Understanding how to control depth of field in your photographs is one way to take creative control of your images.
When an image has a large depth field it means that everything from the foreground to the background is in focus.
A shallow depth of field is when only a portion of the image is in focus. This can mean anything from the background being slightly soft to only a razor thin slice of the image being sharp.
As an artist you don’t always want your images to appear exactly as your eye saw the scene. You want to draw the viewer’s eye to something in particular, to emphasize one thing over another in an image.
So, how do you do that? I’ll show you …
Depth of field is controlled by three things:
Aperture is the size of the opening through which light travels to your camera’s sensor and it is expressed in f-stops.
The smaller the f-stop number the larger the opening. That’s kind of confusing isn’t it? It’s because the f-stop number is actually a ratio. But I don’t want to get into too much mathematics here, just remember it’s like a fraction and 1/2 is bigger than 1/8 so f/2 is bigger than f/8.
The larger the opening the more shallow the depth of field. In other words the amount of your scene that will be sharp, measuring from the object you focussed on, is smaller.
These two images illustrate the effect of aperture on depth of field. (Please click on the images to view larger versions.)
Just to make it even more complicated, all this changes with every lens you have!
So the setting of f/4 ,for example, is actually different depending on the focal length of your lens. f/4 on a telephoto lens like a 200mm is actually a larger opening than f/4 on a 50mm lens.
That means when you use your telephoto the depth of field is smaller than the same setting on a mid-range lens. (Remember large opening means shallower depth of field.) Conversely f/4 on a wide angle lens has a much bigger depth of field than a mid-range lens.
Remember the area of the scene that is sharp is measured from the subject you are focussed on. So if you are making an image of a cityscape and the whole city is miles away, you can get away with a larger aperture opening and everything will still be sharp because the whole subject is in the distance.
On the other hand if you focus on something that is a few meters away and part of your scene is miles away then you will need a small aperture if you want everything to be sharp.
When you are photographing be conscious of how you want the background in your image to look and take control of the depth of field!
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