Reciprocity problems with Portra and Ektar

[JLP]

Not yet enough experience with Ektar yet but i shot an 8x10 sheet of Portra 400 at 320 last Saturday morning exposure time was 4 minutes no compensation. Negative looks and scans perfect.

 

[Josh Harmon]

Then if the metered time is 5 seconds, then ƒ(x)=-5.5391+2.5568x+.0046x^2 =18.44 seconds if my arithmetic is correct. Which is way too much compensation for both Tmax 100 and Fuji Acros.

If a*x^b+x then f(x) = 8.62 seconds then, according to the manufactures data sheets TMax is overexposed by 1/3 stop and Acros is overexposd by 2/3 stops.

If a*x^1.62+x then f(x) = 10.75 seconds.

Thomas

The function does not work with short exposures such as below 10 seconds, it is more accurate in the longer 30+ second range. Should have posted that in my other post sorry.

-Josh