Ignoring reciprocity (which you can pretty much do with Fuji Acros or using paper negatives in large format box cameras), a good formula for converting a light meter reading into an accurate exposure for pinhole cameras is as follows:
1) First, make sure you know the ISO of the film or paper you're using.
2) Set the ISO value into your light meter.
3) Meter your scene and reference the exposure time for the largest f-stop that your meter will read.
4) Use the following formula to convert the exposure time for your camera's f-stop value:
(f/R)^2 x Tm = Tc
f=Your camera's f-stop
R=the reference f-stop used on your light meter
Tm= The uncorrected exposure time recommended by your light meter
Tc= The corrected exposure time you'll actually use
Example: The largest f-stop reading on my light meter is f/128. My camera's f-stop is f/280.
Therefore: (280/128)^2 x Tm = Tc
or: 4.785 x Tm = Tc
So, whenever I use my f/280 pinhole camera, I reference the light meter's recommended exposure time for f/128 and multiply that value by 4.785 to arrive at the actual working exposure time. Once I do the complete formula once, I only have to know the multiplier factor and a simple multiplication (using my cell phone, or el-cheapo calculator that I carry with my camera) will give me the correct time. I can also make a chart to reference, rather than carrying a calculator.
The only time I have to redo the entire formula to find a new factor is when I change the camera's pinhole size or focal length, or when I change light meters and the maximum f-stop reading is something different.
Keep in mind when using this formula that the values for your Tm has to be in seconds, and the formula outputs the results in seconds. So, 1/8 second would actually be plugged in as 0.125, etc.
I've found this method to be universally the best method, and works equally well for film as well as paper negatives.
Again, the formula doesn't take into account reciprocity.
Last edited by Joe VanCleave; 07-08-2010 at 11:45 PM. Click to view previous post history.