Depth of Field Aperture

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.

Complete Digital Photography

Home Page > Photography

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 trade off 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.

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?

Patty Hankins & Bill Lawrence are the co-owners of Hankins Lawrence Iimages LLC, a digital photography company based in Maryland.

Photography