A "stupid" question about the brightness and aperture

Started Sep 27, 2012 | Discussions thread
JeffAHayes
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Re: A "stupid" question about the brightness and aperture
In reply to MaxLowLight, Sep 28, 2012

To add to what was said above, each "official" F-stop lets through exactly HALF (or twice -- depending on which way you're going) as much light as the one below or above it, and every OTHER F-Stop is always an exact DOUBLE of the number of the F-stop two below it.

So, assuming you could build an F1.0 lens (I read a post by someone earlier that Canon once had one). Such a lens would be capable of letting through 100% of light. There's some sort of algorithm I don't quite comprehend involved in the escalating numbers for increasing F-Stops, but the next FULL F-stop is F1.4 (which as most know is the "fastest" commonly available lens, although Canon still has a 50mm F1.2 -- a half-stop faster). Then comes F2.0 (twice 1.0 and two stops slower, or 1/4 as much light). F2.8, then F4.0 (and F4.0 again lets through 1/4 as much light as F2.0, or 1/16th as much as a theoretical F1.0 lens)... F5.6 is twice F2.8 and two stops slower, or 1/4 as much light; F8 is twice F4.0 and two stops slower, again 1/4 as much light, and so forth.

Back in the "old days," when we all had film cameras that had manual dials on them, the FULL STOPS were all we had to deal with. These days, digital cameras give us not only half-stops, but even 1/3 stops, so we can set them to things like F6.3 and all sorts of "F-Stops" I could have never even imagined 20 or so years ago.

I generally still keep my K-5 set to half stops, though. To me going in increments any smaller than that is really just overkill, since we can make such major adjustments in post processing that the difference between 1/2 and 1/3 or 2/3 of a stop is entirely inconsequential.

Of course, the amount of exposure remains the same if, and only if you double the length of exposure every time you go up one full F-stop OR double the ISO, or do some combination of the two. There are possibly millions of different combinations that can be used to arrive at various exposure levels by differing aperture, shutter speed and ISO, but many (including me), tend to shoot at specific F-stops most of the time and also often at specific ISOs, and change only shutter speed.
Jeff
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