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Depth of field preview button

It allows you to see and adjust the effect of hyperfocal distance

The arrow points to the depth of field preview button on a Nikon F2 camera. If equipped with one, your camera's button (or switch) may be in a different location.

The depth of field preview button on your camera allows you to check the depth of field before shooting and to make adjustments as necessary. Press the button and the lens stops down to the preselected aperture to let you see through the camera's viewfinder how much is in or out of focus at the selected aperture setting.

It is particularly helpful when using hyperfocal distance for your photography.

How can an image that doesn't look to be in focus when looking through your camera's viewfinder actually be in focus when you take the picture? This occurs when you apply hyperfocal distance to determine what will be in focus. If you have a depth of field preview button or switch on your camera, here is an exercise you can do to help you understand how the principle of hyperfocal distance works.

LEARN HOW HYPERFOCAL DISTANCE WORKS

You may wish to copy the following and paste it into a word processor to print it out so it is handy when you carry out the exercise.

Set your camera's aperture to ƒ/8. Look through the viewfinder at a bright highlight on a subject, say a tiny point of reflection from something shiny, and turn your lens' focusing ring so the highlight is way out of focus. The highlight will appear as an enlarged disk of light.

Now begin to bring it into focus, and you will notice it progressively reduces in size until it reaches the position of exact focus, when the highlight has changed from a disk to a point. If you keep on turning the focusing ring of your camera past the position of exact focus, the highlight soon expands again into an out-of-focus disk of light.

You are seeing that the highlight starts as a point when in focus and increases in size as a disk on either side of the point of sharp focus.

Good. Now bring it back into focus again, then turn your focusing ring only slightly one way or the other, so that the point of light becomes a tiny disk, but the image is still fairly sharp, although actually out of focus.

Now activate your depth of field preview button or switch, and you will notice how the tiny disk has become an in-focus point of light again. So, even though your picture is not in apparent sharp focus as you view it through the lens, it comes into focus because the aperture closes down and brings it into acceptable focus.

Although the image you see in your camera's viewing screen will appear to be out of focus, using your depth of field preview button will show that it will actually be in focus when the picture is taken.

The depth of field preview button is helpful when you wish to have shallow depth of field (as shown above) or a great amount of depth of field where everything is in focus.

Remarkable, but it works.

You can see it because, although your camera focuses with the aperture wide open, the lens automatically closes to the selected ƒ-stop when you activate your depth of field preview button and also, of course, when the shutter is depressed.

The depth of field preview lets you see what the film or digital camera's sensor "sees" when the exposure is made.

Using the depth of field preview button at a small aperture like ƒ/11 or ƒ/16 darkens the overall scene in your viewfinder. This is not unusual, although it does sometimes make it difficult to visually assess the amount of depth of field. It occurs because the small opening lets in less light than a larger, wide-open aperture does.