Dave Cohen wrote: > Shutter speeds are another fiasco entirely. Anyone care to dig into > that one? Sure, why not.
Shutter speed adjustments are just like aperture adjustments -- the idea is to double or halve the amount of light striking the film. Since there is a linear relationship between the time the shutter is open and the amount of light striking the film, shutter speeds are a breeze, as compared to apertures, where the amount of light is related to the square of the diameter of the aperture. So, I suppose long ago, somebody figured it would be nice if your 'standard' time measurement was 1 second. If we want to get faster than that, then we just keep cutting the time in half, so we go 1/2, 1/4, 1/8, etc. Notice the nice progression of powers of two in the denominator. Now, beyond 1/8, things get goofy. Prior to that point, the speeds indicated on your dial match our little series here, but the next step is where things get different. If we continue to cut the time in half, we would get: 1/16, 1/32, 1/64, 1/128, 1/256, 1/512, 1/1024, etc. Of course, our speed dials show: 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000 I guess camera designers figured people would flip out if they saw a shutter speed of 1/512th or something, so they just called it 1/500th. Now, does any of this matter? Let's see, the biggest discrepancy is at the 1/15 vs. 1/16. Here, we are talking about the difference between 66.666... msec and 62.5 msec, meaning the indicated speed is about 6.7% longer than we would expect. In terms of f-stops, this is about .0931, or about a tenth of an f-stop of error. At the fast end, 1/1000 vs 1/1024 is the difference between 1000 usec and about 976.6 usec, or about 2.4%, which works out to about .034 f stops -- hardly worth mentioning. So my guess is, the goofy shutter speed numbers are just there out of convenience. In reality, I've NEVER seen a camera that could hold a tenth of a stop accuracy, or a human eye that could detect a tenth of a stop of exposure difference. Therefore, I conclude the preceeding has been nothing but an excercise in academic curiousity. Personally, I'm still waiting for a camera with shutter speeds marked as slow, fast, and smokin'. Now, is a kilometer EXACTLY 3280 feet 10 inches, or what? ;) Jim "Bored Mathematician" Gilley E-mail: [EMAIL PROTECTED] See "Grumpy's Home Page" at http://www.eesoft.com --> SPORRS: Serious Photographers of Railroad Related Subjects
