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I want to take a picture of a (almost) pitch black landscape at night. Like a mountain or trees at night. The object will not be moving and I will have a tripod.

I am not sure what is the best way to get the best exposure and image quality.

I know that high ISO gives noise, but also that long exposures can give fix field noise.

Do I want to up the iso above 100 to avoid fix field noise of long exposure?

Or since because I am using a tripod, should I just do 100 iso with a REALLY long exposure time, like > 1 min?

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First of all, your camera has certain limits set in place by the manufacturer, but you'll generally not want to push those, so that you can get better quality photos. So although you might have an ISO HI 2 setting, it's going to be super noisy. Generally, you'll want to stick to as low an ISO as you can get, without compromising other elements of your photo.

If your subject(s) are truly not moving (e.g. rocks, grass at longer distances, other solid objects), you can literally put your camera on the tripod and let it run for a few minutes (or even hours) if you need to. In this case, a low ISO would be better because you'll have less noise, without compromising your image. Note that I'm not going to get into lighting here, but be wary of "tiny" lights that are brighter than everything else -- they'll definitely show up in your images.

However, some "stationary" objects actually move slightly. A good example of this would be trees. While they don't generally get up and walk, you might be surprised how much the branches sway in the wind. If it's completely calm outside, you might be able to ignore this, especially if the trees are further away. But if there's even the slightest touch of breeze, you'll want a significantly shorter shutter time so as not to get blurred tree outlines. In that case, a higher ISO would be justified. They make noise reduction tools for a reason.

One other field to be wary of is the sky. Since we're rotating, the stars aren't going to stay in the same positions, so even relatively short exposures of ~10 seconds could potentially be problematic. (A general rule is about 1 pixel of blur for every 4 seconds of exposure with a wide angle lens at 24MP). For this area though, I'd just encourage you to try it out once and see how much of a difference it makes in your photo. It may not be noticeable for what you're using it for (or it may!).

In short, the lower you can get your ISO without compromising your artistic intent, the better. With long exposures though, you have to be careful about things that you don't generally percieve as moving objects moving (trees, stars, grass).

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Highest ISO at widest lens aperture (for example, ISO 3200 at f/2.8) is a good starting point for the Milky Way. This will still need an exposure of at least 20 or better 30 seconds, which will still be blurred a bit by Earths rotation (stars causing short trails) if on a fixed mount. A very short focal length (very wide angle) is much less problem, but any telephoto try will be terrible about star blur if on a fixed mount.

Aside from the concept of adequate ISO and lens aperture for exposure, there is a rule of thumb called the 500 Rule, which says the longest exposure time to not show objectionable trailing is 500 / focal length. It is NOT about exposure, but about star trail blur. But that may be far too short a time to provide decent exposure. So some call it 600 / focal length to compute a longer time, but that just makes it worse. Howver, it is really too simple minded, because sensor size and resolution is also a factor (of trail length in pixels).

I have a Milky Way calculator (about trail length, NOT about adequate exposure) at https://scantips.com/lights/stars.html which compute star trail length in pixels (after knowing sensor dimensions, both mm and pixels).

One of its results is that the rule: 412 / focal length computes a trail equal to the standard DOF CoC dimension (which from DOF definitions, assumes is not noticeable).

However, you will likely need a longer exposure, and a tracking mount will do it right. One suitable and very inexpensive DIY tracking mount has pages of examples at https://www.google.com/search?q=barn+door+hinge+tracker+astrophotography

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    OP is asking about shooting the foreground for a composited Milky Way shot. – scottbb Aug 1 '18 at 21:05
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    Yes, this answer does not at ALL answer the question. – ScottF Aug 1 '18 at 21:18
  • I have to agree now, sorry. My skimming skill must be rather poor, I didn't sense that before. – WayneF Aug 1 '18 at 22:45

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