Canon Mode Dial
Aperture is the size of the opening in the lens while a photo is taken.
Hello my name is Geo. I imagine your first thought is who am I in the world of photography. Well, I am nobody famous, just an average guy whose passion is photography. With my passion for photography I would like to embark on a journey of exploring the secrets of fantabulous photography with you, the reader.
The aperture mode on your camera is a semi manual mode. When shooting in aperture priority you specify the aperture and ISO; the camera's metering determines the corresponding shutter speed. You also have control over the white balance, metering and picture style. I suggest since you are in the process of learning to shoot in manual modes that you leave your camera in the evaluative metering mode, AWB (auto white balance) and standard picture style settings. These settings actually all do very well. Once you are more advanced you may venture into these settings and learn your options.
One thing that causes many newer photographers confusion is that large apertures (where lots of light gets through) are given a F/stop with a smaller number like F/2.8. A larger F/stop as a F/22 has a very small opening greatly restricting the amount of light that gets through.
Depth of Field (DOF) is what aperture is fantabulous at creating. A F/stop of F/2.8 will keep your focus point nice and crisp while everything around that focus point is fuzzy. This is an artsy thing that could be created in Photoshop, only capturing this effect in camera creates a much more natural look and feel which can be almost magical.
A F/stop of say F/22 has an infinity type of effect where everything is in focus. I have a couple of not so magical shots I took in my kitchen on the right. The fuzzy one on top was shot with a F/stop of F/2.8. The bottom photo was shot with a F/stop of F/22. Which you can see is more in focus overall. I shot these two photos inside on a dreary day with minimal light. F/2.8 let quite a bit of light in so I was okay with a low ISO. When choosing your F/stop you do have to consider your light source. With the photo I shot at F/22 I had to raise the ISO to 1600 because a F/stop of F/22 greatly limited how much light was coming through the lens. You can see the various F/stop openings in my illustration on the right. F/16 is the smallest opening there, so you could imagine that a F/stop of F/22 is even smaller than the F/16. Very little light would come through.
The best way to understand aperture is to get your camera out and do some experimenting. Go outside and find a spot where you have items close to you as well as far away and take a series of shots with different aperture settings from the smallest setting to the largest. You will quickly see the impact that it can have and the usefulness of being able to control aperture. Depending on your light source you may have to increase your ISO.
Some styles of photography require large depths of field (and small Apertures).
In most landscape photography you will see small aperture settings (large numbers) selected by photographers. This ensures that from the foreground to the horizon is relatively in focus.
In portrait photography it can be very handy to have your subject perfectly in focus but to have a nice blurry background in order to ensure that your subject is the main focus point and that other elements in the shot are not distracting. In this case you would choose a large aperture (small number) to ensure a shallow depth of field..
Macro photographers tend to be big users of large apertures (small numbers) to ensure that the element of their subject that they are focusing on totally captures the attention of the viewer while the rest of the image is completely thrown out of focus.
Thank you for listening, Geo Beck.