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__________________________________________________________________ ADavidhazy
> Many years ago, I had a copy of the 'official' f/stops, as promulgated by
> the (then) ASA. This chart listed the f/stops from f/1.0 through f/64 in
> 1/4 and 1/3 increments, and was quite useful in calculating things like
> guide numbers, exposure increases/decreases and DoF ranges. Can you point
> me toward a source for such information?
I have not been able to locate a reference to this but here is my version of
it. I rounded off where it seemed appropriate to me. The difference between 1/3
and 1/4 stop is very small indeed - as well as between 2/3 and 3/4. Often when
one rounds off these end up giving the same number so I did not round off all
the time ... although I probably should have in spite of the fact some numbers
then for 1/4 and 1/3 or 2/3 and 3/4 are the same. Note that when the f-stop
number increases by 1/4 stop the exposure decreases by 1/4 stop assuming a
constant exposure time.
(use courier font to see the table arranged properly)
1 1.4 2 2.8 4 5.6 8 11 16 22 32 45
+1/4 1.09 1.5 2.18 3 4.4 6 8.9 12 17.8 24 35 48
+1/3 1.1 1.6 2.2 3.1 4.5 6.3 9 12.5 18 25 36 50
+1/2 1.2 1.7 2.4 3.4 4.7 6.7 9.5 13.3 19 26 38 53
+2/3 1.25 1.8 2.5 3.6 5 7 10 14 20 28 40 56
+3/4 1.3 1.8 2.6 3.6 5.2 7.3 10.4 15 21 30 42 58
+ 1 1.4 2 2.8 4 5.6 8 11 16 22 32 45 64
... and here is a slightly adjusted set up with just 1/4 stop increments:
1 1.4 2 2.8 4 5.6 8 11 16 22 32 45
+1/4 1.1 1.5 2.2 3 4.4 6 9 12 17.8 24 35 48
+1/2 1.2 1.7 2.4 3.4 4.7 6.7 9.5 13 19 26 38 53
+3/4 1.3 1.8 2.6 3.6 5.2 7.3 10 15 21 30 42 58
+ 1 1.4 2 2.8 4 5.6 8 11 16 22 32 45 64
Finally, just a practical observation: f numbers on lens barrels are somewhat
inaccurate anyway as far as setting them is concerned.
OK ... now here is the _method_ whereby ANY increment could be determined:
To find any desired increment above a particular f number you square the
reference f stop and find the log of that number. Add to that log the value
.075 for a 1/4 stop increment, .10 for a 1/3 stop increment and .15 for a 1/2
stop increment. Now find the antilog of that log and then the square root of
the number will be the new f-stop. (BTW, .075 is 1/4 of .30 - which is 1 stop
in terms of LogH or Log Exposure)
For example: let's add 1/3 stop to f/16 ... (or 1/3 stop less exposure).
16 squared is 256 ... the log of 256 is 2.40 ... to this add .10 and that
becomes 2.5 ---- the antilog of 2.50 is = 316 and the square root of this is
17.78 or 18 (17.78 is exactly it ... most tables would list 17.8 or more
likely 18)
OK ... now how about 1/4 stop bigger than f/16 ... (or 1/4 stop less exposure)
16 squared is 256 ... the log of 256 is 2.40 ... to this add .075 and that
becomes 2.475 --- the antilog of this is = 298 and the sqrt of that is = 17.27
or more than likely 17 in a table.
NOTE that like "regular" f stops the numbers associated with fractional f/stops
also double in size as you increase in two stop increments!.
Andy
Andrew Davidhazy, Professor
School of Photo Arts and Sciences/RIT
andpph@rit.edu www.rit.edu/~andpph
