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Affecting Depth Of Field
Q) I have two questions for you. First, in the
September 2004 issue, you stated that depth of field is related to
distance, focal length, aperture and size of the print. I agree with
the first three parameters, but I don’t see how the size of the print
affects depth of field. Once depth of field has been captured on the
film or digital media, it’s fixed in relation to other objects in the
photo and shouldn’t increase or decrease with the size of the print or
size of the projected image, which is essentially a print shown on a
wall or screen. Am I missing something here?
Joe Berke
Via e-mail
A) I can understand your confusion regarding depth of
field. If something is in focus in a specific “range,” then it’s in
focus, right? Why should print size matter?
Welcome to the world of optics. The human eye can’t distinguish very
small degrees of unsharpness, so the true definition of depth of field
is the range of distance around the focused subject that’s acceptably
sharp (note: acceptably sharp!).
That’s where the final print size comes into play. Larger print sizes
help the human eye see the unsharpness. (It’s generally accepted that
depth of field is calculated using both an assumed film or sensor size
and an assumed print size. By assuming a print size, the amount of
magnification is entered into the formula for determining depth of
field. Technically, this is an extreme oversimplification of the
science, but I hope the explanation helps.)
What does this formula mean to the average photographer? A 4x6-inch
print has a different appearance of sharpness and unsharpness than a
12x18-inch print, resulting in what looks to the viewer to be different
amounts of depth of field. (In reality, for many photographers, it
isn’t the depth of field, but slow shutter speeds that capture camera
jiggle that then make for unsharp images at larger print sizes.)
USB Effects
Q) My second question relates to the USB connections. I was
told by a camera company tech support person that I should unplug any
USB devices from the computer when I’m not using them. He said the
computer runs better when it doesn’t have to look for USB devices that
are turned off and not in use. Is this true?
A) As to your question regarding USB, we need to recognize
that the tech person is probably used to dealing with customers who are
having problems with the USB connection between camera and computer
(this is one of the reasons why I always recommend using a media reader
instead). The tech’s answer is useful for solving USB problems, which
makes the computer run “better,” in a sense.
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