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I currently own a Contax 139 which, unlike many other models, doesn't feature a switch you can lift the mirror with. I'm planning to do some night stars bw photography with very long exposures (i.e. about 4- hours).

Could the mirror movement itself blur the picture?

If so, does the blurring effect anyhow worsen or decrease if the exposure is very long? Does adding a weight to my tripod help mitigate the effect?

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    Have you already solved the problem that through the 4 hours you'll need to traverse your camera across a third of the sky? – MooseBoys Jun 28 '17 at 3:02
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    One of the most important effects I see for outdoors long time exposure stability is the camera strap: remove it, any kind of wind will catch in it and move the camera. – PlasmaHH Jun 28 '17 at 8:23
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    @MooseBoys depends if you're using it to look at some specific planet/star/nebula or just want one of those star-trail photos. – Nick T Jun 28 '17 at 21:29
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    Yes actually @MooseBoys I'm going for the star trails – Eugenio Jun 29 '17 at 7:15
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Mirror slap is an issue in "medium" long exposures - from 1/30 or so til about a second or two.

Tripod shake is issue in "long" long exposures - from about a second upwards.

For shooting stars you can safely forget about the mirror slap. It will last about a second of the four hours exposure. Nothing of importance will be captured in such a short time.

Weighting your tripod makes good sense. A good policy is put a small hook on your tripod's central column (most have a hole in it) and hang your camera bag on it. Plus of course use a wire release and take care not to accidentally kick your tripod in the dark.

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    Depending upon format and viewing size, mirror movement can affect even shorter exposure times. With 35mm/FF cameras it can affect Tv as short as 1/125 or 1/160. – Michael C Jun 28 '17 at 11:07
  • You can forget about the mirror slap, but you must remember that the Earth is constantly moving through space while spinning about its axis, and that this has significant implications if you're going to point a camera at the sky and try to take a 4 hour long exposure. – aroth Jun 29 '17 at 9:47
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Along with weighing down the tripod, using a cable to release the shutter will help reduce camera movement.

Also, if you want to guarantee no mirror shake happens during the exposure, hold a black object in front of the lens while triggering the shot, then pull it away for the duration of your exposure.

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Mirror slap will last some fraction of a second. This is completely irrelevant in a multi hour exposure.

Weighing down your tripod is a good idea nonetheless, since movement from wind will be a bigger risk.

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    If you want to guarantee no mirror shake happens during the exposure, hold a black object in front of the lens while triggering the shot, then pull it away for the duration of your exposure. – digijim Jun 27 '17 at 18:26
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There's always the old Hat Trick:

  1. Take your hat (or anything blocking light, a piece of cardboard, the dew cap of your lens etc.). Hold it in front of the lens to prevent light to reach your film.
  2. With the hat still in place, release your camera's shutter. Wait until mirror slap and tripod shake have faded.
  3. Remove the hat without disturbing the camera.

You basically put another shutter mechanism in front of your camera's shutter which is afflicted with vibrations.

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