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Digital Sensors and Long Exposures

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    Camairish
    Camairish  81276 forum posts Scotland
    15 Jan 2012 - 2:40 PM

    For bulb exposures (eg night shots) can I damage the sensor through overheating after a certain length of time?
    Also, I usually have the in camera noise control switched off - would it be better on for longer exposures of eg over 30 seconds?

    Many Thanks,

    Ian.

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    15 Jan 2012 - 2:40 PM

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    sherlob
    sherlob e2 Member 82314 forum postssherlob vcard United Kingdom125 Constructive Critique Points
    15 Jan 2012 - 4:25 PM

    I've not heard of a sensor overheating through the use of bulb. Rather, the problem of a long bulb exposure in my experience is one of noise. I tend to find the longer the exposure the more the noise in the image (each camera sensor is different so you will need to experiment with your own kit). I'm pretty sure the noise is of a certain type too - if I recall correctly its 'colour' noise. This can prove handy when correcting it in LR or PS (or your image editor of choice). You'll find reference to many cameras having a noise reduction feature for bulb exposures. This takes a second identical exposure immediately after the 1st - but with the shutter closed. The camera then tries to map the noise on the 1st image by using the second as a template. Personally I've never found this of benefit - especially as you are left waiting for the camera to finish the process - on a long exposure this can be painfully slow and lead to you losing any precious light left...

    Hope this helps,

    Adam

    EddieAC
    EddieAC e2 Member 9679 forum postsEddieAC vcard United Kingdom2 Constructive Critique Points
    15 Jan 2012 - 4:39 PM

    I don't think you can damage the sensor through long exposures although you will probably get noise and maybe some hot pixels in the image. There are numerous long exposures posted on flickr of over an hour long.

    With long exposure noise reduction you have to wait while the camera performs a second dark exposure of the same shutter duration and then subtracts it from the exposure it has just captured. A 30 sec exposure becomes 1 minute therefore it may be impractical to use bulb for 30 mins.

    If necessary you can leave this setting off and take this dark frame yourself by leaving the lens cap on, covering the viewfinder and taking an exposure with the same settings. You then stack this dark frame on your main exposure and set the blend mode to difference. This may mean you could take 20 different exposures of 1 minute in length and use only one dark frame for all of them.

    Last Modified By EddieAC at 15 Jan 2012 - 4:51 PM
    User_Removed
    15 Jan 2012 - 4:43 PM


    Quote: For bulb exposures (eg night shots) can I damage the sensor through overheating after a certain length of time?
    Also, I usually have the in camera noise control switched off - would it be better on for longer exposures of eg over 30 seconds?

    Many Thanks,

    Ian.

    Lets tackle this!

    - no you can't damage your sensor by using bulb mode. However you will notice that images become unusable and noisy.

    YES noise is generated by long exposure overheating. - This is the primary cause of "random noise".

    The long exposure noise reduction is ****!

    The way to do it is:

    Take several shorter exposures e.g. (1min) and then combine them in photoshop.
    So 30 exposures of 1 min will give you a noise-free 30 min exposure. Smile

    macroman
    macroman  1115312 forum posts England
    15 Jan 2012 - 6:40 PM

    Or use film. Grin

    Camairish
    Camairish  81276 forum posts Scotland
    15 Jan 2012 - 7:01 PM

    Thanks guys - good tip swwils - I might well give this a go.
    Funnily enough I will be shooting some film too..................lets not get into reciprocity failure!!!

    Ian.

    sherlob
    sherlob e2 Member 82314 forum postssherlob vcard United Kingdom125 Constructive Critique Points
    15 Jan 2012 - 7:55 PM


    Quote: Take several shorter exposures e.g. (1min) and then combine them in photoshop.
    So 30 exposures of 1 min will give you a noise-free 30 min exposure.

    Eh? This might work for a star trail or something similar, but how will it work to get detail? Surely, if a subject needs a 30min exposure - combining 30 x 1 min exposures won't give more light, it will just give the same dark image 30 times. Result when combined = a dark image. Can any one confirm...

    User_Removed
    15 Jan 2012 - 8:04 PM


    Quote: Take several shorter exposures e.g. (1min) and then combine them in photoshop.
    So 30 exposures of 1 min will give you a noise-free 30 min exposure.

    Eh? This might work for a star trail or something similar, but how will it work to get detail? Surely, if a subject needs a 30min exposure - combining 30 x 1 min exposures won't give more light, it will just give the same dark image 30 times. Result when combined = a dark image. Can any one confirm...

    Very true.

    User_Removed
    15 Jan 2012 - 9:27 PM

    Aslong as your individual exposures meet the minimum threshold you can stack (additively) and combine with a dark-frame subtraction no problemo.

    You can do the reverse of this called a subtraction stack to make everything but changes be removed in a frame.

    Last Modified By User_Removed at 15 Jan 2012 - 9:36 PM
    sherlob
    sherlob e2 Member 82314 forum postssherlob vcard United Kingdom125 Constructive Critique Points
    16 Jan 2012 - 10:17 PM

    Perhaps do a tutorial Swwils - you've lost me on the processing. I'm used to stacking exposures for light trails, but even if I stack say 60 thirty sec exposures I don't get any fg detail unless I expose for the fg (or light paint). It would be useful technique but more info needed.

    ade_mcfade
    ade_mcfade e2 Member 1014788 forum postsade_mcfade vcard England216 Constructive Critique Points
    16 Jan 2012 - 10:51 PM


    Quote: Aslong as your individual exposures meet the minimum threshold you can stack (additively) and combine with a dark-frame subtraction no problemo.

    You can do the reverse of this called a subtraction stack to make everything but changes be removed in a frame.

    sounds interesting...

    Too much jargon/assumed knowledge for me to follow it though

    strawman
    strawman  1022006 forum posts United Kingdom16 Constructive Critique Points
    16 Jan 2012 - 11:05 PM

    A technique for image stacking in photoshop using a plug-in. It is more often done for astronomy.

    User_Removed
    17 Jan 2012 - 12:15 AM

    Similar kind of method; but it is quite lengthy. All it is is knowing the variables you are working with and what you are trying to reduce (noise).

    It involves cunning use of the image calculator though, not a blend mode.

    For instance I know images are a matrix of RGB values from 0-255. - I know that if I see a R value of 255 with values of 0-50 within 1-px around it than that pixel is most probably noise and can be subtracted from the stack before it goes above threshold.

    ofc some digital cameras do a good job of this automatically. - but none can avoid sensor heat.

    Last Modified By User_Removed at 17 Jan 2012 - 12:16 AM
    ade_mcfade
    ade_mcfade e2 Member 1014788 forum postsade_mcfade vcard England216 Constructive Critique Points
    17 Jan 2012 - 10:42 AM

    quite involved then Wink

    I've a mate who's just started on astro photography and it sounds like more of a "technical" than "creative" pursuit

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