Thanks Bill,
I stand corrected. My opinion was based on an assumption that film
responded in a linear way. That is not the case. The reciprocity failure
is indeed caused by the inability of film to continue to respond to
light in a linear way when exposures are long. And you are also correct
in saying that different films require different correction. However,
that being said, It should also be pointed out that the variation
between film stocks is minor. The curves tend to fall off in a very
similar fashion. Consider that numerous charts have been published that
will bring you darn close for almost any film. They always call for one
stop at one second, two stops at ten seconds, and three stops at 100
seconds. You can find one in Ansel Adams' book "The Negative."
Now if those charts can serve well for most purposes, I would think
that this factor might be built into more advanced metering systems. I
know I've shot exposures as long as two minutes with my LX and no
correction with good results. I'm curious about this, so later today,
I'll try checking the LX aperture priority exposure time against my
recently calibrated Pentax spotmeter in low light with the LX stopped
down for a long exposure.
Paul
William Robb wrote:
>
> Paul, I think you might be misunderstanding reciprocity failure.
> At some point, the film's response to light goes non linear.
> Effectively, what happens is that at (example only) a one minute
> exposure, your 100 speed film is still a 100 speed film, but at
> a 5 minute exposure, it is suddenly a 25 speed film. No meter
> can account for that, because it is an individual film
> characteristic.
> William Robb
> ----- Original Message -----
> From: "PAUL STENQUIST" <[EMAIL PROTECTED]>
> To: <[EMAIL PROTECTED]>
> Sent: Saturday, January 06, 2001 7:44 AM
> Subject: Re: Vs: Macro Metering & Exposure Compensation
>
> > A TTL light meter will correct for any exposure variation that
> involves
> > loss of light through the lens and shutter, including
> reciprocity
> > failure. (Or perhaps I should say that since a TTL meter
> measures the
> > light reaching the film/sensors inside the camera, it goes to
> follow
> > that it does not have the same problems with long and short
> exposures
> > that a meter measuring ambient light encounters.) Compare the
> readings
> > of your TTL meter to a handheld meter for an exposure of ten
> seconds or
> > so, and you'll see exactly what I mean. The ability to correct
> for
> > reciprocity and extensions was one of the principle benefits
> of SLR/TTL
> > cameras. And it is a huge benefits with a camera like the LX
> that will
> > measure very low levels of light. No heacache invoking
> calculations
> > necessary for those two minute exposures.
> > Paul
>
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