Understanding the Digital Depth of Field

The digital depth of field refers to the distance that is shown across the photograph or image. This actually refers to what is in focus and what is not. Depth of field can be altered to create a background that is out of focus and a selected object that is in focus. Many individuals do not realize that this is not a property of the lens, but of how you take the picture. The depth of field relates specifically to the print of the image; it is impossible to measure the depth of field like measuring the focal length or aperture of the lens.

Focus

There are not 2 different focus areas. The focus occurs only over 1 plane. Everything else in the photo that is outside of the focal plane will be out of focus in some way. While it may seem like the depth of field is related to the background blur, it actually is not. The depth of field cannot be used to determine the blur of the background.

Sharpness

The depth of field relates to the portion of the image that is considered acceptably sharp. When all of the image should be sharp, then a large depth of field is necessary. A small depth of field is used when the subject of the object should be sharp and then de-emphasize the background and foreground of the image. This sharpness is determined by the distance from the camera to the subject. The lens f number, lens focal length and the format size are used to determine the depth of field.

Depth of Field Calculation

If we think about the 3 items that determine the depth of field, it is possible to see how each works. If two different format sizes are used with the same angle and f number and the final picture is the same size, the smaller format size will have the larger depth of field. This is frequently used for digital SLR cameras.  For these 2 images to have the same depth of field, then the final image size will be different. Cropping an image will change the depth of field as will other aspects. The basic equation used to calculated the depth of field is the hyperfocal distance (H), which is equal to the focal length (f) squared, divided by the lens f number. This equation is H = f2/Nc.

Limited DOF

Limited depth of field is used to achieve selective focus, a technique used frequently in nature photography. A specific object in the picture is emphasized. Normally, the f number of the lens will be blurred so the background is blurring. This selective focus can be difficult to achieve if the background is close to the object that should be in focus. With selective focus, it is also possible to use the depth of field to determine how unsharp the rest of the photo should be.

Lens Aperture

The lens aperture is very important to depth of field, as this is factored into the f number of the lens. The aperture refers to the diameter that the lens opens. By reducing this diameter, the depth of field will increase. However, this change will also decrease the amount of transmitted light and increase diffraction. Therefore, aperture can only be used so much for depth of field.