Is this normal (Bulb Mode)

Discussion in 'Photography Beginners' Forum' started by reshp1, Apr 3, 2007.

  1. reshp1

    reshp1 TPF Noob!

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    I'm messing around with the longer exposure settings on my camera (Pentax K110D) and bulb mode. After taking the picture, the camera takes a long time, roughly the same as the exposure time, before writing to the card. Is this normal for digital SLR's? Does this mean that if I try doing 12 hour star exposures, I'll have to wait another 12 hours before being able to view the picture and/or take other pics?
     
  2. zioneffect564

    zioneffect564 TPF Noob!

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    im not a pentax user but my bulb mode will keep the shutter open as long as the button is depressed
     
  3. reshp1

    reshp1 TPF Noob!

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    No, mine does that too. I mean after the shutter closes, there's a big delay before the camera writes to the card, like the same amount of time as the exposure. So I've taken like 2 minute exposures and it takes two minutes after the shutter closes before I can view the pic and/or do anything else with the camera
     
  4. zioneffect564

    zioneffect564 TPF Noob!

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    ohh well im pretty sure that its noise reduction running at least it is on mine
     
  5. xfloggingkylex

    xfloggingkylex TPF Noob!

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    exactly, disable noise reduction and you'll get much quicker write times. then apply noise reduction post process.
     
  6. fightheheathens

    fightheheathens TPF Noob!

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    this is normal in all digital cameras i believe.

    from what i understand, the camera is actually taking another exposure but with the shutter closed. In theory this should be a totally black pictures no matter what the exposure. However this is not the case, some pixels are hotter then others and will not appear to be black in this exposure. The camera detects this and uses software to fix these hot pixels in the actual image. Hence the long delay after a long exposure.
    what this means is that unless you have external power source for your camera the longest exposure you can take is around 1 hour. 1 hour exposure with 1 hour noise reduction gives you two hours of battery. Some cameras might be able to go longer, but from what i have heard the longest exposure you will get is around 1 hour to 1:30 before the battery dies during the noise reduction part.
    what this means is that film is better then digital for star trails if you want to do super long exposures. Also at really long exposures film is less noisy.

    lastly, if you wanted to do 12 hour star trails, you would have to do that north of the arctic circle in the dead of winter. You have to do them when it is mostly dark so about 1 hour 30 min before or after sunrise/sunset. the only place i know you can get that ammount of darkness is way far north in the middle of winter which may pose problems with film and digital....my FM2 is reported to work in -40C/-40F but im not so sure i would...or my film would...or a digital would.... :p
     
  7. reshp1

    reshp1 TPF Noob!

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    Oh cool, you're right. Turned off the NR and it wrote to the card immediately. I think I'm going to try a 10 minute or so exposure to see if it'll take another 10 minutes to do the NR or if it maxes out at some delay.

    EDIT: DIdn't see all the other responses. That makes sense that the camera takes an equal exposure with the shutter closed and "subtracts" that from the real exposure to remove hot pixels and sensor exposure from heat.
     
  8. Garbz

    Garbz No longer a newbie, moving up!

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    The reason though that this is a good idea which can't be reproduced properly with post processing is because of the purple bleeding the sensors suffer when the exposure time gets longer. From what I have heard and read this is because of the CCD/CMOS sensor heating up. If you can afford the spare time for noise reduction for images longer than a few minutes I use it.

    The other option which I had limited sucess with after my battery died just before NR finished, take a second picture later with the exact same shutter speed with the lenscap on (and in a cupboard to avoid light leaking). Then do add the second picture as a difference layer in photoshop.
     
  9. shorty6049

    shorty6049 TPF Noob!

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    This is called "dark frame subtraction" It is used a lot in astrophotography because of long exposure times and hot pixels. One way to do it yourself, is to just take one frame with the lens covered for the same amount of time you'll be taking the photo for, then take as many photos as you want with those settings and manually subtract that frame in photoshop from any picture you took at those settings.
     

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