Re: Compensating Reciprocity Failure

Here is a math equation that "fit" the data for this HP5 film given by
Ilford:

Corrected exp = ( 0.103333 * X * X ) + ( 2.22143 * X ) - 2.08333

X is the measured exposure in seconds

for instance, to find the corrected exposure for 120 seconds (2 minutes):

Corrected exposure = ( 0.10333 * 120 * 120 ) + ( 2.22143 * 120 ) - 2.08333

Corrected exposure = ( 1487.9952 ) + ( 266.5716 ) - 2.08333

Corrected exposure = 1752 seconds or roughly 29 minutes

I think Benno was saying that Steve Simmons' "Using the View Camera"
suggests an exposure of about 28 minutes (give or take 10 secs :-). The
equation gives you 29 minutes. One minute exposure when the exposure is
that long won't make any difference, be sure of that, besides, with pinhole
apertures I rather give a bit more than less, after all I've never had an
overexposed pinhole made negative, but I do have many underexposed ones!!.

For up to 1/2 minute you have corrected exposures given by Ilford. You
could make a table of corrected exposure for up to 5 minutes (this is the
max you wanted, I believe) just by plugging different values of "X" or
measured exposures in increments of 30 seconds.

Hope it helps,

Guillermo
Received on Thu Aug 24 09:11:48 2000