To begin, let’s define “aperture“. An f-stop is a measurement of the aperture. For example, f/1.4, f/2.8, f/5.6, f/8, etc. These numbers are a measurement of how wide the camera lens’ diaphragm opens up in order for light to pass through. Because these are fractions, the smaller numbers are actually the biggest openings. In the list above, f/1.4 is the largest aperture. Choosing the right aperture is important for two reasons:
1) The aperture affects the depth-of-field, and
2) The aperture controls how much light enters the lens.
In regards to depth-of-field, large apertures such as f/1.4 have a very shallow depth of field. A shallow depth of field is great for portraits because it creates beautiful bokeh, which is a blurred background. Portrait images tend to look better when the focus is on the subject, and the background elements are blurred or not distracting. Although a large aperture is great for portraits, the photographer must be careful to make sure that the depth of field is adequate for his subject and the effect he is trying to achieve. Because group photos require a greater depth of field to make sure everyone is in focus, a slightly smaller aperture such as f/4.0 would usually work best.
The amount of light that enters the lens and hits the camera sensor is very important. By changing the aperture – the shutter speed and ISO setting are affected. For example, if the aperture is changed from f/1.4 to f/4.0, then a lot of light would be lost. To compensate, the shutter speed must be increased to a slower speed in order to take in more light, or the ISO setting must be increased to make the camera sensor more sensitive to the light. On the other hand, if the aperture size is increased from f/4.0 to f/1.4, there would be a lot more light entering the lens. The shutter speed would then be decreased to a faster speed, or the ISO setting would be adjusted in order to decrease the camera sensor’s sensitivity to light.
Keep in mind that you should be shooting on the Av mode on the SLR camera’s mode dial, in order to be able to adjust the aperture on the fly. Let’s take a look at some example images below and consider what aperture works best for each image.
When you look at a landscape, do you expect everything to be in focus or just one part of the scene? Most likely, you’d expect everything to be in focus! There is a lot of natural beauty in the natural world. A depth of field that covers all or the majority of the scene is ideal in order to capture the whole scene. What about choosing the right aperture? Although the exact perfect aperture to use for a landscape is different in every scene, a good starting point is to set the aperture at f/11. If you need more depth of field, set it somewhere between f/11 and f/16. I personally wouldn’t go past f/16 because then the lens suffers from diffraction, which softens the image noticeably.
Gorgeous bokeh of night lights over a body of water… How was this effect achieved? It isn’t as hard as you might think! The city lights are out of the lens’ depth of field, therefore the lights become large circular shapes (the shape depends on how many diaphragms your lens has). The photographer just needs to set an f-stop with a low depth of field, such as f/1.8 or even f/2.5. Play around with different f-stops to achieve your desired effect.
Choosing the right aperture is very important with portraits. (In this case, an animal portrait). The depth of field is often very minimal in portraits. Focusing on the eyes, the depth of field extends from the neck and to the end of the giraffe’s mouth in the photo above. Depending on the lens and how far away you are, the f-stop setting would change. Let’s pretend the photographer was using an 85mm lens on a full-frame camera body, 20 feet/6 meters away from the subject. A good starting point would be to set the aperture at f/4.0. Be careful not to overdo it though! A low depth of field needs to have pinpoint accuracy when focusing.