# Stair trails and long exposure noise reduction



## Pinchers of Peril (May 7, 2015)

I've never tried to do any "star trail" photography so I was looking for some tips. I know that the 5dIII has long exposure noise reduction, but I know it takes the camera about twice as long to process the image when this setting is turned. Since I'll most likely be post processing the image afterwards anyway I was wondering if there is any real benefit to this. Also, does this only apply to jpeg or does it do semething if you are shooting RAW? I know that I could also take multiple exposures and just stack them to minimize noise as well, but this seems tedious to me and I'd rather shoot one long exposure try to edit out the noise afterward. Like I said I've never tried this so I'll be curious how it turns out. I just wanted to see if any of you people had experience with it. Thanks.


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## Pinchers of Peril (May 7, 2015)

After doing some research I found that the long exposure reduction actually does affect RAW files so that is good to know. Still curious if any of you out there have actually used it and how the results were for you.


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## timmy_650 (May 9, 2015)

I have found it better to take many photos and stack them in PS. If you are going for one shot why are you worried about noise. I would think you would be under iso 800 if you are going for one long shot. The problem I have found with one shot is that plane keep getting into my shot and if it is a long shot it is a lot harder to get rid of them.


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## Pinchers of Peril (May 11, 2015)

Yeah I imagine planes would be pretty annoying. Even though my ISO would be above 800 I still feel like having an exposure of almost an hour is sure to heat up the sensor and cause noise. I don't have software for stacking pictures and so I don't have much experience with that. It also sounds like a ton of work ha ha.


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## timmy_650 (May 14, 2015)

I use PS and stacking is super easy. It is a lot easier than trying to calculate setting for a super long exposure. All you have to do is click Load files into a stack and then go over and choose lighten and then you are done. I Started like you and have having trouble with everything and sitting out in the cold for like 30 mins and have your picture not come out is really annoying. Then I thought about getting a film camera so make my life easier. Then I learned how to stack and haven't looked back.


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## jrista (May 14, 2015)

Pinchers of Peril said:


> After doing some research I found that the long exposure reduction actually does affect RAW files so that is good to know. Still curious if any of you out there have actually used it and how the results were for you.



LENR does affect RAW, as it's done at the time the light exposures are taken. With older cameras, a single dark frame will be exposed (shutter closed) at the same settings as each preceeding light frame, then subtracted from that light frame, before the light frame is written to the memory card. With newer cameras, the camera will usually wait for a few RAW images to fill the buffer, then take a single dark frame that is subtracted from each of the pending light frames in the buffer before they are written.

There are a couple drawbacks with LENR. It is effective in removing dark current, and thus the thermal signal (hot pixels) will be removed from each light frame. The first drawback is that every frame has random noise, and if you subtract two random distributions from each other, the standard deviation grows (in other words, the image gets noisier, even though the hot pixels were removed). It won't get a lot noisier, but when it comes to astrophotography, your working with tiny signals to start with, so the increase in noise is a factor to consider.

The more grievous issue is that LENR is done inline with your light frames. You have to wait before starting the next light frame while the dark is being exposed and subtracted. This is a problem for astrophotography in general, as total sub count has a direct impact on how much you can reduce noise by integrating the frames. With most cameras, you lose half your light frame time by using LENR. With a couple of Canon's newer cameras that support "Multiple LENR", you might lose a third of the time. 

The problem is a more explicit issue with star trails imaging, however. With star trails, you need to keep exposing as continuously as you can to avoid gaps in the trails. With LENR, you'll usually get consistent gaps, as the exposures are as long as the LENR time between them. 

The solution is to simply take a set of darks before you start, or after you start, once the camera has reached it's equilibrium temperature (darks need to match the temperature of the lights). Just take the darks at the same ISO and same exposure time as each star trails frame. Take about 10 darks, and combine them together with averaging to produce a master dark (note, this must be done BEFORE debayering the dark frames.) You now have a master dark that can be used to calibrate every single star trails frame you took. No more gaps.


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