how to translate exposure factor figures into reality?

Discussion in 'Pinhole Photography' started by little golan, Apr 20, 2010.

  1. little golan

    little golan Member

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    I am ready with three pinhole cameras for the world day. Using the 'pinhole calculator' I have the f # and the exposure factors. I have a light metre that will let me say iso 3.2.... but I cant quite understand how to change my information into exposure times. I could use film, I can use the calculator for that, but I want to use paper. So can anyone help me tranlate it?
    F333 / exp factor 229.1
    F230 / exp factor 109.3
    F358 / exp factor 264.8.
    thankyou.
     
  2. RalphLambrecht

    RalphLambrecht Subscriber

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    Why ISO 3.2? Why exposure factors?
    Does your lightmeter give you EVs? If yes, try this:

    http://www.darkroomagic.com/DarkroomMagic/Camera_files/PinHoleDial.pdf

    find more here:

    http://www.darkroomagic.com/DarkroomMagic/Camera.html
     
  3. little golan

    little golan Member

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    Thankyou Ralph. Why 3.2? because that's as low as my Gossen Digipro metre will go , and I think thats the ISO for paper? Ilford Multigrade. Exposure factors??? well Im floundering here. I measured the pins /pinholes and the focal lengths, fed them into the pinhole calculator and it came out with these exposure factors. I have it in my tiny mind that I have to do some sums........??!! then I will have the exposure times Im Looking for.
     
  4. little golan

    little golan Member

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    Also, my metre does have EV, what is this?
     
  5. Poisson Du Jour

    Poisson Du Jour Member

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    All this stuff about exposure factors and equations is naff ... it's putting you in at the very deep end and it's very unnecessary. Use your exposure meter to read the scene, extrapolate the result to the pinhole and shoot and forget about fancy fractions and mathematical gymnastics.

    I'm learning of many people getting too involved with the mathematical side of pinholes, including something called Raleigh's Theorem (?). It's absolutely not necessary (it may well be when designing a pinhole).

    An EV is a logarithmic expression of the constant quantity of light equal to a set shutter speed and aperature value. Think of it that with each change of the Av/Tv quantity of light is doubled (or halved).
     
  6. RalphLambrecht

    RalphLambrecht Subscriber

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  7. DWThomas

    DWThomas Subscriber

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    Assuming you are talking about Pinhole Designer, the idea is to see what shutter speed your lightmeter shows for f22, then multiply that by the exposure factor.

    Reading: 1/25 ( = 0.04 seconds) @ f22
    x 229 = 9.16 seconds @ f333.

    Note that many films exhibit reciprocity failure at long exposures, so even more time than the calculated value may be needed. Paper may have less of a problem, I've never tried it.

    The important thing is that if you get anywhere near close, you should get something usable. From what I've read over the past year or so, an ISO index down around 3.2 to 6 is likely a good place to start for paper.
     
  8. little golan

    little golan Member

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    Thank you every one for that I understand now, and will do some sums. Poisson du Jour, I dont quite agree with you, it isnt naff. I have been bumbling along with trial and error with my pinholes for too long now. I Like experimenting, but its better to learn from experiments. I also like the concept of 'not knowing for certain', and anti- precious that comes with a pinhole camera but I dont want to waste time and paper. Someone who can, has worked out the sums so that I can use the calculations,and they have put their labour of love on line for free. I do like to use recycled containers for my cameras and it amuses me, that the biscuit tin, and the hole made with a sewing needle, now has a calculation on it. :smile:
     
  9. Tom Miller

    Tom Miller Member

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    The best way to determine proper exposure is to test. This is especially important with paper. In the pinhole camera I use the most with paper negatives, the fastest black and white paper exposes perfectly in 30 seconds in bright sun; the slowest paper I've found exposes perfectly in 5 minutes. This is more than a 4 stop difference.

    I use a light meter set to read EV. Bright sunlight is EV 15.5 at 100 ISO. If a scene meters EV 13.5, then expose for 2 stops more. For example, the 30-second paper would need to be exposed 2 stops or 4 times longer or for 120 seconds.

    Here is the most lucide explanation of EV that I've found. It is for lens photography, not pinhole; but explains the principles well.
    http://www.chem.helsinki.fi/~toomas/photo/ev.html

    The same idea works with film. Even though all films' ISO ratings are consistent for lens photography, the reciprocity failure of films can vary widely, so testing films is valuable as well.
     
  10. Rich Ullsmith

    Rich Ullsmith Subscriber

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    I agree with Tom and Gary. The problem with metering a scene (sure, you have to start somewhere) is suppose you start with f64 and then extrapolate the reading out to your f333, or whatever. Even a quarter-stop anomaly in your initial reading will be grossly exaggerated by the time you get to f333. That incorrect reading will now be compounded by reciprocity failure.

    I think you will get better results sticking with the same film (Tmax 100 is great) and the same developing scheme, and keep a few notes on light conditions (bright sun, hazy sun, etc.). This will get you close very quickly.
     
  11. DWThomas

    DWThomas Subscriber

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    Hmmmm -- actually, since stops are ratios, I believe after the extrapolation, you'll still only be off a quarter stop. But the basic situation is true that there are enough variables tangled up here that some simple tests are likely the quickest way to get there.

    The fudged metering works pretty well for me with film, but since paper doesn't normally spec an ISO and may have some spectral issues as well, it would be necessary to figure out an ISO from tests to get any real precision.

    Myself, once I start pinholing, I tend to toss precision out the window. :tongue:
     
  12. Chan Tran

    Chan Tran Member

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    I wouldn't use the exposure factor at all since the aperture is known (i.e. f/333) the film speed or rather paper speed is known as ISO 3.2. What else does one need except for the reciprocity characteristic of the paper?
     
  13. DWThomas

    DWThomas Subscriber

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    I doubt many meters can give a direct reading for f/333 or f/256. My Digisix stops at f/32 and my Sekonic L-508 ends at f/128. Pinhole Designer can actually output a table (Excel spreadsheet compatible) with a list of settings compensated for the exposure factor and optionally, reciprocity. The latter is only for some films and a bit suspect in at least one case I've tried.

    One notes the shutter speed indication for f/22, uses that to index the table and get a value of how long to hold the button down. Such a table easily fits on a small card. Obviously one can mentally diddle the f/22 and shutter speed values by stops and come up with a new shutter speed, but the chart is simpler (based on my one day a year experience!)
     
  14. Tom Miller

    Tom Miller Member

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    Reciprocity failure is a big problem. Each paper and film has different reciprocity failure characteristics. Tech specs for film usually include reciprocity compensation factors; but since they are not needed for enlargements onto paper, the manufacturers don't publish them. The easiest way around the dilemma it is to test. Also, the 3.2 ISO mentioned in the original post seems to be little golan's light meter's lowest setting. People usually rate paper anywhere from ISO 2 to ISO 6. One advantage of testing is that when the basic daylight exposure for the tested camera / paper or camera / film combination is known, adjustments to the it, like giving three or four more stops of exposure, are linear. For practical purposes, doing the test in the pinhole range incorporates reciprocity in determing the basic exposure and no further compensation is needed.
     
  15. RalphLambrecht

    RalphLambrecht Subscriber

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    In the absence of testing, paper typically has a reciprocity failure of 1/12 stop per stop of exposure.