I haven't yet mastered all of the nuance of metering on large format. I use a spot meter, a Minolta Spotmeter F to be exact.
Basically, trying to meter a tone in the scene, correct the indicated exposure to place it in the appropriate zone, apply bellows factor and/or filter factor compensation, and apply reciprocity failure compensation all in my head, I have a tendency to either forget something or get one of the corrections backward, and more often than not I screw it up and end up with an ugly sheet of film. I also tend to shoot more slide film than black and white, so it's even more critical.
I know a lot of people use exposure records to keep notes about their shots, and that looks like an excellent way to work through exposure calculation as well as keep records for development compensation and notes for the future.
In that spirit, I started working on an iPhone app to help me calculate exposure and serve as an exposure record. Right now all I have is the calculator part, but I have some interesting ideas for the future of the app.
Here's the current calculator UI. Note that it's ugly and quick, the final version hopefully will be MUCH more polished than this.
The way the calculator works at this moment :
1) spot meter the 'key' tone in the scene (with ISO set on meter to your desired EI and with meter set to read in EV instead of aperture/shutter)
2) input EV from meter and zone you would like to place it in
3) input filter factor and bellows factor in stops
4) calculator shows the EV that would result in a proper exposure.
5) slider allows you to select paired aperture and shutter combinations. The aperture and shutter will only ever show in standard stops, from f/0.5 to f/128 on the aperture side, and any number of whole seconds if greater than 1s, or 1/2, 4, 8, 15, 30, 60, 125, 250, 500, 1000, and multiples of 1000 on a log2 scale on up as necessary. It then calculates what EV is represented by the whole shutter stop and whole aperture stop, and finds the amount of error, and then displays that as a positive or negative third stop on the aperture side.
The idea was that my shutters only allow me to select whole stops on shutter speed, but the apertures are continuously variable.
Right now the big thing I'm missing is reciprocity failure. Obviously the big challenge there is that it's very film specific. Since the final app is going to be an exposure record, I am probably going to try to have a user-configurable list of films, and for a given record, you will select a film and that will set the EI and affect the reciprocity curve. The problem is that there's not exactly a standard database of reciprocity characteristics, and I don't know how I'm going to go about allowing the user to alter that curve. I'll probably use a t = a + x^c type equation, and i guess I can allow the user to set a and c for a given film, but I dunno how many people are going to have the desire or ability to determine and tweak those kind of curves. I think for the purposes of using the calculator right now, I'm going to use a simple t = x^1.48 type curve for anything over 1s.
As I expand the app to include more of an exposure record type functionality, I'll add the ability to record multiple spot meter readings and place them in zones, and to designate one reading as the 'key' reading that determines exposure. The rest of the readings will be used to show an overall contrast in stops and to indicate compensation requirements for development.
Another interesting thought I had for the final version of this app: many exposure record forms I've seen have an area to draw the scene. On the iPhone, the user could use the camera to take a picture of the scene which will be attached to the record, and if it's a 3GS, the exif data in the photos that it captures contain ISO/aperture/shutter info, and I can display what EV the iPhone's camera thinks the overall scene is, and use that as a sanity check against the recorded and calculated readings.
Anyway, I'm mostly rambling about ideas now, but if anyone has any thoughts on how this can be more useful, please feel free to let me know. Also, if anyone with an iPhone/iPod touch/iPad is interested in playing with it, pm me and I can set up an Ad Hoc build for you to beta test. Just bear in mind that the state of it at this point is by no means sexy, polished, or bug free
Ian
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