom number) of a fraction with the focal length as the numerator (the top number). For example, a 50mm lens with an aperture setting of f/8 would have an aperture size of 50mm/8, or 6.25mm. A larger number means a smaller opening and vice-versa. Simple, right?
Lastly, you need to know that the size of the opening determines two things:
- The amount of light allowed to pass to the sensor when the camera shutter opens.
- The size of the cone of light passing through the lens.
The first of these two items is how aperture size affects exposure. A larger opening (smaller f-number) allows more light and vice-versa. That means that wider apertures allow faster shutter speeds and narrower apertures require longer exposure times.
The second, while a little harder to explain in technical terms, is depth of field. This refers to the distance toward and away from the camera from a point of focus in which objects will still appear to be in focus. A smaller aperture extends depth of field, and a larger aperture reduces it.
Those are the basics – everything you need to know to start using aperture creatively. Here's a short, but very cool FocusEd video that may make it even easier to understand and you'll get a laugh or two along the way:
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