7D: Grainy sky pics

Started May 2, 2012 | Discussions thread
JonJT
Forum MemberPosts: 70
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Re: 7D: Grainy sky pics
In reply to Sailor Blue, May 4, 2012

Whether you use HA-ETTR or "regular" ETTF, the resultant image, with non critical highlights nearly or actually clipping, may not provide the OP with the final exposure he wants. It may well be too exposed. Using the image he posted as an example of the exposure he wants, he'd have to bring everything down in post because, that image does not require the full dynamic range of the 7D sensor to capture, as it is exposed. HA-ETTR would produce a much brighter image. This is where the exposure reduction occurs, and that is only if the op actually wants to change the exposure.

You don't need high ISO values to take sky-scape pictures at night. With a sufficiently fast lens (as you noted), you can take pictures of the night sky with relatively low ISO settings and long-ish shutter speeds; certainly low enough that ISO noise isn't that much of an issue.

Sailor Blue wrote:

JonJT was right, ISO value and exposure do effect noise (grain) in an image, but wrong in suggesting that you reduce your ISO or exposures.

ETTR is the best way to expose anything with a digital camera. HA-ETTR is an easier way to do ETTR than the traditional way of using the histogram.

http://www.dpreview.com/articles/8391014823/ha-ettr-an-easier-way-to-expose-to-the-right-using-the-cameras-highlight-alert

Your problem is that in order to acquire star images you are having to use high ISO values and long exposures. This means that you have a combination of noise from using the high ISO and thermal noise.

There isn't anything you can do about high ISO noise. The noise reduction in post processing is more flexible than the in-camera noise reduction so that is the method I would recommend even though it is a lot of work. Using a lower ISO and then increasing the exposure in post processing will give you more noise, not less.

Temperature is basically defined by how fast the atoms of a substance vibrate. If they vibrate with enough energy in a semi-conductor like your image sensor. you can get electrons that break free. These free electrons are read as a bit of local exposure, i.e. noise.

Thermal noise is always present but only becomes noticeable with long exposure times. Astronomers use image sensors chilled to the temperature of liquid nitrogen, −196 °C (77 K; −321 °F), to minimize thermal noise and allow them to use exposure times of hours.

Chilling your camera below about -18°C (0 °F) will not work - you will simply freeze up the mechanics and may damage your camera and lenses.

There are two ways to reduce your noise problem. First, get a faster lens, which will shorten your exposure times. Second, take multiple images and stack them - the noise will be reduced by the square root of the number of exposures, i.e. if you stack 16 images you will have 1/4 the noise.

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