At 11:45 10/31/99 , Olaf wrote:
>to using numbers that are powers of 2. I therefore couldn't help but
>noticing the close relation of such 2 powers to the shutter speeds used on
>the OM cameras (and probably other brand cameras too). Look:
>
>speculation), but maybe someone knows anything about how these shutter speed
>timings work and if indeed there is some correlation between the shutter
>speeds and the range of 2 powers...
>
>Anyone?
Olaf hit the nail on the head. The entire exposure system is *completely*
powers of two, including apertures, ASA (now ISO) film speeds and EV
(exposure value).
Apertures in full stops are in increments of "half" powers of two:
2^0 = 1
2^.5 = 1.4
2^1 = 2
2^1.5 = 2.8
. . . and so on. Simply multiply the previous one by the square root of
two to get the next one. For half-stops use 1/4th roots of two. The
reason for half exponents of two for full stops of aperture is the amount
of light is determined by the *area* of the lens opening which is
proportional to the square of its diameter.
ISO film speeds are in approximate 1/3rd powers of two:
2^4.66 = 25
2^5.66 = 50
2^6 = 64
2^6.66 = 100
2^7 = 125
2^7.33 = 160
The EV is the exponent of 2 used for the shutter speed plus twice the
exponent of 2 used for the aperture. EV = 0 defined as f/1 at 1 second.
Example: shooting Portra 160 outdoors in bright sunshine (not what I would
really use this film for) using f/11 (2^3.5) and 1/250th (1/2^7) gives an
EV of 14 (2*3.5 + 7). Switching to the magical f/8 (2^3) and 1/500th
(1/2^8) by trading a stop of each gives an EV of 14 again (2*3 + 8).
-- John
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