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Alan Mund Offers a Correction to Inside the Camcorder Lens

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Charity Pays

Just a note to commend you on your editorial entitled "A Video Exercise in Good Will" (Viewfinder, February, 1997). Assisting charities can be truly rewarding, and it is certainly good practice for the amateur videographer. You may also get a bit of word-of-mouth business out of it as well, such as an occasional wedding. I have assisted our church for several years now, and found it to be very worthwhile.

I must also add that I thoroughly enjoy your publication. It always seems to be where I'm at concerning gear or technical issues. Keep up the good work.

Mendal Slack
Internet


How Deep Is Your Field?

I feel that your article "Inside the Camcorder Lens" by James Hawes in the November 1996 issue leaves out an important piece of information about depth of field.

Different focal lengths vary in their depth of field only when you're shooting from the same distance. The resulting shots are widely differing in composition and image size. When you shoot the same image size with different focal lengths, you will discover that the depth of field for each focal length is exactly the same for all. Only the perspective varies with each focal length when you're shooting identical image size.

For example, to shoot a head-and-shoulders shot with a wide-angle lens, you'll have to move in closer, and thus lose some depth of field. If you're shooting with a telephoto lens, you'll have to move back, and thus gain some depth of field.

When you're dealing with depth of field, like most things, you don't get something for nothing. Since image size is of primary importance in composing your shot, I find this aspect of depth of field worth mentioning.

Alan Mund
Vista, California

The three parameters that affect depth of field are the iris, the distance to subject, and the focal length of the lens. All else being equal, the smaller the iris opening, the greater the depth of field. The larger the distance to subject, the greater the depth of field. And the shorter the focal length of the lens, the greater the depth of field.

Let's put aside the iris' effect and examine the effect of distance to the subject and lens focal length.

In your same-image-size example--the two head-and-shoulders shots--it sounds like you're saying that the difference in depth of field between the wide-angle-lens shot and the telephoto-lens shot is exactly compensated by the change in depth of field due to the different distances to subject that these shots require. Hmmm...an interesting theory. We do agree that these two parameters have opposing effects on depth of field, but, after much in-depth research (pun intended), we couldn't verify that distance to subject has as great an effect as focal length. In our experience, focal length has a much greater affect on depth of field than distance to subject.

Please forgive our skepticism. And if you find verifiable proof of your contention (a mathematical formula would do nicely), please share it with us.

--The Editors


Preserving Images for Posterity

I have read with great interest Matt York's Viewfinder columns in Videomaker Magazine, and I appreciate his interest in preserving those precious moments in his family life for posterity.

It is sad, however, that by choosing videotape you are destined to relegate these memories to the scrap heap because of videotape's limited archival stability. Most experts think 10 to 15 years is about what you can count on. Unfortunately, millions of others who place their trust in video cameras are being similarly deluded.

I just projected 8mm Kodachrome movie film that is 40 years old, and has been stored at ordinary room temperature, and the images were as bright and clear as when they were first taken. Why don't you urge your readers to shoot Kodachrome 8mm or Super 8mm film if they are interested in long-term preservation of their memories? To do anything else is to mislead a gullible public taken in by marketing glitz, simplicity and false economy.

Seymour Rosen
Marianna, Florida

We're glad to hear that you enjoy the Viewfinder column, Seymour, but we think your condemnation of videotape as an archive for family memories is premature.

First, we should point out that that film isn't immune to the ravages of time, especially color film. While the silver oxide in film (which creates the grayscale portion of film images) is longlived, the dyes used in color film fade relatively quickly. This is why you hear a lot of hoopla from Hollywood when they restore color movies from the '40s and '50s.

As for videotape, experts disagree as to its archival capabilities, but we think 15 years is a very conservative figure. Some of us have videotapes from the '70s that look and sound just as good now as the day we recorded them.

But, if you still feel a little skittish about your videotape's shelf life, you can now transfer your videos to a digital video format like DV without losing any quality in the process. And, in a few years, we'll be able to transfer our precious videotaped memories to compact disc very affordably, where they'll last for generations.

--The Editors

Tags:  August 1997
Donald
Keller
Fri, 08/01/1997 - 12:00am