Aperture is the opening that allows light through to the sensor or film in your camera body. The cool folks in the biz also refer to it as f-stop.
What is it ?
A diaphragm, much like an iris of the eye, controls the size of the opening to the sensor. The diaphragm is not to be confused with the shutter. In the context of your eye the aperture is your iris and your eyelid is the shutter. You use the iris to control how much light you let into your eye per unit of time and the shutter to control when and for how long that light is allowed to reach your retina.
In the world of cameras, the lens aperture is specified as an f-number, technically defined as the ratio of the focal length to the effective aperture diameter. Simply rearranging this equation we get aperture diameter defined as a ratio of focal length to the f-number, f/2, f/1.4, f/4 etc. Ok, here comes the tricky bit so stick with it, when we want a large aperture diameter (wide aperture) the math tells us that this will happen when the f-number, the denominator, is the smallest therefore f/2 is a wider opening than f/4. Since ‘focal length (f)’ is what I can best describe as a run-time variable that changes from picture to picture some clever chap long ago decided to forgo the ‘f’ in camera convention for aperture description. So in your camera you use the f-number to adjust your aperture.
Figure showing the aperture, focal length and f-number association
Wow .. too much huh .. lets take stock. Aperture is defined as focal length/(f-number), but convention says that you ignore the ‘focal length’ … so aperture is defined by just the f-number, the denominator. To get a large opening of the ‘iris’ the denominator has to be as small as possible at a given focal length. If you have followed me through the mine field then you would say ‘Ah huh’ when I tell you that the smaller the f-number the wider the aperture.
How it’s used ?
Once you realize that aperture is like the iris of your eye, knowing how and when to use aperture to your benefit is easy.
Let’s say I am at a Broadway show, ‘Wicked’, and I want to take a picture of those dancing witches on stage. Thinking back to the Basics Part I, we know for a given exposure (EV) setting camera decides for you what aperture, shutter speed and ISO to use. Ignoring ISO for the moment; the camera will set an aperture-shutter speed combination to collect the required exposure (light) to take the picture. The best thing in this situation would be to manually set the aperture as wide as possible and let the camera do the legwork in selecting the appropriate shutter speed.
With the aperture at its widest you collect the most light per unit of time allowing the camera to have the fastest shutter speed meet the EV setting. Why does that matter you ask? … well think about it, if you are shooting an action sequence and have a long shutter speed the image is going to be blurred. This is one of the most common problem people face when taking pictures without a flash, they get a collection of blurred images. Manually setting the aperture is one of the ways to address this issue.
This shows how the shutter speed increases for higher f-number Wide aperture also gives you a narrow depth of field but that is a can of worms for another day. See the grass image above to get an idea of aperture and depth of field relationship.
Another pleasant side effect of using large aperture, depending on who you talk to, is vignetting. When you have the aperture wide open the bulk of the light funneling thorough the lens hits the centre of the sensor and gradually falls off to extremities of the aperture opening.
An example of vignetting, see how the eye is guided to the helecopter
How to work the aperture ?
Shooting in Aperture priority mode is how you tell the camera that I will pick the aperture and you can pick the rest of the bits like shutter speed etc.I personally tend to shoot predominantly in Aperture Priority mode (AV) telling the camera what aperture to use for a given EV and let the hardware pick the best shutter speed.
Hopeful this blurb gave you an insight into the basics of aperture and made you more confident to through around words like f-stop and aperture.
In summary:
“The smaller the number f-number the wider the aperture”
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