This week, let's take a closer look at one of the most fundamental controls on your camera, the Aperture.

The Aperture is an adjustable opening inside the camera – usually in the lens – that controls how much light can flow through the lens.  Think of it as a valve on your bathtub.  With a small opening less water flows into the tub so it takes longer to fill it up. 

A larger aperture was used to separate this cat from the background and a longer lens helped separate me from her teeth.

Your lens’ aperture works the same way only with light instead of water.   With a smaller opening, it takes longer for the sensor to record the scene you are photographing.  Open the aperture all the way and you don’t have to have the shutter open as long for the sensor to record the image.

To accurately control the flow of light through the lens we must be able to measure how much light passes through the aperture. Now, you'd think that would be pretty straight forward. Just measure the diameter of the opening. But the laws of physics don't allow us to do that. You see, the light gathering capability of any lens is controlled not just by the aperture opening but also the focal length of the lens. 

A shallow depth of field is a great way to draw a viewers attention to your subject. In this case the background is soft enough to do that but not so blurry that you loose the important environmental elements of the image. 

If I had a 20mm lens with an aperture opening that had a diameter of 3.57mm, then the effective aperture value would be f/5.6.  If I put that same diameter aperture on an 80mm lens then the effective aperture is f/22.

To get my 80mm lens to gather the same amount of light as my 20mm lens with an aperture diameter of 3.57mm, I'd have to create a larger aperture opening with a diameter of 14.3mm, then the effective aperture is now the same at f/5.6.

How apertures are labeled can be quite confusing.  The values are a function of both the focal length of the lens and the diameter of the aperture and are expressed as a fraction like f/22 or f/5.6 etc.  The smallest aperture opening will have the largest denominator.  f/22 is a much smaller aperture than f/4, for example.  On the camera you may not see the “f/” notation and may only see the number.  You may also hear the aperture referred to as the f-stop.

Regulating the flow of light through the lens is not the only thing that the Aperture does.  It also plays a critical role in determining how much of your photograph is in focus.

When the aperture is changed, you also change the Depth of Field in your photograph.  This is the distance between the nearest and farthest objects in your scene that appear sharp or in focus.   By shooting at a very small aperture you’ll find quite a bit in focus whereas if you open up your aperture you’ll minimize the depth of field.

The aperture is one of the most powerful tools that any photographer has so take time to understand how it works. 

1/3 Stop Scale

Digital cameras allow you to adjust the aperture size in increments as small as 1/3 of a stop. This gives you much better control over the exposure but may also present a confusing array of f/numbers for the aspiring photographer. 

To truly master exposure controls you need to have a keen awareness of these values - especially in mixed lighting scenarios. One such environment is when you must balance the ambient with your flash. In this case adjustments are often made in fractional stops. 

To add to the confusion, many light meters will present aperture values in decimal - 1/10 stops. All this can be quite intimidating to the photographer. 

This is not something that you want to learn or practice when you're standing in front of a client who is paying for your professional services. Even if you're not a professional, it can be quite intimidating standing in front of 17 extended family members while you're tinkering with your camera - all the while Uncle Louie is using you as the butt of his jokes. 

Certainly experience will help. Set time aside to practice and learn how all this works. Practice, practice, practice.