Exposure latitude with 4x5

[rossomck]

I was using a brand new Chamonix 45N2 camera today, with a mixture of 400 and 125 rated mono film. I have 2 apps on my iPhone for exposure - one is a reciprocity timer, the other is called film timer. But they give vastly differing exposure times.
For example -
let's say my light meter said 2 min exposure for Ilford 125 rated film. The reciprocity app changes a 2 min exposure to 21 min 27 secs????? (this is for 90mm focal length and minimal bellows extension). The film timer App (which only has Delta 100) changes a 2 min exposure to 4 mins. Where am I going wrong??? Which App is right, and which App should I use?
Surely the difference between these two suggested exposure time is madness???

[domaz]

My reciprocity calculator indicates an adjusted time of 1165 seconds for a 128 second exposure for FP4+. So pretty close to the 21 min figure. What the heck are you taking a picture of though? A cave?

[vinny]

Ilford has a reciprocity chart. I believe it's on their website.

[Joanna Carter]

Whatever you do, don't take Ilford's reciprocity chart for Delta 100 too seriously, it appears to be the same chart as for FP4 and several other films.

The truth is that many films have very different reciprocity to each other, especially "T-grain" films like Delta 100 and Fuji Acros. Fuji Acros has so little reciprocity, it is not worth bothering about (about 1/2 stop from 120 secs to 1000 secs).

Delta 100 is certainly not in need of a large reciprocity correction and I would say that the app that gives you 4 mins for a 2 mins exposure could well be right but, in the absence of any sensible chart from Ilford, you are going to have to try it and see.

[engl]

Reciprocity adjustment is film specific, and the most reliable information is from the reciprocity charts from the manufacturer. Assume any app or second hand information is incorrect until you've confirmed it against manufacturer charts.