March/April 2006 HelpLine
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?
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.)
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.