DIGITAL CORNER
By Bob Dean
Depth of field
I know depth of field is not just a digital topic but in keeping with our plan to address some of the basics I thought I’d discuss the concept. Depth of field is defined as the distant in front and behind the main area of focus that is acceptably sharp. If we look at how an image is “projected” on to the camera sensor we see:
The minimal acceptable sharpness is something called a circle of confusion. If you are really interested in the details check out http://en.wikipedia.org/wiki/Circle_of_confusion
We all know that the aperture setting impacts depth of field. The reason can be seen in the following diagrams:
Light waves travel in straight lines and as we reduce the diameter of the lens opening we really extend the depth of field.
So what can we take away from this that we can apply to our everyday shooting? Well consider the following:
a) The smaller the f stop the greater the depth of field
b) The greater the magnification, the shorter the depth of field.
c) Depth of field is typically 1/3 in front of the point of exact focus and 2/3 behind. This changes as we increase magnification and the good rule of thumb for macro shooting is 50/50.
Go out and play with depth of field. Try images close and far, shoot the same subject with different f stops and different magnifications. You’ll develop a whole new set of creative tools.