How to Set Aperture for a Blurred Background Using a dSLR Camera. Photographs with blurred backgrounds make the subject stand out when you compare them to snapshots with everything in focus. The blurred background can take regular photos from average to extraordinary. By changing the aperture setting you can adjust how much, or how little, of the background remains in focus. Creating this effect is fairly easy using the Aperture Priority mode on a dSLR camera.
Turn your camera on after installing the lowest aperture lens on the camera body. Aperture is sometimes referred to as an f-number. The f-number of a lens is shown in ranges with some of the popular portrait lens apertures starting between f/1.2 and f/1.8.
Set your camera to the "Aperture Priority" mode. This mode allows you to select the aperture while the camera automatically selects the correct shutter speed for your selection. If you don't know which setting is Aperture Priority, consult your camera's manual. On Canon dSLR cameras the setting is AV on the dial; many other dSLR cameras use A to signify Aperture Priority mode.
Test your aperture by starting with a few high f-number settings, such as f/20, and take a picture. You can change the f-number using a dial that is located towards the shutter button on Canon dSLR camera or towards the viewfinder on other dSLR camera brands.
Step your f-number down and take another picture. f/10 shows a small difference when you compare it to f/20. Notice how the background is becoming more blurred as you widen the aperture of the lens. The lower the f-number the wider the aperture, so the more blurred the background becomes.
Set the aperture to an f-number just above the lowest setting. You can see a remarkable difference in the depth of field if you take a picture at f/1.4 and compare that to the first picture you took at f/20.
Decide how blurred you want the background. Practice and personal photography style can help you learn and decide what aperture you like for different photos.