Reciprocity failure is a well known feature of film photography. At certain level the relationship between aperture and exposure time breaks down and is no longer proportional. This effects kicks in at exposure time of about one second. In order to expose the film correctly the exposure time must be adjusted upward from what was metered.
The exact amount of adjustment varies from film make to make – the sadly discontinued Fuji Acros (may the manhood of the manager who signed off the decision shrink and shrivel) was known for requiring almost zero adjustment, on the other hand the Fomapan sports truly atrocious level of reciprocity failure. This level of reciprocity failure can be in fact made into an asset in long exposure film photography, but deserves another post of its own.
I am fond of long exposure photography. But in pursuing this sub genre of film photography I have found the charts and tables provided in manufacturer’s data sheets inadequate to my needs. I proceeded to make my own tables, mainly by interpolating the manufacturer’s data.
A while back I wrote a blog post about calculating the necessary adjustment using tools of advanced statistical programming. This was fun, but it was applicable to one make of film only. I have revisited the project, and created instead an application that interpolates the exposure adjustment dynamically. It is written in R, using the Shiny package to make the web interface. The app can be used to interpolate adjusted exposure times from any data provided.
The columns of measured and adjusted exposure times on the left can be edited as necessary. I have seeded the app with data for generic Ilford film (Ilford is unusual in the fact that they propose the same adjustment for different emulsions – e.g. FP4+ and HP5+).
The table of measured & adjusted exposure times is spaced in half exposure steps – a sequence with factor of square root of two. This means exposure doubles in every second column (to avoid the hassle of having to use decimal numbers I skipped 1.41 seconds, commonly rounded to 1.5 sec).
The chart and table of adjusted exposure times are recalculated automatically. The table can saved in csv format, and processed further in your favorite spreadsheet editor.