Newbie questions about iso, shutter time settings

[Run45]

Hi,

I am really new to DSLR photography but recently got a Canon 100D and am aiming to get some night-sky pictures in light polluted London.

So basically just using the camera + tripod to shoot the night sky from my garden.

 

I have a  basic photography question:

If I assume the following "fantasy"/"idealistic" conditions :

    1) Having a perfect still tripod

   2)  Having 0 noise increase if you increase ISO

   3) The earth not rotating

Then would "long exposure + low iso" result in the exact same image as "shot exposure + high iso"  ?

If that is the case should I just do the following for night sky photography ?

1) f/n  ... choose lowest possible number n to gather as much light as possible.

2) calculate the longest possible exposure time so I do not get star-trails (there are some rule of thumb formulas I found online).

3) increase iso until I reach a non overexposed image

 

So the only real variable here is the ISO setting.

But 3) gives me some grief. If I increase the ISO  then sometimes the hight sky looks too bright (although I still can make out a lot of starts) ... Is that usually fixed in Photoshop etc (if then how ?)  ?

How should the sky look like in the raw image :  dark + some stars OR bright + lots of stars ... I am not sure what and what not can be fixed with post processing ... 

 

Thanx for any comments in advance.  

 

[Alien 13]

The standard answer is to look at the histogram (available in the camera menus) and aim to get the peak about a third of the way from the left. I often go much higher maybe two thirds or more but it depends on the target, one or two overexposed stars are easily fixed in post processing but large formations will loose detail.

The simplest way to correct for a bright sub/image is to apply an s-curve but fixing the start and end points in levels and then adjusting the mid for best balance will get you close.

Alan

[Run45]

thanx ... will try tonight to check the histogram tonight (if the weather is good .. cant see out of my office window ;-) )

[billyharris72]

If you are shooting from a fixed tripod with a sensibly sized lens (say up 100mm) then you should be fine for exposures of about 400 / focal length in seconds before you start to get serious issues with star trails (experiment with this though, as it depends on the size of the camera sensor and where in the sky you are pointing it).

For the sort of exposure lengths you are dealing with you want your focal ratio (f-stop) as low as it will go (though going really low makes the focus more critical, so play around from F1.8 to about maybe F5). The exposure time really benefits from a low f ratio - halving the ratio means you can quarter your exposure and vice versa.

High ISO is good but will lead to noise. Depending on your camera, probably ISO 800 or 1600 will be the best compromise. You can shoot darks (or with short exposures just turn on noise reduction) to manage this somewhat, but you will always get a grainier image as ISO increases. You will also find that there comes a point where skyglow starts to wash out the image (worse if there is any light pollution). In this case either back off on exposure time or drop your ISO. I would not stop down if you have good focus, as you don't gain anything from that.

As for how the sky should look in the raw, I tend towards slightly bright with more stars, but not so that it's really washed out. Again, I can't stress this enough, play about with your options and see what comes out. Don't judge your images on unprocessed RAWs - the default rendering of RAW images on sofware is often quite unsuitable for astro images. Have a play with each one in Photoshop or Gimp (just with colour levels) and see what looks best.

[D4N]

1) Low f number comes at a cost, you have the lens wide open so you get lots of light but this also means you will have a harder time focusing as the sweet spot for focus gets smaller (hence high f number gives you more depth of field). Often somewhere between f2 and f4 works but it will depend on the lens.  Low f number will also make any optical defects more obvious causing aberrations in the image.

 

2) Yep and then come down a little bit from that to be sure.

 

3) Gain is very camera specific, there may even be negative gain settings available that you really don't want to be using (for example if your cameras sensor is equivalent to ISO400 with no gain applied then ISO100 would actually be negative).  At higher gain settings there will be more noise but so long as you aren't below the unity gain level you aren't actually adding any more signal, noise and signal are both increased by gain.  In theory the camera has captured the data so you should be able to get out it in post processing so long as nothing is over exposed.

There are also other effects that kick in at different gain settings, there may be active noise reduction that may look nice but also has a habit of considering stars are noise and removing them from the image.

[billyharris72]

Never thought about the histogram - will give it a go myself.

[MARS1960]

If you have found some online rules of thumb then you know that shooting at say 100mm will allow exposures of up to 5 secs before you get trailing so what i would do is set exposure to 4secs, pick an iso of around 1600 and then check the histogram, etc,etc. As for f-stop, start wide open and try a few different iso settings, then stop down and try again, etc etc, all the time sticking to the 500 rule.

[MARS1960]

Also this may be of some help.

http://www.astropix.com/HTML/I_ASTROP/SETTINGS.HTM

[Run45]

thanx everyone for those tips ...very useful.