Hi Ettienne,
Glad to hear it's working!
The light meter measures incident light so is similar, in principle, to an incident ambient light meter. Concepts like "18% grey" apply only to reflected light meters.
Because the TSL235 are not calibrated from the factory and there are so many different paper responses, you will need to calibrate your specific sensor to your specific paper. Very roughly, the process looks like this:
- on the timer, press * for FOCUS mode, and the light level will be displayed on the screen in stops if the sensor is plugged in
- make prints of a step wedge (e.g. Stouffer) at various contrast levels; for each print, record the light level reported by the meter under each step of the wedge
- if you have a reflective densitometer, measure the density of your test prints, otherwise just note the steps that correspond to the major (0, II, V, VIII, X) boundary zones
At that point, you have notes that reveal, for each contrast setting, the relationship between total exposure (light level indicated on the meter plus the number of stops of exposure time used to make the test prints) vs print density.
When you want to make a print:
- put the neg in the enlarger and choose the enlargement factor
- use the meter to measure the light level at important points (highlights, shadows, faces, etc) on the print
- decide what zone you want those points to be at
- decide what contrast you need, based on the metered differences, your zone choices and your notes gathered earlier
- compute the exposure time you need
Things to beware:
- if you meter wide-open then stop down to make the exposure, don't forget to adjust your exposure time accordingly
- lenses mostly are pretty accurate in that one click is one stop of attenuation, EXCEPT:
- wide-open is often a bit darker than it should be, so the first click of the aperture is less than one stop difference
- wide-open or near it, there is often vignetting, so readings taken near the edge of a print will be darker and the first click-stop or two will have less effect near the edges
You can use the meter to confirm (and calibrate-out) the above issues. While the meter is slightly more accurate a higher light-levels, it's probably easiest to meter your prints at the working aperture instead of wide-open unless your negs are really super-dense and you can't get a good reading.
A useful formula is:
Exposure_Stops = Time_Stops + Meter_Stops - Aperture_Stops
where Time_Stops is the value you program into the timer to make an exposure, Meter_Stops is the meter reading and Aperture_Stops is you stopping the lens down between meter reading and exposure (e.g. if you meter at f/5.6 and expose at f/11, then Aperture_Stops is nominally 2 but you might measure it on your specific lens to be 1.8 or something).
Assuming you're not making stupidly long exposures where the paper gets into reciprocity failure, then the density on the paper is a function ONLY of your contrast filters and Exposure_Stops.
For example (I'm totally making up numbers here, they will NOT correspond to your paper), let's say you print a test wedge at f/8 and 3.0 stops of exposure. When you metered that step wedge, the first step with the slightest bit of tone had a meter reading of 2 and the first step that was fully black had a meter reading of 7. Therefore you can say that, for this particular contrast setting:
Exposure_Steps(white) = 3+2 = 5
Exposure_Steps(black) = 3+7 = 10
Now you want to make a print. You find a part of the image that should be completely black and the meter reads 6.5 with the lens wide-open (f/5.6) and we want to achieve Exposure_Stops = 10 at f/11 (Aperture_Stops=2).
Time_Stops = Exposure_Stops + Aperture_Stops - Meter_Stops = 10 + 2 - 6.5 = 5.5
Next to check that the contrast is reasonable, you measure a highlight and it reads 2.5.
Exposure_Stops(highlight) = Time_Stops + Meter_Stops - Aperture_Stops = 5.5 + 2.5 - 2 = 6.
d'oh! Your tables tell you that you want only 5 to get borderline-white so this check tells you that your highlight will come out off-white. You could put up with it (it might be OK), you could increase the contrast or you could do some dodging to reduce the exposure there - all decisions for after you've seen the first test-print.
Now you program 5.5 stops into the timer and make the test print. It should come out with borderline-black at the shadow location you metered, and not-quite-white at the highlight location. Make some artistic decisions about contrast, dodging and burning, and iterate until you get the result you were aiming for.
