Here is some background information that will be helpful for Thursday's topic. Reading it before the session would help things solidify.
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Definitions*:
Circles of Confusion - When a projected ray of a point of light strikes the film plane in a size larger than a point the cone of projected light is called the circle of confusion.
Depth of Field - The distance between the near and far points in the scene that register as small enough circles of confusion on the film to be seen as sharp points under normal viewing conditions.
Hyperfocal Plane - The plane at which (and beyond which) elements in the subject are deemed to be sufficiently in focus to appear sharp (i.e., depth of field).
Hyperfocal Distance - the distance beyond which all objects can be brought into an acceptable focus. (And an alternative way of saying it - The hyperfocal distance is the closest distance at which a lens can be focused while keeping objects at infinity acceptably sharp.)
The Basics*:
- In reality, it is only possible to critically focus one subject plane, and this subject plane must be parallel to the film plane. Objects further and closer will not be precisely in focus, although they may be considered to be acceptably sharp.
- The focusing distance, the aperature used and the focal length of the lens affect the depth of field. The smaller the aperature, the greater the depth of field. Each time you double the f-stop, you double the depth of field. The longer the focal length, the shorter the depth of field. Wide angle lenses have greater depth of field than telephotos.
- The "one third rule" states that at any point along the subject plane which recedes from the camera, the hyperfocal distance will be closer to the camera by one third of the distance. Again, this is subject to focal length and magnification ratio. The closer the camera to the subject, the narrower the depth of field.
- If you focus closer than infinity, you may have a hyperfocal plane behind the focus point as well as in front of it. The circle of confusion at both hyperfocal planes are equally out of focus.
- If the subject is too thick to get everything in acceptable focus, the photographer normally favors foreground objects because of their prominence. The eye normally anticipates closer items as being sharper than distant items.
- When the focus of a lens is set at infinity, the near limit of the depth of field is referred to as the hyperfocal distance for the aperature used. If the focus is then set at the hyperfocal distance, the depth of field will extend from one half the hyperfocal distance to infinity. When object at infininty must appear sharp, focusing at the hyperfocal distance gives the maximum depth of field. The hyperfocal distance is closer to the lens when the lens is stopped down.
*Information for these discussions and to develop the attached .pdf was taken from: The Ansel Adams Guide - Book 1; Basic Techniques of Photography and Scheimphlug, Depth of Field and Hyperfocal Distance; by Steve Simmons and Ron Wisner; View Camera; November/December 1990.
Check out the following URLs. They provide good information on the subject and the first one provides a calculator you can tailor for your (SLR?) camera and lenses to print a chart of depth of field at various distances and aperatures.
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