Yeah. I'm still waiting for approval.
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I have one question, but it doesn't need a new thread. I metered f/22 4 sec. iso 5. After a lot of testing of different exposure times I ended up using 8 minutes exposure time when metering f/22 4 sec @iso5 to get a decent exposure. Is it correct that I need to calculate my exposre using about f/2800 iso5?
I read somewhere that the iso for paper is about 5, but it doesn't make sense when I needed 8 minutes exposuretime.
Not sure I am reading the question correctly -- was this with a pinhole camera? Pinhole cameras typically have an aperture up around f/200 or smaller.
The typical way to deal with pinhole exposures is to make a chart using the shutter speed from a reading for f/22, then multiply it by a factor that relates the pinhole f-stop to f/22. For an actual pinhole aperture of f/220, the exposure relative to the exposure at f/22 would be the ratio of 220/22 squared (I made it easy for myself :D) = 100. As such, a reading of 4 seconds would be multiplied by 100, 400 seconds, is 6 minutes, 40 seconds which is in the general order of magnitude of 8 minutes.
With film, these long exposures are affected by reciprocity failure, often quite drastically, as in requiring 3 or 4 times the metered length. For that one can make charts to include the reciprocity compensation. There is a Windows program called Pinhole Designer that can do some of these calculations and generate a chart for a number of films.
A little more detail about what you were actually doing might help.
DaveT
An enlarger is a good way to check your pinhole for hole size and roundness -- just put it in the negative carrier (cardboard and tape usually needed) and crank the head up to project a 10x or so image of the hole.
Pinhole focal length Vs negative size isn't something much talked about but is key to getting good results. The optimum is to use a focal length that is as short as possible while covering the media. Setting the focal length at 50% of the normal focal length is usually the limit. Naturally, at a wide angle the thinness of the pinhole is critical - the metal should come to a razor edge. Emery cloth should be used as the final abrasive to polish off the hole.
The bread truck camera, if it took 3x4" film (guessing at its size) would need a focal length of 2.5" or so; instead it looks as if it may be 6" - more suited to an 8x10 camera.
Using a long focal length is equivalent of blowing up the central 1/6 or so of the negative to make the final print - not a good thing when you are starting out at fuzzy.
DWThomas: Yes it was with a pinhole camera, made out of a cardboard box. I metered with my normal lightmeter @iso5 f/22 so I would now how the light was, the reading was 4 seconds.
I used Ilford Multigrade RC IV deluxe as negative. ( developed in Agfa Neutol 1+9 for 1 minute)
Then I started out testing exposures, first 1 minute exposure, then 2 minute, then 4 minute and then 8 minutes. 1 and 2 minute was way too dark, 4 minutes I was getting there and 8 minutes was a maybe a minute too much. So 6 minutes 40 seconds like you suggest sounds perfect.
Do I understand you correct that if I use this method each time for this pinholecamera I can just multiply my lightmeter reading @iso5 f/22 with 100?
Regarding converting a meter reading to one's pinhole camera's f-stop:
(Fc/Fm)^2 * Tm = Tc
Where:
Fc = f-stop of camera
Fm = f-stop used on meter
Tm = exposure time recomended by meter
Tc = converted exposure time
In your example, Grainy, I didn't see where you gave us the f-number of the pinhole camera, but let's guess at a number like f/200. So here's the hypothetical calculation for the values you provided:
(200/22)^2 * 4 = 330 seconds, which is 5-1/2 minutes.
Provided that you keep using the F/22 reading on your light meter, and provided that the f-stop of your camera doesn't change, you can perform part of this calculation ahead of time, thus:
(200/22)^2 = 82.6
So, you would write down this multiplier of 82.6, take it with your camera, and multiply it by the meter reading (in seconds) to arrive at your corrected exposure time. Keep in mind that this formula does not account for reciprocity failure. In my experience, paper has very little issues, whereas film does.
NOTE 1: In the above example I assumed your camera's f-number was 200. You need to measure the diameter of your camera's pinhole, in millimeters, and divide that number into the camera's focal length, also in millimeters, to arrive at an accurate figure for your camera's actual f-number. Then, plug that number into the formula to get your actual working correction factor.
NOTE 2: I'm assuming that you're shooting paper negatives in daylight illuminated scenes. Remember that paper is, for the most part, only sensitive to UV and blue, and a very slight amount of green. Thus, if you meter an indoor scene that's illuminated by artificial lighting, the meter reading will not be accurate (because the meter's spectral response is much wider than the paper's), and you'll have to manually figure out how much extra exposure to use. For this reason, I recommend using panchromatic film (rather than paper) for artificially lit scenes.
NOTE 3: I've found that for using paper negatives you should try to be more exact with developer temperature and dilution (and freshness) than you might when using paper for prints. For instance, I use Freestyle's Arista brand grade 2 RC paper for negatives. I use Ilford's Universal paper developer, freshly mixed at a 1+15 dilution, and develop the negatives with the chemistry at 68f. That being said, once you start using the formula for converting your meter reading, and once your development process is consistent, then you should run a series of calibration tests to find out your paper's actual working exposure index; the value of "6" you provided may or may not be accurate.
To do these tests, mix up your chemistry at the proper dilution and temperature, in the daytime, and then shoot a series of exposures of a day lit outdoor scene that has a mix of shadows and highlights, using different exposure indexes for each shot. Start at about EI=2, working up to EI=12, making the calculation from the above formula for each shot. Write the metered exposure index on the back of each paper negative with a Sharpie marker. When you're done processing the negatives, you'll be able to review them and easily figure out the best exposed negative, and its corresponding exposure index.
Good luck, keep us posted.
~Joe
Assuming your f-stop equivalent is f/220, yes. But I was just guessing a typical number. Joe VanCleave in the post above has shown you the accurate way to do the math, and pointed out some possible pitfalls re: tungsten lighting. The smallest f-stop I can set on my Gossen Digisix is f/32, the meter has a ring you set and then read off combinations of shutter speed and f-stop. Using f/22 and a multiplier is just a handy way of extending that scale to the teeny tiny apertures.
In my first response at the beginning of this thread I posted two links that get you to a bunch of information about all sorts of pinhole considerations.
Thanks a lot for informative reply Joe. I will defently try this. I haven't measured the size of my pinhole yet because I don't have anything to measure it with. The only thing I know is that it's less than 1mm.