Re: Compensating Reciprocity Failure

> Does anyone use ILFORD HP5 Plus sheet film for pinhole photography?
> If yes, I need advice.
> The manufacturer has supplied my with a reciprocity failure chart
> that end with 35 seconds metered time, transforming into 200 seconds
> actual exposure time. On my last shooting meter reading was 2 minutes.
> A wild guess was to compensate for 5 minutes, but both negatives
> came out quite underexposed.

Here is where remembering a bit of secondary school science helps.

> D) I juts like it

The best reason.

> Do any of you HP5 users (assuming there are a few worldwide)
> have any personal experiences to share?
> I need corrected exposures times, let's say from 1 to 5 minutes.

What you have to do is extrapolate the data Ilford gives you. It may not
give you an exact behavior of the film but it would certainly beat a wild
guess. What you do is take paper, pencil and a ruler. Following the ilfor
data http://www.ilford.com/html/us_english/pdf/HP5_Plus.pdf repeat the same
graph at the left bottom corner of your paper, then extend the curve
(remembers it is a curve not a straight line) to the top of the paper, now
you make marks on the horizontal axis (measured exposure) from 35 secs
(where Ilford data left) until the right edge of the paper, you do also the
same with the vertical axis, from 200 until the top of the paper. Now you
have a graph that can give you a better guess of what the exposure should
be.

Another way is to find a math equation that'd "fit" closely to the given
data and that we can use to find any other corrected exposure, given the
measured one. I will give you such equation later.

Guillermo
Received on Thu Aug 24 08:43:17 2000