What Is Aperture Priority
In order to understand Aperture Priority, you must know what aperture is. Aperture is the photography term that refers to the size of the camera’s lens opening at the time a photo is taken. It is one of the three factors that determines how much light is allowed to enter the camera, the others being the shutter speed and ISO.
By way of example, let’s say you are in a room with a single window, the only source of light for that room. If the window is quite large you get lots of light. If, however, the window is very small, you will get much less light in your room.
The aperture of a camera is just like the window. A small aperture lets in less light than a large aperture.
The idea is to let in just the right amount of light. If the window (aperture) is too small, there will not be enough light for a good photo. When there is not enough light, the picture is too dark, or underexposed.
If, on the other extreme, the window (aperture) is too large, the picture will be too bright and there will be parts of the photo that are totally washed out (no color or detail). This is know as overexposed.
Aperture, also referred to as the f-stop, is designated as a number preceded by the letter f. So you might see something like f/1.4, f/4.0, or f/11 as the aperture of a picture. Each of these numbers relates to a different size opening of the camera lens.
Here is a chart showing the relationship between aperture sizes.
Did you notice that the larger openings have the smaller numbers? It seems weird that an aperture of f/8 would be tiny compared to a huge aperture like f/1.4. This is one thing you have to get used to in camera-speak. Large numbers referring to aperture are indications of small apertures and vice versa.
Now, Aperture Priority is when you use the camera’s aperture as your main control (priority). The photographer tells the camera what aperture to use for the picture and the camera software determines the shutter speed to get the correct exposure.
(aperture instructional images from Wikipedia – http://en.wikipedia.org/wiki/Aperture)
Using Aperture When Taking Pictures
One of the easiest ways to take control of your camera and the results you get is by using the Aperture Priority setting.
You can control several things through Aperture Priority.
First, you can control the depth of field. Increasing the size of the aperture reduces the range (or depth) of focus. When you reduce the depth of field, the parts of the photo in front of the plane of focus and the parts of the photo behind the plane of focus will be blurred.
The relationship between depth of field and aperture can be quite techie and complicated. The explanation here should be sufficient for basic understanding, but if you would like a simple tutorial on a very complex subject, please look at Peter Carey’s Aperture explanation.
This is a photographer’s trick to get your attention to focus on the main subject of the picture. The sharp area of focus draws your attention to it, and the blurred area kind of blends into the photo, unnoticed.
Photographers use this concept when taking portraits because it is important to draw full attention to the subject of the photo rather than having the viewer’s eye distracted by other parts of the picture.
Another area where depth of field is important is in close up, or macro, photography.
Taking a close up with a wide aperture does the same thing as the portrait idea. It limits the plane of focus so that just the area you want to draw attention to is in focus. For instance, you can isolate the stamen of a flower and keep the petals from drawing the viewer’s eye.
Conversely, when you want to get more of the picture in focus, you want more depth of field, you use a smaller aperture. Again, think about the picture of a flower. If you want the entire flower to be in focus, you simply use a smaller aperture.
Another thing you can control with Aperture Priority is the Shutter Speed.
Larger apertures let in more light, so the shutter speed can be much faster. You can use this concept to freeze the action of a picture. Want to get a sharp photo of you son in action on the soccer field? Use a large aperture.
What if you want to slow the shutter speed? Use a small aperture.
common example of wanting a slow shutter speed is taking pictures of a waterfall or a white-water creek. You want to smooth out the water and have it look really silky. Slow the shutter speed by increasing the f-stop of the aperture (remember, the larger the f-stop number, the smaller the aperture).
Another example for slowing the shutter speed to get a desired effect is that soccer picture. Instead of wanting to stop the action for a clear picture, you may want to get a blurred picture to indicate fast movement. Car racing is another place this technique is used as well.
You can use your imagination to think of ways to use the idea of Aperture Priority in your photography.
To recap, here is a quick list of ways to use aperture priority:
- Shallow depth of field (for blurred backgrounds) – use a large aperture (small f-stop number).
- Deep depth of field – use a small aperture (large f-stop number).
- Freezing action – set a large aperture so that you get a fast shutter speed.
- Blurring action – set a small aperture so that you get a slow shutter speed.