Calculations for fstop enlarging timing

Discussion in 'Enlarging' started by craigmj, Aug 24, 2004.

  1. craigmj

    craigmj Member

    Aug 24, 2004
    Hi All,

    I'm trying to build an enlarger timer that can do quarter-stop exposure calculations. Working on the web (and glancing at Tim Rudman's f-stop calculation table), I've concluded that the relevant formula is:

    [t2] = 2 ^ (-1* ( ([scale] -1) * log2 ([f]^2) + [scale]*log2(1/[t1]) ) )

    [t1] = initial time
    [f] = f-stop
    [scale] = the amount by which one wants to scale (e.g. 1.5 would offer a 1/2 stop increase)
    [t2] = the final time required (keeping the f-stop constant)

    This is based on the idea that EV = log2( [f]^2 ) + log2 ( 1/[t] )

    I've checked my values for Dr Rudman's table, and I'm correct per his table for 2 seconds basic exposure and above, when INCREASING f-stops.

    BUT, 3 questions:
    1. If I start with a basic exposure of [t1]=1s, log2(1/[t1])=0 and therefore no matter how large my [scale] value, my [t2] remains 1. (i.e. My formula only works for values over 1s initial exposure)

    2. My formula does not handle decreasing exposure. If I have a [scale] of 0.5, and [t1]=2s, it suggests [t2]=1.4s, whereas of course 1s is correct: half the exposure.

    3. My formula includes the f-stop, and is (heavily) effected by the f-stop. In fact, at an f-stop of 2, my times DECREASE as I attempt to INCREASE the exposure - clearly nonsense. Dr Rudman's table seems, therefore, based on the f-stop being f1, and not changing. I could simply exclude it for the purposes of my calculation, but I would like to understand why it is included in the calculation, and the logic of excluding it before I do so.

    Many thanks for any help,
  2. Jon King

    Jon King Member

    Sep 20, 2002
    New Hampshir
    Medium Format