Long exposures...

Discussion in 'Digital Photography' started by vandi, Apr 24, 2007.

  1. vandi macrumors member

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    Atlanta
    #1
    Ok. i know the basics for long exposure photos with digital. I have a Canon xTi (400D). Anyone know if there is a way to get an exposure longer than 30 seconds. I know about the 'bulb' feature. but you have to hold the release button. i want to press it, walk away for 20 mins or longer, then come back press it again to close it. Anyone know?
     
  2. beavo451 macrumors 6502

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    Jun 22, 2006
    #2
    Remote shutter release. They usually have either a lock or they open the shutter with the first click and close with the second click. I don't know what model works with your Rebel (I shoot Nikon).
     
  3. ChrisA macrumors G4

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    #3
    You will have a LOT of noise in the image after 20 minutes. You will certainly want to take another 20 minute exposure with the lens cap on so you can subtract this from the first exposure. You can even take multiple "dark frames", average then together then subtract the average.

    One common method is the "hat trick" Hold a hat or something in front of the lens to block the light. Trip the shutter and hold it in place with a mechanical clamp, maybe a length of wire bent to the right shape. then remove the "hat" from in front of the lens. Black cardboard makes a good hat. Wait a few seconds after tripping the shutter before unblocking the lens for the camera to stop shaking.
     
  4. Zeke macrumors 6502

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    #4
    Please don't do this. Only badness can come of it.

    What you're looking for is bulb mode. You need a remote shutter release that has a timer on it (or else you get to stand there holding the trigger for 20 minutes) that will open and close it at the right time. Noise will be a problem but that can be reduced by the aforementioned methods (either image stacking for star trails, separate exposures for noise subtraction, etc...).
     
  5. vandi thread starter macrumors member

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    #5
    no offense ChrisA, but that doesn't sound like such a good idea considering the amout of $$ i just dropped on this camera. Unless we misunderstand what you are suggesting. Zeke, that is exactly what i was looking for. are you refering to a 'remote control'? Such as the RC-1 from canon?
     
  6. Silentwave macrumors 68000

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    #6
    Doesn't the XTi feature Long Exposure Noise Reduction? I don't know how it works on the Xti in particular, but on Nikons it automatically does a dark frame of similar length to remove noise.
     
  7. wmmk macrumors 68020

    wmmk

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    #7
    Hmmm....

    I've never really understood the subtractive noise thing. My K100D is set to do it, but I've never really understood what it going on and how it's helping anything...
     
  8. Zeke macrumors 6502

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    #8
    Yeah the RC1 will work well. In bulb mode, 1 push opens the shutter, the next closes it. It doesn't have a timer so you have to use a watch.

    Yes, the Canon has long exposure noise reduction which takes an equal length picture with the shutter closed to subtract from the first. It works pretty well but there are better options for removing noise.
     
  9. vandi thread starter macrumors member

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    #9
    lovin it

    awesome awesome awesome! i cannot thank you and everyone else enough. i now know what my next order to B&H will be. thanks again!:D
    oh and if you dont mind....what are some noise reduction methods i can google? unless you feel like typing? i'm joking, you don't have to explain unless you want. just method names or programs. whatever. thanks
     
  10. Lovesong macrumors 65816

    Lovesong

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    #10
    Hmmm... vandi, sorry to butt in, but did you read the manual. The reason I'm asking is that you might not have to go out and spend $40 on a remote shutter. I don't know about the rebel, but the 5D allows you to have a self-timer in bulb mode. Basically you set the dial to Bulb, and then the self timer. The timer runs out, and the mirror lifts and holds until you press the shutter again. Some may say this will induce shake. I seriously don't think that they understand how this whole thing works. First off you want to capture for 20 minutes (what purpose this may serve, the world may never know). A couple of seconds will not effect your shot, as it's averaged.
    This is the basis of ChisA's suggestion. You are exposing something for 20 minutes- unless it's a black hole, you'll get a nice white frame (you might as well shoot a white wall at f/22 and out of focus). By reducing the amount of light that is averaged out by a factor of 2, you might actually get the effects you're seeking and not blow out your chip. Using the hat trick WILL NOT damage your equipment. Anyone that suggest that, is simply misunderstanding the concept of it.
     
