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Film - Digital

Depth of Field preview

As a means of calculating the depth of field for any given distance lenses often have a series of markings on them that relate to the aperture setting and are read of onto the distance scale of the camera.


Excluding compact cameras, or view cameras, many professional 35mm film or digital camera cameras have a feature called a depth of field preview button - marked by the red arrow on this camera. When this is pushed in it stops the lens down to the working aperture.

However, when we stop the aperture down to the working aperture, the illumination of the scene in the viewfinder is dimmed because of the reduced light coming through the lens, but the depth of field in scene is viewed in relationship to the selected aperture for the exposure.

When we use a Single lens reflex camera the working aperture that we have selected is only automatically set when the shutter goes off and then it is returned to wide open to let as much light through as possible for ease of viewing. While this gives us a brighter image in the view finder for focusing etc. it does not let us se what the depth of field is like ( areas of focus – if we stop the lens down to a smaller opening like f16 - the area of focus becomes larger ).

This can be useful to see both what is in focus and also out of focus in the image.

For instance, if you want to experiment with images which have no sharp areas in the image ( out of focus images) it is useful to have a camera with this facility.

You can also purchase medium format SLR cameras with a depth of field preview button.

Mouse over to view

View through the camera view finder as the depth of preview button is depressed - notice how the illumination diminishes but the wire in the foreground becomes sharp as the working aperture comes into play.



Three other factors play a part in depth of field:

subject distance

focal length



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