Reciprocity formulas (for Ilford films)

[tgtaylor]

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

Thanks.

Mathematically model it and write the equation - Ilford has done all the work for you. Then it's just a plug in problem quickly solved with a scientific hand calculator. Alternatively you can ask Ilford for the equation I'm sure that they will oblige.
Thomas

[BradS]

Mathematically model it and write the equation...
Thomas

Which is what this is...

choose the points (0, 0), (22.5, 100), (35, 200) and you get the equation:

t_a = 0.1015873t_m² + 2.15873t_m + 0

[jwaddison]

You can make an equation yourself by googling and downloading a free copy of Curve Expert. You provide data points from the graph.

[StoneNYC]

Get an iPhone...

Then get this...

https://itunes.apple.com/us/app/reci...459691262?mt=8

Problem solved

Someone else did the work for you

He is also updating it soon, it's a member here on LFF, lots of new films coming. I have no affiliation other than I use this app all the time and can focus on thinking about the shot instead of doing math.

[Ed Bray]

One of the reasons I like using Adox 25 is its great anti reciprocity attributes, 1 sec = 1.2x, 10 secs = 1.3x and 100 seconds = 1.5x this compared to FP4+ where 10 seconds requires 30 seconds and 30 seconds requires 155 seconds and 100 seconds is off the graph by some large margin.

By the time you get to an exposure time of 30 seconds for FP4 you will be shooting at the same time as for 100 seconds with Adox 25 (both 150 seconds ish) from then on, you are better off using Adox 25 as it will give you a shorter exposure time regardless of the initial ISO.

[StoneNYC]

One of the reasons I like using Adox 25 is its great anti reciprocity attributes, 1 sec = 1.2x, 10 secs = 1.3x and 100 seconds = 1.5x this compared to FP4+ where 10 seconds requires 30 seconds and 30 seconds requires 155 seconds and 100 seconds is off the graph by some large margin.

By the time you get to an exposure time of 30 seconds for FP4 you will be shooting at the same time as for 100 seconds with Adox 25 (both 150 seconds ish) from then on, you are better off using Adox 25 as it will give you a shorter exposure time regardless of the initial ISO.

Fuji Acros100 can be exposed for 2 minutes before any reciprocity adjustments need to be made, FYI

[Ed Bray]

Fuji Acros100 can be exposed for 2 minutes before any reciprocity adjustments need to be made, FYI

Not available in 5x7 though

[StoneNYC]

Not available in 5x7 though

OH! Yes point taken... But isn't TMY-2 available in 5x7? I just looked at B&H and they only have TXP in 5x7 and TMX in 5x7 but not TMY-2... Hmm freestyle doesn't have it either...

Though I don't see ADOX 25 either... I see ADOX CHS II 100 but I can't imagine that's better than Ilford, but I also would guess that TXP doesn't have as good an anti-failure than Ilford, but I don't have the chart on hand to know for sure.

Anyway that app I listed does work

[evan clarke]

Ok, All. Forget all this scientific calculating..GET ENOUGH EXPOSURE AND PHOTOGRAPH SCENES THAT WILL PRINT. Log . bs has no place in making fine prints.

[Jerry Bodine]

Ok, All. Forget all this scientific calculating..GET ENOUGH EXPOSURE AND PHOTOGRAPH SCENES THAT WILL PRINT. Log . bs has no place in making fine prints.

Point well taken. To each his own, but how do you know when exposure is ENOUGH? Open shutter, go have a few pitchers of beer, shoot some pool, come back the next day to see how we're doing?