Depth of Field: Setting the Aperture to Control the Look of Your Photo

By Patty Hankins & Bill Lawrence

Have you ever seen a photograph where the subject of interest is perfectly in focus and the background is blurred so as not to distract from the subject? Have you wondered how the photographer managed to get the image that way? Chances are, the photographer was controlling the depth of field – which is one of the tools that can make the difference between a snapshot and a “wow!” photograph.

Depth of field is the term used to describe the distance between the closest distance and the farthest distance that is in focus when you take your photo. For any particular focal length lens and distance the lens is focused on, the determinant of the depth of field is the size of the lens aperture (opening), measured by the f-stop. The smaller the aperture (the larger the f-stop number), the greater your depth of field, meaning that a larger distance in front of and behind the point you focus on will remain in focus. If you have a number of elements far apart that you want to all be in focus, then a small aperture (large f-stop number) will help get everything in focus. The tradeoff is that the smaller apertures let in less light, so the shutter speed has to be slower to compensate – this can be a problem if your subject is moving quickly.

Large apertures (small f-stop numbers) result in small depth of field. This can be very useful technique if you want to highlight only part of the image or blur out distracting objects in the background. The eye will gravitate to the area in focus. So if you have a flower in the middle of a field you’d light to highlight, you can draw people’s eyes to that specific flower by using a large aperture. Then the flower is in sharp focus and most of the background will be out of focus.

One thing you need to be aware of if you use a small depth of field, you have to be careful to focus accurately on the point of interest. With the small depth of field, any mistakes in focus will result in the main point of interest being blurry. Patty uses this technique frequently for her flower photography. Choosing a large aperture (often using an f-stop of about 2.8) for a small depth of field allows her to focus on a specific flower while blurring the distracting background. You can see some sample photos of flowers taken with large apertures and small apertures below.

If you want to try this technique with your own photos, you need a camera that allows you to set the aperture – and that has the ability to set a wider aperture to get the smallest depth of field. In general, it is easier to get a small depth of field with one a 35mm camera or a digital SLR than with one of the non-SLR digicams. In general, the digicams have smaller sensors, and use shorter focal-length lenses; this results in a large depth of field even with small apertures. It’s still worth giving a try if you are using a digicam, but the effect won’t be as dramatic

To adjust your depth of field, you need to be able to set the aperture. This means getting the camera out of full automatic mode. You can use either aperture-priority automatic mode (where you set the aperture, and the camera chooses the shutter speed), or in full manual mode, where you set both the shutter speed and the aperture. Depending on the length of the exposure (shutter speed) – you may need to use a tripod to avoid camera shake affecting your image.

Try a couple of shots of the same subject with the largest and the smallest aperture (smallest and largest f-stop numbers, respectively) to see how different the photos look.

Controlling the depth of field is a simple technique that can dramatically affect the look of your photos – and make people want to know – how did you do that?

******************************************************************************

The following pair of photos was taken of the same flower with the tripod in the same position.  The only difference is one was shot at a small aperture (large f-stop number) and one was shot with a large aperture (small f-stop number).  The first photo was taken at f16 (a small aperture, but the particular lens used will go smaller to f32).

Copyright 2005, Patty Hankins.

While the flower that is the central subject is pretty, the background is distracting.  So Patty shot the photo again with at f2.8 (the largest aperture or smallest f-stop number the lens used was capable of).  The photo below shows how using f2.8 gives a much smaller depth of field.

Copyright 2005, Patty Hankins

This photo nicely shows off the tulip against an almost abstract background of out of focus flowers.  The background is much less distracting, and the eye is drawn to the central subject of the photo.

There are also times when a wide depth of field that results from a small aperture (large f-stop number) is desirable, when you want everything from near to far in focus.  Try shooting using different aperture settings on the same subject, and see how the photos differ!