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  1. #1

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    Using Light Meter inside Camera Obscura

    Dear World,

    I'm trying to get my head around the following, any clarification would be greatly appreciated!

    - I've turned a room into a Camera Obscura, and I'm trying to use it to expose paper negatives (standard Ilford RC paper).
    Is there anyway I can accurately calculate the exposure with a light meter standing inside the Camera Obscura?

    I typically rate my paper at ISO 6, then shoot wide open. My lens on the Camera Obscura is f9, however if already standing inside the camera, I'm assuming the aperture on the light meter is irrelevant?

    So a working example would be as follows;
    - Set light meter to ISO 6, f1.0
    - take reading at film plane within Camera Obscura
    - expose for indicated time

    Am I on the right track??

    Cheers,
    Matt

  2. #2
    ic-racer's Avatar
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    You don't need to know the aperture number when using a film-plane meter. You set the ISO and it tells the shutter time.

  3. #3

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    Sorry, just to clarify, I'm using a standard incident Light Meter. A Sekonic L-358.

  4. #4
    ic-racer's Avatar
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    Quote Originally Posted by Leave Me Here View Post
    Sorry, just to clarify, I'm using a standard incident Light Meter. A Sekonic L-358.
    In that case the readings will not allow an accurate prediction. You will have to do some test prints to 'calibrate' your light meter readings.

    If you had something like the Horseman film plane meter, you could just hold it near the wall and read the exposure time, though, you would need to read the display from the wall side of the meter.

  5. #5

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    As said in the post above, what about metering the light falling on the wall where you will finally put your photo paper? Shouldn't it give you an idea about the exposure?

  6. #6

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    http://www.sekonic.com/Products/L-358/Features.aspx#3

    Read the sections on Incident Light Metering and Ambient Light Metering.

    As ic- said, it might only get you into the ballpark, but seems quite easy to do.

  7. #7

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    p.s.

    set meter to ASA 6
    Lumisphere retracted
    Place meter on "film plane"
    Determine light value
    Read time for f/9 (or whatever the correct aperture is... you say both f/1 and f/9))
    Expose paper for that amount of time... compensating for reciprocity failure if required.

    Process, evaluate, and adjust for perfection on subsequent exposures.

  8. #8
    ic-racer's Avatar
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    The usual meters out there estimate the film plane light intensity using an equation (too difficult to type here) that takes into account the following:

    b = Constant with units of lx⋅cd–1⋅m2
    θ = Angle between subject and lens axis
    N = Relative aperture (f-number) of lens
    F = Lens flare correction factor
    f = Focal length of lens in m
    V = Lens vignetting factor
    Ls = Luminance of subject in cd⋅m–2
    T = Lens transmittance factor
    u = Subject distance in m

    So, yes you could solve the equation for the above variables then predict the exposure.

    Much easier to just do some test strips.

  9. #9

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    Thanks for the replies. A bit more research and I've discovered essentially what I'm dealing with is Bellows Extension Compensation on a large scale. I've jumped from 4x5" where I never needed to worry about it, to UULF 1mx1m upwards, where it is essential.

    Now I'm standing inside my camera, trying to work out the exposure with my light meter, when possibly what I should have been doing is working out the focal length or magnification, and apply these figures to the Belows Extension Compensation calculation.

    Ideally, I'd still like to try and somehow meter the exposure from inside, but again, as I am taking the reading from the film plane, the light will not pass through any aperture, and thus that figure should be irrelevant. So when setting/reading the meter, that's what I proposed f1.0, because its the largest the meter will go.

    Hope I havent confused anyone! Love to see or hear of some examples of workflow doing shots 1m x 1m (40"x40") or larger.

    Cheers,
    Matt.

  10. #10

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    Quote Originally Posted by Leave Me Here View Post
    Thanks for the replies. A bit more research and I've discovered essentially what I'm dealing with is Bellows Extension Compensation on a large scale. I've jumped from 4x5" where I never needed to worry about it, to UULF 1mx1m upwards, where it is essential.

    Now I'm standing inside my camera, trying to work out the exposure with my light meter, when possibly what I should have been doing is working out the focal length or magnification, and apply these figures to the Belows Extension Compensation calculation.

    Ideally, I'd still like to try and somehow meter the exposure from inside, but again, as I am taking the reading from the film plane, the light will not pass through any aperture, and thus that figure should be irrelevant. So when setting/reading the meter, that's what I proposed f1.0, because its the largest the meter will go.

    Hope I havent confused anyone! Love to see or hear of some examples of workflow doing shots 1m x 1m (40"x40") or larger.

    Cheers,
    Matt.
    Use an incident light meter with the flat diffuser. If the light is bright enough to get reliable reading then you're ok. Set the ISO at 25 times the paper speed in your case about ISO 150. Get the reading in EV. If you get EV 0 then use 1 sec exposure. For every EV step higher than 0 halve the exposure time. It should give you exposure of zone V. Measuring this way is really like a spot meter but using an incident meter. Oh you can set the aperture on the meter at f/1.0 then the shutter speed is what you are going to use.

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