Depth of field is the distance range between the nearest and farthest object or person in a scene that will appear in acceptable sharp focus. Three things govern depth of field: lens opening, lens focal length and distance at which the lens is focused. Knowing how to increase or decrease depth of field is an important tool for making a photograph something more than a snap shot. (As an example of narrow depth of field I have attached “Details No. 18″ [close up of cross}, and, as an example of maximum depth of field I have attached "Rocky's Footsteps" [Philadelphia Art Museum].)
The larger the lens opening, the less depth of field you get at a given distance. The area in focus increases as the lens is stopped down. To check this, note the depth of field scale on your lens barrel.
There is a series of aperture numbers repeated on both sides of the center index mark. As you focus, depth of field for any lens opening is indicated between the f/ number on the left and its corresponding number on the right. Focus the lens at different distances and watch the depth of field range increase or decrease according to the distance you set.
Keep in mind that the indications on the camera depth of field scale are only approximate, but adequate to guide you while shooting. Depth of field tables are available for each lens focal length, should you want to study them.
If your lens barrel does not have depth of field markings, you can stop down the lens and look through the view finder.
Don’t let the name scare you. All this is is a way to focus using the depth of field scale on your lens. It varies with focal length, with f-stop and with the photographer’s need for sharpness, but it is easy to use.
You simply focus by setting the infinity mark on the focusing scale at the mark for the f-stop you’re using. The lens is now set at its hyperfocal distance. As long as you photograph things within that range, you don’t need to change the focus at all.
Lens focal length:
The diameter of the lens aperture plus the focal length of the lens are both determine depth of field. There are complex formulas to calculate depth of field, but, for our purposes, you should remember this — the shorter the focal length of a lens, the greater the depth of field it produces at any given aperture. Here is a comparison to illustrate this for two lenses, each set at f/8 and focused at seven feet — for a 35mm lens, depth of field runs between 5 feet and 12 feet, for a 135mm lens, depth of field funs between 6.3 feet and 7.8 feet.
Note: While it is true that these two lenses produce quite different size images of a given subject form seven feet, it is also true that if you move back far enough with a 135mm lens to get the same image the 35mm lens produces at seven feet; depth of field will be equal.
With any lens, the closer you focus to an object, the less inherent depth of field. In reverse, the farther you focus from lens to subject, the greater the depth of field.
For more information about depth of field, and to find depth of field calculators, as well as hyperfocal distance chart makers, I recommend a visit to johnhendry.com:
- Introduction to DSLR photographyJuly 10, 2013