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Thread: Proper exposure for stars, no moon?

  1. #11
    Richard M. Coda
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    Re: Proper exposure for stars, no moon?

    I'm no scientist but I seem to recall hearing (from Rod Klukas) that exposures have to be less than 8 seconds or there WILL be noticeable movement. This was for the moon, so I'm not sure if that applies to stars as they are tinier.
    Photographs by Richard M. Coda
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    "Speak softly and carry an 8x10"
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  2. #12

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    Re: Proper exposure for stars, no moon?

    Around dusk while sun is just up you can easily see stars against the blue sky. They are very bright and don't suffer from night time. That means you can expose for sunny 16 rule or thereabouts and the stars will appear on film. Fact that it is night is irrelevant since you are not photographing the blackness, you are exposing the bright objects.

    The problems arise when you want to include the landscape in the image. That means you have one subject which is very bright and one which is very very dark, especially if there is no moon. Then what is the sky doing. Is it light polluted because if not then it will be black too. That means there is nothing to give a contrast edge to the land sky boundary. So in reality it depends what you are actually photographing, land and stars or, land, sky and stars or just stars.

    On my rough calculation for 150 lens on 4x5 film, you will get around a 0.025mm of movement over an approx 2 second exposure. That is 25% of 0.1mm circle of confusion. All very approximate but the maths is simple to do. If you want very sharp stars then go with the sunny 16 rule but don't expect any land features to show up except the horizon if there is plenty of light pollution around you.

  3. #13

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    Re: Proper exposure for stars, no moon?

    motors are required for telescopes because the angle of view is very small so the stars move across the field very fast by comparison to the sort of lens focal lengths we use.

  4. #14

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    Re: Proper exposure for stars, no moon?

    And finally, I'm referring to the human visible constellations which we all know. If you are wanting to photograph the much dimmer non human visible stars, then of course a motorised unit is required.

  5. #15
    umop episdn
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    Re: Proper exposure for stars, no moon?

    I've done this with my 4x5. With film at least, this kind of photograph (with pinpoint, sharp stars) can't be done with a single shot on a tripod.

    There really isn't a fixed formula for photographing stars. There are too many variables. Wide angle or telephoto lens, maximum f-stop of the lens, sensitivity of your digital camera or film, portion of the sky you're photographing, and how much local light pollution you have are going to radically affect the results.

    Generally, however, exposure times for stars are going to be several minutes. A regular tripod just won't work. You'll need a equatorial telescope mount to track the stars to keep them sharp.

    The photo below I made with my 4x5 camera. It was done with two exposures on a single sheet of color transparency film; one for the foreground and sunset (on a normal tripod) and the stars at home with the camera riding atop a simple equatorial telescope mount. I'd be glad to go into detail if you'd like - just send me a PM.

  6. #16

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    Re: Proper exposure for stars, no moon?

    As already said, in the astrophotography the f number is not decisive, the actual physical aperture dimension is, because of the point light source the stars are. Forget about any sunny f16 rule...
    Also, the star trailing depends on declination (latitude) beside the focal length ect... There is a mathematical formula for this.

  7. #17
    Richard M. Coda
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    Re: Proper exposure for stars, no moon?

    That's a nice image, Danny.
    Photographs by Richard M. Coda
    my blog
    Primordial: 2010 - Photographs of the Arizona Monsoon
    "Speak softly and carry an 8x10"
    "I shoot a HYBRID - Arca/Canham 11x14"

  8. #18

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    Re: Proper exposure for stars, no moon?

    I think the question is, how long an exposure can he get away with and NOT create star trails?

    This might help: http://velatron.com/dca/Tripod/

    Edit: Just found the rule of thumb for this: 600 / FL gives the maximum exposure time (in seconds) to avoid obvious trailing.

  9. #19

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    Re: Proper exposure for stars, no moon?

    My experience trying to shoot star trails, 10+ years back, is that reciprocity failure will kill you if you use conventional b/w films or 100-speed color film. In my case, an f/8 w/a lens wasn't fast enough. Perhaps with 400NC, TMY, or TMX?

  10. #20
    Peter De Smidt's Avatar
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    Re: Proper exposure for stars, no moon?

    Acros and G100x (Kodak color slide) films don't have reciprocity failure until exposure over 2 minutes.
    “You often feel tired, not because you've done too much, but because you've done too little of what sparks a light in you.”
    ― Alexander Den Heijer, Nothing You Don't Already Know

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