light meter for long exposures 10mins plus

Discussion in '35mm Cameras' started by ww, Apr 11, 2004.

  1. ww

    ww Guest

    can anyone recommend a light mter for exposures lasting 10mins to a couple
    of hours cheers
    ww, Apr 11, 2004
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  2. ww

    Jani Guest

    I'd be interested to know this too. A couple of years ago I was trying to
    find a meter that can measure up to exposures lasting several hours but
    find one. So I ended up designing and building one myself. The trouble with
    extremely small amounts of light is that the meter becomes sensitive to even
    the smallest amounts of light so just a small change in amount of light
    bounces the exposure time up and down quite a bit and one still has to do
    some guessing between these values.

    Jani, Apr 11, 2004
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  3. As I recall the Gossen Luna-Pro was (is?) tops in that area.

    BTW without adding film speed and aperture the exposure time does not
    tell us a lot about your requirements. ;-)
    Joseph Meehan, Apr 11, 2004

  4. There is no way to accurately measure it, for several reasons.

    First, the photon energy is too low for most contemporary methods to
    register. The only way is by measuring a cumulative change over a period of
    time. It's cheaper to do this with a chemical reaction, say, with film
    emulsion... ;-)

    Second, even if you could get an accurate reading (with astronomical
    tools for example), reciprocity failures of films makes this an arbitrary
    number. All films become less sensitive to light for longer exposures, and
    usually not evenly across the color spectrum, so adjustments have to be
    made in all cases. Most times, drastic adjustments, especially with
    exposures that long. And this isn't even considering that many films, in
    low exposure conditions, become grainy as hell.

    And finally, in the conditions you're most likely to be using such a
    reading, i.e., a starfield at night, the conditions are changing throughout
    the time period. Stars move, skyglow shifts and changes with activity in
    the closest city, reflective clouds or haze roll in, and so on.

    Not to mention, any kind of contemporary meter is going to give a
    reading for 18% grey. My light meter at the farthest end of the scale will
    get an accurate reading from bright moonlight, but what this means is that
    I now have an exposure setting to make something in moonlight exposed as if
    in daylight. Not the effect I'm usually after. If you want your skies (or
    whatever) to look middle-tone, then such a meter reading is useful. Chances
    are you don't ;-).

    One roll of film, exposed for different times, will give you a
    guideline, and show you some of the pitfalls. For instance, stars aren't
    exposing the same position on film for more than 30 seconds - they've
    moved. But skyglow will. So for a one-hour exposure, skyglow can overpower
    the stars, even though the stars appear much brighter to the naked eye.

    Fuji Provia 100F gives brilliant star colors and exceptionally low
    grain. But it works absolutely terribly with the amber-colored sodium
    pressure lamps used in most US cities nowadays, and will render the color a
    sickly green. I've got some tests on Fuji Sensia to check out soon. Kodak
    Portra 400 has the largest recip failure I've seen, is total crap for long
    exposures. Fuji Superia 400 can get a good exposure, but produces prominent
    grain in low-exposure areas (dark portions of sky). So you really have to
    experiment to get good results.

    Oh, yeah. Provia 100F can be pushed, rated at a higher ISO, which
    supposedly makes it more sensitive to light and allows a shorter exposure
    or smaller aperture. However, the recip failure is so great in cases like
    this that it actually makes no difference whatsoever - you might as well
    use it as normal and save the extra porcessing fees for a push.

    Good luck!

    - Al.
    Al Denelsbeck, Apr 11, 2004
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