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What is Aperture?

August 1, 2011

Lately I have found myself explaining camera settings to people and I’ve found that many times they don’t quite understand the “exposure triangle”. The exposure triangle consists of aperture, shutter speed and ISO and how they interact to produce the proper exposure for a photo. In this blog post I am going to attempt to explain, as simply as possible, what aperture is. In future posts I will explain the other two and hopefully bring it all together. I’m not going to get all technical because, quite frankly, I don’t know all the science behind it. I just know what it does.

First off, the aperture in a camera lens is the opening through which light passes. Think of it like the pupils in your eyes. Just like a pupil, the aperture can change size. The wider the aperture opens, the more light that can pass through it. Conversely, the narrower the aperture, the less light that can pass through. Simple, right?

The confusion tends to come from the aperture value used to denote a wide opening versus a narrow one. The term f-stop is used to describe the diameter of the aperture opening. A lower f-stop number means a wider opening. A higher number means a narrower opening.
Check out this diagram below showing some common f-stop values and their relative size to each other. Notice, for example, that an f-stop of f2.8 is wider than f5.6.


All you really need to do is remember that the lower the number is, the wider the opening, and thus more light entering the camera and hitting the sensor. I think sometimes this just seems to be the opposite from the way we normally think, so it confuses people.

The amount of light hitting the sensor is clearly part of what makes up the exposure of an image. In a low light situation such as inside a room that is not well lit, or in a church where you may not be able to use a flash, it helps to have a lens that has a wide aperture setting. Using the widest aperture will allow the camera to pick up as much ambient light as possible. The more ambient light the camera lens can gather, the faster the shutter speed can be and still produce the correct exposure for the photo. I will write more about shutter speed in a later post, however the faster the shutter speed, the better chance you will have of making a photo that isn’t blurry due possible movement of the camera while holding it in your hands.

The aperture setting also has another effect besides the exposure. It affects the depth of field in the photo. Wider apertures produce a shallower depth of field. This means the subject is in focus while the background is out of focus. Narrower apertures produce more depth of field so that more elements of the image are in focus.

Here is an example of a photo made using a wide aperture. Notice how the background is very out of focus. This is the result of using an extremely wide aperture of f1.2.

Canon 5D, EF 85mm f1.2L @ f1.2 1/200 ISO 800

Try experimenting with different apertures. Set your camera to use the Aperture Priority mode (usually “Av” or “A” on the mode dial) and try different apertures. Watch how the camera adjusts the shutter speed automatically to make the exposure. Notice how the shutter speed slows down as your aperture number gets higher (narrowing the aperture more and more is called “stopping down”). Take multiple pictures of the same subject using different apertures and notice how the background comes more into focus as you “stop down”.

Using different aperture settings allows you to get creative with the look of your photos by changing the depth of field. At the same time wider apertures allow you to shoot in low light and still use a decent shutter speed to make sharp images. I will talk more about shutter speed soon and describe more about how it relates to the aperture. I hope this helps. :-).


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