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Apologies if this has been done to death, but a quick search of the forums didn't show what I wanted.
I have just returned from Cefalonia where I had the opportunity (at long last) of trying for a star trail. The night appeared good - lots of stars, a very small moon (well out of the shots composition). I set the composition with some foreground and plenty of sky - due to geographical local I couldn't set a northly perspective - but I didn't think this would matter - the view was across the sea so no obvious sources of light polution were present.
I set the camera (5D) to an iso of 100, and to f11. Opened the shutter to bulb using the cable release and gave a 40 minute exposure. I did some light painting on the key forground features, and then sat bac and waited. The result an absolute mass of noise. The light painting hadn't even rcorded (this was probably due to my relying on a hand held led torch purchased from the local supermarket - not the brightest beam).
I know digital sensors are prone to random noise on long exposures, but I reall y thought I would capture something! Anyone advise me where I went wrong? Should I be reverting to film for such shots?
Rgds,
Adam
Hi Adam.
I'll try and shed some light (pardon the pun) on the noise problem for you.
You get it for various reasons.
1. Long Exposures.
2. Low Light Levels.
3. Temperature of the sensor.
So if you are in a hot country in the dark and trying to do a really long exposure you have the recipe for a very noisy photo.
On the Canon 20D there is a setting for long exposure noise control.
What it does is to take another photo without actually opening the shutter, (known as a dark frame in astrophotography) which is used with the original frame to reduce the noise level in camera.
They do this by subtracting one image from the other, so if the noise is in the same place on both shots it is removed or reduced.
You may have had better results if you had opened up the aperture a little and let a bit more light in so reducing the exposure time.
There isn't much you can do about cooling the camera sensor except try and keep the whole camera cool during the day.
It is possible as Luis suggests to take a whole load of shorter duration shots and combine them together (called stacking) in software such as RegiStax.
HTH
Doug 
I don't know how bright the moonlight was, but I suspect that F11 would have underexposed quite significantly. F4 would have been a better bet
You will see a lot of noise in star trails - it's the nature of the game. When you photograph is daylight, the ratio of signal (light) to noise is high, but at night the ratio of signal to noise is much lower, because there is so much less light. F11 would have cut down the light still further.
Make sure you have a fully charged battery before you start. I'm not certain, but it seems to me that low battery power could contribute to poorer noise performance.
Quote: On the Canon 20D there is a setting for long exposure noise control.
What it does is to take another photo without actually opening the shutter, (known as a dark frame in astrophotography) which is used with the original frame to reduce the noise level in camera.
They do this by subtracting one image from the other, so if the noise is in the same place on both shots it is removed or reduced.
That feature is on the 5D too. but having tried back to back tests at night (5 min exposure), I found that the 'noise reduction' feature gave worse results than a 'normal' shot.
Plus 80 minutes is right at the limit of the 5D battery life span. Don't want to wait 79 minutes and find the battery runs out....
You don’t need to use the in camera NR, you just need a dark frame of the same length and as close to shooting temperature as possible and you can do the dark frame subtraction in PS.
For example, if you are shooting 30 min exposures, make a dark frame of the same length (shoot 30 min with lens cap on and in a dark room so there is NO light with the temperature as close to the estimated shooting conditions as possible) same camera so same pixels and sensor, you can then use this frame for all 30 min exposures to do the subtraction in PS.
Repeat the process for all the exposure lengths you are likely to do and or with different camera bodies, and archive them in a folder for later use name them 30_min_exp_5d etc.
PS work.
Paste the dark frame over the light frame and select blending mode difference.
End result, isn’t perfect, nor is in cam NR but a good reduction of noise, results vary from camera to camera, large prints probably not possible up A4 can be OK.
Cheers Guys,
Plenty of food for thought. To be honest I decided to follow the advice I had read re: camera settings. I was tempted to have a second go with an increased iso and a wider aperture - but as it was already well passed one in the morning I decided to call it a night.
I have had noise problems with the 5d before when photographing lightening - in that case the shutter speed was much shorter (probably about 5 mins tops). I remember trying the noise reduction technique - it consummed batteries at an alarming rate and didn't really seem to reduce the noise.
So Simon given Doug's point about random noise do you reckon your PS technique definately cuts noise down?
Finally, would working in film make a difference?
Adam
The PS dark frame works well with hot pixel colour noise as that is consistent and pretty sensor specific, doesn’t really help random noise that much, as it well random so there is little or no consistency. Still a fair improvement on no NR though.
With film you don’t get increased grain with exposure length (just with film speed) like you do with digital, but if you screw up you don’t know until you get in the dark room, some film works better than others, been a long tome though cant remember which is better/worse. Obviously lower ISO film performs better grain wise.
http://photo.net/photodb/photo?photo_id=6962358
Digital Star Trail, very well done and by a guy who will have no qualms sharing his techniques! In short, a series of three-minute exposures all blended together seemed to be his route, with a select bit of light-painting and lots of warm coffee...
All the best,
James
(p.s. - Film is champion of this anytime, it'll just take a few goes to get right...)
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