Shortest exposure for Canon XTi/400D when using "Bulb"

Discussion in 'Canon EOS' started by alan_rockwood, Feb 23, 2018.

  1. Does anyone here know what is the shortest exposure one can make using an external input when the camera is set on "Bulb"?

    This is kind of backwards from what a lot of people ask, which is how long can you make an exposure.

    What I have in mind specifically is when using the external connector to control exposure rather than the shutter release button.

  2. Not 100% sure if this is what you're after.
    When I use an external intervalometer connected to the bulb port, when I just want an external shutter button to avoid vibration (but no bulb setting) I just set it to manual mode (or which ever mode you prefer to use) and dial the exposure settings I want on the back of the camera.

    If you do want to use bulb, I guess the minimum exposure is how little you press down the button.
  3. Thanks for the reply benjaminrussell,

    To provide a bit more context, in the application I have in mind the camera would be mounted to a modified copy stand. There would be repeated exposures using different shutter speeds. For various reasons I wish to avoid touching the camera during the process, one being the time involved in that process, another being to avoid disturbing the image registration during the process, and a third being to avoid the likelyhood of human error in the manual process.

    This leads me to look into adjusting the exposure entirely through the external shutter release port. I would generate and send the shutter control signals to the camera from some external device, either a computer or a dedicated circuit.
  4. If you have a computer w/ the appropriate version of DPP on it, the easiest thing to do may be to shoot tethered.

    Precise control of exposure length through the external shutter release in B mode may not be possible. While you could easily fab a device which can output the correct signal, the camera's ability to read that signal appropriately may not be adequate... after all, B has never been a precise way to get any finer resolution than 1 sec intervals..
  5. Marcus Ian, shooting tethered with DPP is an interesting idea. However, I don't think DPP works with the Rebel Xti/400D. (Canon EOS Utility)

    Regarding using B, is the lack of precision when using less than 1 second due to fundamental limitations of the B setting or to the absences of well-calibrated fingers to operate the B setting?
  6. I just tried a simple experiment with my Xti. I set the camera on B and flicked the shutter release as quickly as I could with my finger with the camera set to an aperture setting of f/4. Then I found a shutter speed that gave the closest exposure match to the first shot. It turned out to be 1/160 second. I repeated this several times.

    So, if we assume that the camera would perform similarly using an external shutter release, I am pretty sure I can get the shutter speed down to about 1/160 second when using B, and possibly even faster.

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