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Photography Question 
Janet Bradley

star trails settings

Hi! I used to take star trail pics with my 30mm Nikon years ago and had relatively nice results.. I'm trying them again with my D90 and I cannot get the sky to be black.
here's what I'm doing:
new moon, clear skies, very dark location.
bulb setting, F5.6 manual focus, ISO L0.3 (I cannot seem to find goes from ISO200 down to L0.3)
and left lens open for 1 hour. The pic shows a light grey sky. I got the 15 degree arc of stars but I want the black sky. What am I not doing?? I really appreciate any advice.. I have 2 more nights of the perseids. -jan

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8/12/2010 7:04:25 AM

Gregory LaGrange
BetterPhoto Member Since: 11/11/2003
  I'm feeling that when you did it before, you did it with negative film. And although most things carry over when you go from a film camera to a digital one, some things don't work quite as well.
You don't have the exposure latitude that you had with film, for one. Another is contrast and saturation that is built into the film, that's something you usually have to do yourself with digital. If shooting film for altruistic as some people like to claim, then what need would there be for a film company to make a film best for portraits, best for great color and scenics, etc...
Also, what you see as a completely pitch black night to your eye, with a very long exposure might actually be coming out normal with light from the moon, street lights, distant buildings.
Digital can also have a different point of reciprocity failure. And that means that with exposures outside the normal range, like super long ones of an hour, the normal matching of less light with more time doesn't follow the typical measurements. Such as, you figure what does it take for a 30sec exposure at night. So you make your calculations of doubling it to one minute, then two minutes, 4 minutes...and you get it up to what do you set for a one hour exposure. Because of reciprocity failure, you would actually have to a much longer or shorter exposure.
As far as the pictures you're getting now, if your star trails are coming out good enough, you could use the pictures you have now and work on the contrast and darken the sky.

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8/12/2010 2:04:02 PM

Lynn R. Powers   Jan,

You are talking apples to oranges comparing the perseids and star trails
but I will try to cover both. First of all I have no idea what Lo.3 is. Nikon books talk in engneering not photo terms. The standard lowest ISO on the Nikons is 200. You want to use an ISO of 800-1600. I just saw some nice photos of the Milky Way taken at 20" at ISO 800. For brighter stars(trails) use a higher ISO. Also use a wide angle lens, at least 18mm on your camera and open the lens all the way or at the most closed one stop.

The backgrount grey skies does sound like light pollution. Either from nearby lights, Zodiac light, far off city light pollution or if near the mountains alpineglow. A little bit of stray light exposed for a long time records a lot of light on the sensor. Digital cameras do not suffer from reciprocity failure like film does.

According to APOD, after midnight to dawn will be the best time to capture the persieds although the entire night should be pretty good. You might want to get some sleep early and be on sight at 2:30AM. (Someone once told me that there was such a time of day. :-) )
I would suggest shooting at ISO800 for 30". You will need a large card. Using a cable release just push the button after each shot. I am sure you know better than to wait until you see the streak in the sky to try to take it then. You may want to go for one minute exposures.

For star trails it is recommended to take several photos of 10 minutes each and stack them together. Some people do it for six minutes each and some do it for a minute each and end up with stacking 60 photos atop one another. You have to have the proper program to stack photos. CS3 the extended version was necessary but I am unsure if it is in CS4 or CS5.

For one hour exposures and star trails you have to make sure there is absolutely no potential light pollution, take the photos at midnight or later and pray.


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8/13/2010 11:07:04 AM

Kathryn Wesserling
BetterPhoto Member Since: 4/21/2005
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  Thought you might appreciate some additional information - the Link is the newly arrived SnapShot newsletter.

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9/14/2010 12:02:47 PM

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