The depth of field refers to how much of your image is in focus. If you have a shallow depth of field less of your image will be in focus. This can help to draws the eye to the subject of your image. If you have a large depth of field it means that more of the image is in focus. This is a great technique if you have more than one subject in your image.
OK, first things first, let's cover some terminology you may have seen.
Let's look at some examples of what the same set up would look like shot at different aperture values. For this I have placed Dylan in the foreground and Millie in the background. I have set my camera to Aperture priority mode as this is the setting that I am wanting to control. The lens I used was my 50mm prime lens as this has the widest aperture range of all my lenses.
The following image was shot with an F-Stop value of 1.8, as you can see Dylan is in focus in the foreground but Millie is out of focus in the background.
As I increase the F-Stop Value Millie is drawn more into focus. This image is shot with an F-stop value of 5.
And finally this image was shot at F/22, the top end of the scale. You can see that the depth of field is very large here and most of the background is also in focus.
The aperture controls how much light the camera is allowing into the image sensor, much like the iris of your eye.
Our first image has a lower F-stop value with the 'iris' being more open to let in more light, therefore the image has a shallow depth of field leaving only Dylan in focus. Our last image has a higher F-stop value with the 'iris' being more closed to let in less light, therefore the image has a wide depth of field allowing both dogs to be viewed in focus.
Below is a graphic explaining how this looks.
Aperture GraphicImage from http://www.imagemaven.com
Now, it can get a bit confusing as aperture is referred to by how open the lens or "iris" is. If you hear someone referring to shooting with a large aperture, they are referring to a low number; and vice-versa.
When the camera is set to aperture priority mode it works out the best shutter speed for you to get a correctly exposed image. It is worth remembering that you can still change the ISO value if you need to (we will cover ISO values in another post shortly). For example, you may need to increase the ISO value if you are in a dark room.
The aperture priority mode is great if you only want to focus on the field of view in your image and you are not worried about motion blur. This mode allows you to control if your background is blurred, which looks great in portraits; but it also allows you to use a large depth of field which is great for landscape images. So, this mode would work better for portraits rather than for action shots.
On most cameras you have the option to change the priority mode, even no point and shoot cameras! This mode will usually be marked as "A" or "Av" on the menu.
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