Reciprocity formulas (for Ilford films)

[BradS]

I pretty much always just estimate long exposures (loosely based on Heroique's table) and usually get a usable negative.

This is certainly the more pragmatic approach. All of the graphs and math, although fun, amount to a grand guess.

[tgtaylor]

I've been using Ilford's reciprocity curve for Delta 100 with great success. Appears to be spot on in my experience.

Thomas

[Cletus]

This is certainly the more pragmatic approach. All of the graphs and math, although fun, amount to a grand guess.

Yes, indeed, but if ever there were an educated guess I will be the first to admit, your's is it!

[macandal]

I've been using Ilford's reciprocity curve for Delta 100 with great success. Appears to be spot on in my experience.

Thomas

So how do you estimate times that do not appear on the curve? When you get to times above 200 seconds/~3 minutes?

Thanks.

[BradS]

Here’s a chart I've shared that's easier and more reliable (in my "KISS" experience).

• Metered 2 sec. – use 4 sec (2x)
• 4 sec – use 12 sec (3x)
• 8 sec – use 32 sec (4x)
• 15 sec – use 75 sec (5x)
• 30 sec – use 3 min (6x)
• 60 sec – use 7 min (7x)
• 120 sec – use 16 min (8x)

This is actually in close agreement with Kodak's suggestion for Tri-X films (Kodak Data sheet F-4017, page 2) which says essentially:

metered - adjusted
1 sec - 2x (add one stop)
10 sec - 4x (add two stops)
100 sec - 8x (add three stops)

I think people have trouble extrapolating this to longer times and feel uncomfortable with the coarseness of the table.

It is easy to express this relationship in an simple equation...
t_a = t_m * 2^{(1 + log₁₀(t_m))}

which can be re-written:
t_a = 2 * t_m ^{(k + 1)}, where k = log₁₀(2)

or, replacing k+1:
t_a = 2 * t_m ^(1.3)


This is probably a pretty good starting place for most films.

[macandal]

or, replacing k+1:
t_a = 2 * t_m ^(1.3)


This is probably a pretty good starting place for most films.

Thanks. I'll use this.

[BradS]

Yes, indeed, but if ever there were an educated guess I will be the first to admit, your's is it!

Thanks.

Thanks. I'll use this.

Cool. Hope it works ok for you.

[Andrew O'Neill]

Formulas are nothing but a guesstamate. You really should do your own testing. I have found Ilford's data to be a bit over the top. I have been using my own tested data in the field for 16 years for HP5 and FP4 and have been satisfied. I also have data for Delta 100 somewhere. If I dig it up, I can post it if you like.

[macandal]

I also have data for Delta 100 somewhere. If I dig it up, I can post it if you like.

That would be very much appreciated. The more information the better. Thanks.

[BradS]

Formulas are nothing but a guesstamate. You really should do your own testing. I have found Ilford's data to be a bit over the top. I have been using my own tested data in the field for 16 years for HP5 and FP4 and have been satisfied. I also have data for Delta 100 somewhere. If I dig it up, I can post it if you like.

I concur. I hope people understand that all that I have done is give an equation for the relationship provided by the manufacturer. In Ilford's case, by choosing three points from the graph in their datasheet and writing the equation for the parabola that passes through those three points and in the and similarily with the Kodak data, all I have done is express in an equation what they say in word and in the table. There is no magic to this. The equations are no better than the data from the respective manufacturer.

As with so many things in photography, take this stuff as a starting point and develop (pun?) numbers, tables, graphs, whatever that works for you...based upon your own empirical evidence.