You challenged Alan Mund (August "In Box") to find a mathematical formula to prove his assertion that if image size is the same, depth of field will be the same. This important fact is so little known that many professionals not only dont know it, but dont believe it when theyre told. Ansel Adams, in various editions of "The Camera," gives the relevant formulas:
near limit of depth of field equals Hu/H+(u-f) and
far limit of depth of field equals Hu/H-(u-f). Where:
the hyperfocal distance which equals F2/fc
F = focal length
f = f-stop number
c = circle of confusion
u = lens to subject distance
For a four-inch focal length lens at f/8, focused at 16 feet and assuming a circle of confusion of .001 inch, we can calculate a far limit of 17.6 feet and a near limit of 14.6 feet. Subtracting these gives a depth of field of 3 feet.
These formulas will work for any distance where they are really needed but the formula for the far limit of the depth will go to infinity as the distance approaches the hyperfocal distance.
Thanks, Bob, and hats off to your excellent scholarship. We owe Mr. Mund a commendation, too, for catching us asleep at our desks. Did you all get that? There will be an exam on this stuff later in the semester, so pay attention and take notes.
Most of the terms in Bobs letter are fairly familiar and often seen in the pages of Videomaker. One exception is the circle of confusion, which describes the way in which spherical lenses tend to splatter light across a tiny area rather than focus them on a sharp point. Its also the realm that you enter when you pay too much attention to the numbers and forget to just shoot what looks pleasing to your eye. The formulas are very nice and all, but its sometimes just as important to forget them as it is to learn them.
Vince Finds a Friend
I read your column about Vince Codac ("Pause," September, 1997)and his need for friends and I got a chill. I have had the same problems with my board, a Bravado 1000 that I purchased over a year ago.
I wrote [Truevision] tech support and they said I should uninstall the Bravado software and drivers and then re-install them. I tried that and it didn’t work. I then was told to remove all of my ActiveX drivers on my PC. No success. I was then told to uninstall the software and re-install Windows 95A. I did that last weekend and still it did not work. I then went in and tried to capture video again and the Bravado codec was no longer there. I just received an e-mail today telling me to reset all of the settings in Windows 95, which I have done, and it still does not work. I cannot get the Bravado codec to re-install.
Although I have gotten responses from Truevision and done everything they asked, I have never had a situation where nothing worked and there did not seem to be an answer.
This is extremely frustrating. Believe me Vince, you have a very empathetic friend in me.
Paul A. Bokros
More ActiveX Trauma
I want Vince to know that hes not alone. I can only add to his ["circle of?" –Ed.] confusion since I’ve also been unable to find a solution.
The problem starts when I place a clip onto the construction window in Adobe Premiere 4.2. Any attempt to use the pointer at the top of the window or to play a clip results in an immediate lock-up. Interestingly, the sample video clips (using Indeo compression) that come with Premiere, do not cause this. Its only with clips I’ve captured in MJPEG that the problem occurs.
I’ve updated the Bravado 1000 drivers from Truevisions Web site. I visit Adobe’s site regularly to download info, tech tips and FAQs. I’ve expunged and reloaded everything several times–the problem remains.
I’ve paid several guys who had appeared knowledgeable. The closest thing to a solution was the guy who loaded the 4.0 version of Premiere. IT ACTUALLY WORKED LIKE ITS SUPPOSED TO!! And then I blew it. I swore I’d immediately back up everything onto my tape drive as soon as I got home. But, like an idiot, I just had to read my e-mail, so I launched Internet Explorer. Guess what? Immediate lock-up with any clip on the construction window. And the guy at the store won’t return my calls.
Thanks, Paul and Alex. My therapist Monty was right. Your letters of commiseration console, proving that a sorrow shared is half the sorrow. For myself, Ive ditched the card and moved on to a less expensive capture board that works with ActiveX. For you, I can only recommend the caring ministrations of Monty.