An interesting site

WayneF

Senior Member
LOL

It is just the log of the ratio squared, divided by log 2.

The same calculation (that shows f/stops difference) applies to f/stops, to inverse square law distances, and to flash guide numbers. So I have a similar calculator in those three places on my site, which shows two decimal places, instead of rounding off to third stops.
 

STM

Senior Member
LOL

It is just the log of the ratio squared, divided by log 2.

The same calculation (that shows f/stops difference) applies to f/stops, to inverse square law distances, and to flash guide numbers. So I have a similar calculator in those three places on my site, which shows two decimal places, instead of rounding off to third stops.

True, but can you do that in your head? I sure can't. I HATE math. And a third of a stop is generally regarded as the smallest increment where you can detect a measurable difference in exposure, even using a densitometer like my trustworthy Beseler PM2 color analyzer. Two decimal points is really superfluous. Most camera shutters and apertures are not any more accurate than a 1/3 stop.
 
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WayneF

Senior Member
Can you do that in your head? I can't.

No, me neither. The calculation is the purpose of the calculator. :) What I am attempting to add to the conversation is that the same calculator and same result can be used for fstops, for inverse square law distance, and for guide numbers. But these all work relating to the square root of 2 (1.414), so third stops sort of blurs it.

If you wanted to know how light fell off at 80 feet vs 40 feet, enter f/80 and f/40 there (as if it were distance), and the answer is the correct 2.00 stops.
Or 4.4 feet and 2.2 feet, same 2.00 stop difference.
Double distance is of course 2 stops, inverse square law.
Shutter speed is linear, not the same thing.

And a third of a stop is generally regarded as the smallest increment where you can detect a measurable difference in exposure, even using a densitomer. Two decimal points is superfluous. Most camera shutters and apertures are not any more accurate that that.

Tell that to Sekonic. :) Their specifications for our light meters is to detect and measure to tenth stops. We have a tenth stop mode, which the specs claim to honor.
Can we set it on our camera? No (or rather, we have to round it off). But we can set our multiple flashes to that precision, honing in on their ratio.

The biggest advantage of tenth stops is: How many stops lighting ratio between metered f/6.3 and f/10? Not that easy, and the purpose of the calculator.

If working in tenth stops, it would read as f/5.6 plus 3/10 stop vs. f/8 plus 6/10 stop. So we DO know this in our head, immediately. 1.3 stops.


Here is a tenth stop table using three significant digits (which are necessary to meaningfully create the table):
Photographic Tables, Aperture f/stop, Shutter Speed, ISO and EV



 
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Browncoat

Senior Member
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WayneF

Senior Member
I never get the purpose of that. There is an old saying about better to remain silent than to remove all doubt. :)


I posted this here yesterday:

There are all kinds of posters in the forum.

The type that add too much detail, when so few want to know any detail about anything.

The type that post stating they don't want to know anything, don't bother them.

The type that have to add the same thing as the previous post said, going on record they knew too I suppose.

The type that just have to post something wildly extraneous, to show how cute they are.


I'm not half trying, I'm sure there are many more obvious categories. :)



I will admit to a couple of them. Do you want the other two?
 
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