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  1. #1
    Rudeofus's Avatar
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    Who uses a sky quality meter for estimating night time exposures?

    A recent thread here described in detail, how people estimate and bracket night time shots. This seems necessary because most light meters don't cover such dark scenarios. From my brief forays into astro photography I came to know about the "Sky Quality Meter" which essentially tells one how dark the night sky really is. Could this help me estimate accurate exposure times for night shots as well? Technically it seems like a very sensitive light meter, all one needs to do is establish a table for converting from their magnitude readings into exposure values. I could even imaging creating different hoods for the light sensor so I could restrict its angular coverage (would obviously require creating separate conversion tables for each hood).

    Has anyone tried this? Does it work for this purpose?

  2. #2
    Steve Smith's Avatar
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    I have never heard of such a thing. This did remind me though of the method my father taught me for metering sunsets: Point the meter straight up vertically. I can remember trying it once but can't recall how successful it was.


    Steve.
    "People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.

  3. #3
    Rudeofus's Avatar
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    Quote Originally Posted by Steve Smith View Post
    This did remind me though of the method my father taught me for metering sunsets: Point the meter straight up vertically. I can remember trying it once but can't recall how successful it was.
    I read that for metering sun sets one points the spot meter about 6 solar diameters off the sun, this has worked very well for me at least. The difference between this and night shots is that with night shots your exposure meter simply says "there is no light at all" when an 1 hour exposure confirms the opposite. It may even be useful for estimating maximum useful exposure time for star trail shots, before the back ground brightness registers on my film.

    If you search for "sky quality meter" you will find countless hits in astro forums, so it seems to be used in this area for predicting faint star visibility. I'm about to plunge down the money and get one, but would love to hear beforehand from people already using this device for night shots.

  4. #4
    Lee L's Avatar
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    Quote Originally Posted by Rudeofus View Post
    I read that for metering sun sets one points the spot meter about 6 solar diameters off the sun, this has worked very well for me at least.
    Interesting idea. Which direction do you use for the offset? Is that recommendation designed to be metered on clear sky?

    6 solar diameters is only 3 degrees, and I've seen published measurements on significant differences in flare among 1 degree spot meters, specifically the Minolta vs Pentax (although this was years ago and I don't recall which specific Pentax). In any case, not having tried this myself, I would expect the difference in flare would mean a variation of at least a stop or two when metering this way. Which spot meter are you using?

    I'm not saying this isn't a good approach, just that the recommendation may be more specific to a particular model of meter than might be apparent at first glance.

    Lee

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    Google for The Ultimate Exposure Computer...very, very accurate for a variety of situations.

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    Quote Originally Posted by PVia View Post
    Google for The Ultimate Exposure Computer...very, very accurate for a variety of situations.
    Or you can just set your Olympus OM2N to "Auto", and press the shutter release (using a tripod, of course)

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    The issue is not a meter to read low light, the issue is that film suffers from Reciprocity Failure...the emulsion response is down at the toe of the curve, not where it is relatively linear in response. So even if the meter said 50 seconds for the ISO speed that was loaded, it might really take much more time than indicated.

  8. #8
    AgX
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    The Recipocity Failure is not linked to any part of the specific curve. You can aim for a certain luminance of the opbject to correspond to a certain density on the film at any part of the specific curve. But under certain conditions (exposures yielding high intensity illuminance at shortes duration and low intensity illuminance at longest duration) the density will be lower than expected.

  9. #9
    Rudeofus's Avatar
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    @Lee: The 6 diameters rule is not a scientifically verified law but just a rule of thumb which I apply for sun set shots with silhuettes and with which I have gotten excellent results on slide film so far. I don't know about dedicated spot meters, as my EOS 3 does this job very well with its 1° spot measurement function.

    @PVia: The ultimate exposure guide just lists a number of light situations together with an exposure estimate for them. It does, unfortunately not tell me how long I can expose star trails before the back ground turns grey or how long to expose a very dark forrest or landscape (this is also where the in camera meter of my SLR falls short). For good negative film it's not a big deal if you are one or two stops off, for slide film being 2 stops off kills the image.

    @wiltw: sure, reciprocity failure needs to be taken into account, but that's something you measure only once for every film (or look up numbers online). As long as I did night time exposures with negative film the results looked great, although I just guestimated the exposure time. Unfortunately slide film isn't that forgiving, so I started looking for a measurement device for extremely low light.

  10. #10
    Lee L's Avatar
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    Quote Originally Posted by Rudeofus View Post
    @Lee: The 6 diameters rule is not a scientifically verified law but just a rule of thumb which I apply for sun set shots with silhuettes and with which I have gotten excellent results on slide film so far. I don't know about dedicated spot meters, as my EOS 3 does this job very well with its 1° spot measurement function.
    Thanks. I believe you'll find that the EOS 3 has a 2.4% spot meter. The viewing angle of the metering area in degrees will change with the focal length with an in camera TTL spot meter. That's why I assumed from your description that you were using a handheld spot meter.

    Lee

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