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Oh no.. !? (Need some help with long exposure & noise)

Discussion in 'Creative Corner' started by hmpws, Dec 11, 2010.

  1. hmpws

    hmpws Mu-43 Regular

    177
    Apr 24, 2010
    Auckland, New Zealand
    I took a long exposure of the night sky (relatively cool outside).

    The settings were:
    14-45mm @ 14mm f/5.6
    60 seconds
    ISO 200
    Long shot NR OFF

    <a href="http://www.flickr.com/photos/tonytse/5250582031/" title="P1000768.jpg by ttse007, on Flickr"> View attachment 154048 "640" height="480" alt="P1000768.jpg" /></a>

    I used ISO 200 because I don't have a remote release to do bulb mode yet :frown: and I needed more light.

    I turned NR off because I read that the noise can be dealt with in post processing but I didn't take a dark frame for dark frame subtraction either :frown:.

    There are quite a lot of noise, hot pixels and some banding?

    What can I do now? Does anyone have any pointers on how to best approach night sky photography? I was thinking that for this particular photo, I needed ideally about 2 stops more light (so 8min @ ISO 100)?

    I can post the RAW up, if anyone is interested in having a look at it. I am not sure where I can host it.

    P.S. a quick astronomy lesson for those interested (finding south in the southern hemisphere):

    <a href="http://www.flickr.com/photos/tonytse/5251200932/" title="P1000768 by ttse007, on Flickr"> View attachment 154049 "640" height="480" alt="P1000768" /></a>
     
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  2. bilzmale

    bilzmale Mu-43 All-Pro

    This type of shot is where you are best to rely on the inbuilt dark frame NR of the camera. It will take a second exposure so time will double.

    The advice about PP for NR applies (imho) to standard exposures.
     
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  3. Djarum

    Djarum Super Moderator Subscribing Member

    Dec 15, 2009
    Huntsville, AL, USA
    Jason
    Which Camera is this?

    Why not shoot wide open(f3.5)?
    I'm suprised at 60s that the stars did not start to streak. The best approach to this is to use a barn door mount or use a astronomy motorized GEM that is aligned to the south pole. If you do this, then you can use longer exposure times.

    Also,Typically dark frames are used to subtract the noise. Usually the camera will do this by default if shooting JPEG, but only uses one frame. If you are shooting RAW, then you will have to shoot this dark frame yourself. For your shot, you would put the lens cap on, shoot a dark frame for 60s, and use that frame as a layer on top of the shot and then you can change the opacity of the dark frame to reduce dark current noise.

    Dark current noise is sensor noise which is generated by the heat of the sensor. It is very predictable and even at base ISO, shows up in long exposures.

    Also, I'm extremely jealous, LOL. I'm too north to see alpha centauri(although farther south towards florida it can be see close to the horizon) and you get to see the Magellanic clouds.
     
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  4. zpierce

    zpierce Super Moderator Subscribing Member

    671
    Sep 26, 2010
    Minneapolis, MN
    Zach
    So that's what the camera is up to when it's doing the long shutter noise reduction? I've always wondered about that. How exactly does that work?
     
  5. Djarum

    Djarum Super Moderator Subscribing Member

    Dec 15, 2009
    Huntsville, AL, USA
    Jason
    Well, there are several different types of noise an image can have. One is called dark current noise. Basically, noise generated by heat while the sensor is reading data or just because there is some leakage current for that pixel.

    This noise is generated in a predictable manner, even at lower ISOs. Basically, hot pixels start to show up in the image. This noise will even appear if no light hits the senor.

    The way that it works is that the camera takes two images. The first is the image of the subject. The second is a dark image for the same exact amount of time with the sutter closed (many cameras do this by default in jpeg mode if the exposure time is long). The software compares then subtracts the two images. Obviously to some degree data is lost for the information that should have been at those locations. This is why typically in astrophotography, several shots of the sky are used and several shots blacks are used and combined to make one image. Usually stacking software is used.
     
  6. zpierce

    zpierce Super Moderator Subscribing Member

    671
    Sep 26, 2010
    Minneapolis, MN
    Zach
    Hey thanks for the info, this is very interesting and I'm still a bit confused. Why would it have to "take" another black image? Couldn't the processor just do the math? I assumed it was doing math processing the image during that "Long Shutter NR" time, however it seemed strange that it was always the same duration as the exposure.

    Along these lines, but slightly different topic, I just got a GH2, anyone have any idea why their bulb mode went down to 2 min from 4 min on previous cams (my G1), which was already fairly low compared to competing DSLRs?

    I'm very interested in low light / night photography, similar to the original poster, I want to get as much info as possible and gain more experience, I love the well done night images.

    Thanks!
    -ZP
     
  7. hmpws

    hmpws Mu-43 Regular

    177
    Apr 24, 2010
    Auckland, New Zealand
    Thanks for the reply. I shall try again next weekend if the weather is nice (and if I get my hands on a remote release).

    I have studied more in-depth about night time photography now. I will see how it goes.
     
  8. ~tc~

    ~tc~ Mu-43 Hall of Famer

    Oct 22, 2010
    Houston, TX
    Because the noise is unique to every situation - temp, shutter open time, even your particular sensor

    I don't know why you wouldn't use the LSNR - yeah it slows you down, but another 4 mins is a helluva lot less than it would take to get (probably) inferior results in post processing.

    Note that it's taking a black frame - it doesn't matter if the camera is moving during the second exposure
     
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  9. Djarum

    Djarum Super Moderator Subscribing Member

    Dec 15, 2009
    Huntsville, AL, USA
    Jason

    I'm not sure I understand the question. I believe if you shoot RAW, the camera does no sort of noise reduction, so it has to be manually done and then processed in photoshop or a program that can deal in layers.

    I know that my e-p1 takes about twice as long to process for long exposure times. I believe it is taking a black frame since I use jpeg. I could be wrong, but I know that with 60s, my shots aren't as noisy as the one you posted.

    Temperature plays a big role because the colder it is outside, the less noise you will end up getting. This is why dedicated astrophotography sensors have special cooling units attached to the sensor.
     
  10. zpierce

    zpierce Super Moderator Subscribing Member

    671
    Sep 26, 2010
    Minneapolis, MN
    Zach
    I'm clear now on it now, thanks. I wasn't understanding the point of taking a black photo but now I understand that it's taking basically a "context sensitive" black photo, accounting for conditions of the sensor at the time the original photo was taken.

    Long shutter noise reduction is definitely done on Panasonics when shooting RAW, because that's all I shoot and it always does it. Similarly, other processing is done on RAW photos in the camera as well such as lens correction. RAW is not as RAW as one would think on Panasonics at least :) 
     
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