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Thread: Bulb and aperture?

  1. #1

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    Frank Deland

    Bulb and aperture?

    When using Bulb as a shutter speed what factors influence the choice of aperture?

  2. #2

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    Andrew

    Re: Bulb and aperture?

    Bulb is just another shutter speed. ISO and DOF still play their same role in the exposure triangle.

  3. #3

    Re: Bulb and aperture?

    Image quality and DOF. Most lenses perform best (IQ wise) a couple of stops off wide open. Given that you don't need the widest aperture for extra light (due to the long exposure) you can maximise image quality. Obviously if you want a specific DOF that will also factor into your choice of aperture.

  4. #4
    rpcrowe's Avatar
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    Richard

    Re: Bulb and aperture?

    Bulb simply opens your shutter when you press the shutter release and closes your shutter when you release the shutter release.

    The term "bulb" originates from old-time shutters which did not have a sync setting for flashbulbs (or flash powder - but that was even before my time). The photographer would open the shutter, fire the flashbulb, and the close the shutter.

    Now the bulb setting is normally used for longer exposure times than that which the DSLR shutter is configured.

    The main factor that influences the aperture when shooting in the bulb mode is the length of time that the shutter is kept open by the photographer. The computation of exposure is exactly the same as when the shutter has control of the exposure time. However, it is not really possible to measure the shorter exposures (say 1 second or less) effectively when using bulb. However when using longer exposures, the exact exposure time is not absolutely critical. If you want an exposure of 10-seconds and expose for 9.5 seconds, you are not significantly off your desired exposure as you would be if you wanted an exposure of 1/2 second and exposed for 1 full second. In each case, you would be 1/2 second off but in the case of the desired 10 second exposure, you would only be off by 5% while in the case the desired 1/2 second exposure you would be off by 100% or 1 - full stop.

    When I first started using bulb exposures with view and copy cameras, I would calculate the exposure by counting "one thousand and one... one thousand and two" and so on. In a longer exposure I would be off a very small percentage. However I would recommend someone who is not used to calculating time this way to use a watch with a second hand...

  5. #5

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    Frank Deland

    Re: Bulb and aperture?

    Thanks for the replies. Originally I thought that if I were holding the shutter open, I should therefore, let in as much light as possible by using a wide open aperture, but now I understand how it is still related to my choice of DOF. At night I so far have best results when I bracket exposures and then later combine images. When using bulb, I first used a watch, but now am comfortable with counting just so I know that I vary the time I hold the shutter open.

    Impressive resume rp,and your images of Yosemite have wonderful color and pleasing compositions. Eventually, I will look at more of your albums.

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