Aperture offers some control of focus blur
The third homework assignment I give students in my digital photography class at the Delaware Art Museum is to separate subject from background by using small aperture numbers; that is, apertures with little depth of field. The subject is in focus while the background is out of focus, blurred. A Japanese word for out-of-focus points of light is “bokeh”.
“Aperture” is a fancy word for lens opening, the hole in the lens that lets light into the camera. Because it varies in size, it can let a lot of light into the camera or very little light.
Technically, aperture numbers are fractions like f/8. An aperture of f/8 is a hole 8 times smaller than the focal length of the lens. If the focal length of the lens is 24mm, the aperture is 3mm at f/8. Most lenses have an aperture range such as f/4.5 to f/22. Because apertures are written as f/…, they are also called f-stops.
Aperture settings determine depth of focus, how many things are in focus, near and far. When the aperture number is large, lots of things are in focus near and far. When the aperture number is small, very little is in focus. Another way of saying this is that small aperture numbers produce small depths of focus while large apertures produce large depths of focus.
Out-of-focus blur and motion blur sometimes look the same, but can look very different. Often both kinds of blur are present to some degree and can be controlled to some degree.
The easiest way to vary your camera’s aperture is to put it in Aperture Priority Mode. For Canon, that’s Av on your command dial; for Nikon and Sony, A. (NOTE: Nikons have “A” and “M” settings on the side of their lenses. This controls Auto or Manual Focus of the lens and has nothing to do with Aperture or Manual Modes of shooting.)
Depth of Focus
You’d never guess it, but in addition to controlling how much light gets into the camera, aperture settings also determine depth of focus, also known as depth of field.
When the aperture number is large, lots of things are in focus. When the aperture number is small, very little is in focus. Another way of saying this is that small aperture numbers produce small depths of focus while large apertures produce large depths of focus.
Out-of-focus blur and motion blur sometimes look the same, but not always. Often both kinds of blur are present to some degree and can be controlled to some degree.
Recipe for out-of-focus backgrounds
(1) Use a low aperture number. This blurs the background more than a high aperture number.
(2) Use a long focal length, not wide angle. If you have a zoom lens that goes from 18mm to 55mm, use the 55mm setting. That is, zoom in. If you have a longer lens, perhaps a 300mm lens, use that. Again, zoom in! If you have a micro or macro lens, try that. Those lenses often have very little depth of focus.
(3) Make sure the subject is much closer to you than the background.
Below are some images in which the subject is in focus and the background blurred. Many were taken with a fixed 50mm lens at f/1.4. In the photo of the horned tomato worm above, the white glow behind the worm is actually a state highway, the asphalt brilliant in the sun. Click on any of the images to see how I paired them with text.