> "Tim Franklin" <[EMAIL PROTECTED]> wrote:
> Its quite easy to get carried away by technical specs.
> EV 1 I have seen defined as being (a) the light given
> off by a single candle, and (b) a situation requiring
> an exposure of 1 second at F1.4 when using ISO 100 film.
Hi Tim,
The depressing thing is that Canon uses an EF 50mm f/1.4
for the AF dark limit spec.
If you use a slower lens, the sensor receives less light
so the dark limit is reached even earlier.
e.g. EV 5 in front of a f/5.6 lens translates to EV 1 behind
the lens at the AF sensor.
According to my calculations, the EV 1 AF dark limit is reached
whenever the meter says Tv=1" @ ASA=100 for ANY lens wide open.
Or Tv=1/2s @ ASA=200, or Tv=1/4s @ ASA=400 ... you get the idea.
Cheers
Julian Loke
P.S. This even ignores the increased difficulty of focussing
at narrower max apertures due to increased DOF!
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