What are Depth of Field and The Plane of Focus?

Depth of Field

A basic definition of Depth of Field (DoF)is: the area of acceptable sharpness within a photo that will appear in focus. In every photo there is a certain area of your image in front of, and behind your subject that will appear in focus. It is the distance between the nearest and farthest objects in a scene that appears acceptably sharp in an image.

If everything is in focus then you have a large DoF. If very little is in focus then you have a shallow/small DoF. It might be easier to remember this simple concept: The lower your f-number, the smaller your depth of field. Likewise, the higher your f-number, the larger your depth of field. For example, using a setting of f/2.8 will produce a very small or shallow DoF while f/22 will produce a big or deep DoF.

The Aperture you use is a big factor in determining how much DoF you get. A small F-stop will produce a small depth of field. A large F-stop will produce a large depth of field. Another factor you need to keep in mind is the length of your lens and how close you are to your subject. All three of these factors determine how much DoF you end up with in your photo. The closer your subject is to the camera, the shallower your depth of field becomes. Therefore, moving further away from your subject will deepen your depth of field.

DoF is not always evenly spaced throughout your image either, it’s usually about one third in front and two thirds behind your focal point, but as your focal length increases it becomes more equal.

DoF is often the most difficult of the three variables to learn, and people often get confused. Try shooting the same simple set up like a flower in a vase and change F/Stop to see the difference you get at each, from the same distance from your subject. Don’t forget to adjust your ISO and Shutter Speed.

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The Focal Plane

Focal Plane and DoF (Depth of Field) often go hand in hand. The Focal Plane is directly dependent on your DoF. The Focal Plane is the level within your image at which everything is in focus. It is the same level as your Point of Focus. Everything beyond that level will fall off into blur. The smaller the F/stop the smaller the focal plane will be, the higher the F/stop the larger the focal plane will be. This is in regards to the horizontal plane, not vertical.

If you are shooting on a wide open aperture (Small number) like an F/2.8, you have an small or shallow depth of field and your plane of focus will be quite small. If you are shooting on a mid range aperture like F/9, then you have a much deeper depth of field and your plane of focus will be a much bigger. With macro, because we are shooting so close to our subject, the DoF and focal plane are much closer together than they would be if we were pulled further back to get more of our scene in our photo, for say a landscape has a much larger focal plane as it may be several Kilometers (or Miles) away, than an object 10cm (or 3 inches)away, even at the same Aperture. Set up an experiment like the one below and try it for yourself. No need for anything too fancy, just enough so you can tell what is in focus and what is not.

Learning the Focal Plane and DoF is often the hardest part of photography and learning what to use in which situation. If you want more in focus, use more (larger) F/Stop. Now just to be really confusing a small number is actually a wider aperture on your lens. It works out this way as F/stops are actually fractions. Another complex mathematical equation,

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