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I was on a hiking for a week into the dense forest of Western Ghats with my Sony slt a58.

I am into Astrophotography.

The place was absolute black with millions of stars. The perfect place for shooting stars and milky-way.

I was getting shot with following setting:

Exposure:

Shutter Speed: 30 seconds

f/number: f/3.5

ISO : 1600

Now the image was with more noise and then i have to decrease either ISO or Shutterspeed.

I have tried using ISO-800, 30s and it was less noisy but bit more darker.

So in this condition what should be more feasible to decrease ISO or Shutterspeed? and why? to get perfect less noisy star photograph?

Note: There is also online calulator that will calculate exposure depending on your camera gear - http://www.lonelyspeck.com/milky-way-exposure-calculator/

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The best way to deal with noise in the situation you describe is to use a form of dark frame subtraction. If your camera doesn't offer such a built in feature, take a few frames during your session with the lens cap on (using the same ISO, and shutter time). Then use an application in post processing to subtract the dark frame from each segment from the light exposures.

Please see How to prevent hot pixels? and What's the best way to deal with hot/stuck pixels in long exposure night photographs? for more about dark frame subtraction.

Another way to deal with your problem is to stack multiple shorter exposures. There are several applications that you might use to do so. Most of them allow you to take a single dark frame at the end. The dark frame is subtracted (which deals with fixed noise associated with the sensor and any "hot spots" it might have as well as banding) and the images are combined, which allows the random noise from one image to the next to be averaged out. Most stacking programs can also auto align the stars as they move across the sky from one frame to the next.

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The SLT A58 has the so-called "long exposure noise reduction" function which performs the dark frame subtraction mentioned in Micheal's Clark's answer. You should choose the shutter speed a lot less to prevent start trails (A few seconds at most, depending on the focal length) and crank up the ISO instead. That will dramatically increase the noise, but you deal with that using image stacking mentioned in Micheal's Clark's answer.

In general, it is better to work with higher ISO images that are more noisy to do image stacking with than lower ISO underexposed images. In the end you'll have to multiply the result in the later case to get to the same result in the former case, but multiplication in post processing typically yields worse results than having started out with the correspondingly higher ISO in the first place.

You should also set the high ISO noise reduction to low, make sure you shoot raw images or raw + jpg. Then with Sony's Image Data Converter convert the raw files to 16 bit tiff files and switch noise reduction off when processing the files. You really do not want to do any noise reduction on the individual images, as you'll then lose the signal from faint stars that are hidden in the noise. In practice, the way the images are processed in camera and with Image Data Converter, even with all the noise reduction setting set to minimal or off, will still lead to some noise reduction being performed, so you'll want to minimize this.

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