Carlos Perez-Palacio wrote:
> So my question is: How much exposure/light is 10 or 13 EV?
The meter calibration equation is
2^EV = A^2/T = LS/K
Where
EV = Exposure Value
A = f-number
T = shutter time
L = Luminance
S = Arithmetic ISO speed
K = Meter calibration constant
Exposure Value can be interpreted either as a combination of camera
settings or as an indicator of luminance. When used in the latter sense,
one must know both the ISO speed and the calibration constant. When using
EV to indicate luminance, most manufacturers, including Canon, assume an
ISO speed of 100. Canon, Nikon, and Sekonic use a K of 12.5; Pentax and
Minolta use 14.
So how bright is EV13 with ISO 100? The "Sunny 16" rule (1/ISO @ f/16)
yields
A = 16
T = 1/100
and
EV = log2(16^2/ 0.01) = 14.64
so that EV 13 is about 1 2/3 step less bright than a normal front-lighted
scene on a sunny day. As another point of reference, EV 0 is 1 second
@ f/1.0. Expressed as a luminance, I'd describe this as mighty dark ...
> Does the EV changes linearly as ASA changes? I mean, if for ASA 100, 125
> f2.8 produces 10 EV, for ASA 400, 10 EV would be 500 f2.8?
When used to indicate a light level, EV usually assumes ISO 100. When you
determine camera settings, however, you need to adjust for ISO. With a
lighting level of "EV 10 with ISO 100," you'd use settings of EV 10 (1/125
@ f/2.8) with an ISO 100 imager, but settings of EV 12 (1/500 @ f/2.8) with
an ISO 400 imager. So in this sense, the change in EV is linear with ISO speed.
Jeff Conrad
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