  11. jlcharles macrumors 6502

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    #11
    I think you misunderstood what he was saying. If you don't have a remote shutter release that's lockable, you have to hold the button down. He's suggesting a piece of wire to hold it. Try a rubberband holding a penny on the shutter. The process would go something like this:

    1. Set the camera to bulb.

    2. Penny on shutter release, with the lens cap still on or something to block the light like Chris said.

    3. Put the rubber band on to hold it in place.

    4. Remove lens cap/whatever you used to block light.

    5. After exposure, put the lens cap back on.

    6. Remove penny to end exposure.

    If you use the noise reduction feature, it figures out where the hot pixels are for that exposure and removes them.
     
  12. vandi thread starter macrumors member

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    #12
    ohhhhhh

    ohhhhhhh. ok. now i understand. ChrisA...i'm sorry. i missunderstood what you were refering too. that makes sense. i thought you were talking about a wire on the actual shutter. i understand the removal and replace of the cap or something dark too.

    to Lovesong. i understand completely what you are saying. I don't know if the xTi will do that, or if the shutter will only stay up as long as the trigger is held. i will try it out. if it works, you are right and i won't need the remote. thanks a lot for the input.
     
  13. volvoben macrumors 6502

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    #13
    If you go the 'black hat' route and use the lens cap be careful placing it back on, I used that technique once years ago and despite the long exposure I moved the camera around enough while wrestling to get the lens cap back on that the camera moved, caught a just-out-of-frame streetlight and it ruined a great night exposure in a drydock (this was pre-digital, hence I didn't know I'd ruined the shot).

    My advice is just to get a remote, my old nikon infrared model was about $25 and before that I used a universal TV remote, the D200 uses the wired type but a cheap release can be had on ebay for about $10. I have no idea what canon's using these days, but if you take more than a handful of long exposures it's probably worth the price.

    Finally, this is completely unrelated to an unmetered bulb exposure, but be sure to use the eyepiece cover when metering a long exposure (or at least stick your thumb over the cover and then hit the exposure lock). Especially when there's light behind you or you're shooting with an IR filter or similar, your exposure will be way off if you don't block light from entering the eyepiece.
     
  14. ChrisA macrumors G4

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    #14
    That's it. But the lens cap is not a good "hat" because it touches the camera. You want something that blocks light but does not touch the camera so you can't bump ithe camera as you remove it. When you open the shutter the camera should "see" a black card. removing the black card (or hat) starts the actual exposure.

    If there is a light source in the image even a small one like a star the slightest bump will spoil the image even if the bump lasts ony a few seconds in the 20 minute exposure. Same for wind
     
  15. ChrisA macrumors G4

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    #15
    Noise" has multiple components. With a short exposure at high ISO most of it is "shot noise" caused by the fact that light comes in discrete packets. With very long exposure most of the noise will be thermal noise generated within the sensor itself. These are other nise source but these are the big ones. The subtraction of a dark fram address the thermal noise issue. It works because dark frames are repeteable and will be about the same for a given camera at a given temperure.

    Chilling the camera will reduce this kind of noise too but that is not practical. You will see some astonomers using very expensive and complex cooling systems to keep the sensor at cryogenic temperature On the Canon Rebel he effect is strong enough that you might see the difference between a 20 minute dark fram taken at night in winter vs indoors You really do ned to take the dark frame right aftr the shot and not wait because the temperture could change
     
  16. Trogloxene macrumors regular

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    Apr 17, 2007
    #16
    What is it that you are trying to shoot for a long exposure? I shoot a lot of low light cave photos, and instead of doing the long exposure I max out the lighting with old fashioned flash bulbs. My Nikon D-100 is terrible with noise on long exposures.

    -T
     
  17. vandi thread starter macrumors member

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    #17
    I kinda wish i'd taken a photograpy class in high school instead of sound now. Everyone is so knowledgeable. I really want to try a star trail shot. I'm just waiting for a clear night and looking for someplace where there is no surrounding light. That's interesting how temperature can affect sensor noise. maybe it will be clear tonight?
     
  18. Plymouthbreezer macrumors 601

    Plymouthbreezer

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    #18
    The D100 is an older model camera - a Cannon XTi or any newer Nikon model (such as a D70s and newer) should do much better with noise on long exposures.
     

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