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Using depth-of-field preview
24 October 2007 10:13
The image you see through your DSLR’s viewfinder is shown using the lens’ maximum aperture (usually f/4 or f/5.6). So, whenever you set a smaller aperture there’ll be more of the scene in focus than you’d originally seen. This can mean that elements in the background or foreground that were blurred and out of focus in the viewfinder may suddenly become distracting in your final shot.
Most DSLRs offer a useful feature called depth-of-field preview, which gives you a much better idea of how the final image will look by stopping the lens down to the aperture you’ve set.
To use it, first compose your shot through the viewfinder as normal but ensure that you’ve set the aperture using either manual or aperture-priority exposure mode. At this stage, the image in the viewfinder will be shown at the lens’ maximum aperture, so you won’t see the effect of the aperture setting.
Now press the depth-of-field preview button. This will stop the lens down to the aperture that you’ve set on the camera, but the image will look much darker than the normal viewfinder image. To see what’s sharp, look through the viewfinder while keeping the button pressed, giving your eyes time to adjust to the darker view.
After a while you’ll be able to make out more detail in the viewfinder. Start looking around the subject to check how much of the image is in focus and whether any distractions are visible. The smaller the aperture, the darker the image, so the longer you need to wait for your eyes to adjust.
It’s much easier to see the effects of this feature if the camera and subject are static, allowing you to make precise changes to the settings or subject before you take the final shot. Mounting the camera on a tripod will help you make sure you don’t accidentally change your viewpoint.