Black bar at bottom with the right sync speed and slight motion blur when shooting with strobes

Discussion in 'Lighting Equipment' started by alan_kovarik, Jan 2, 2021.

  1. I have Nikon D3300. Its sync speed should be 1/200, but when I shoot with strobes, I have a black bar at the bottom of any image. When I change the shutter to 1/160, the black bar is gone. Where could be the problem? I use GODOX SK 400II strobes with Godox XT-16 transmitter. I didnt noticed this problem when I used speedlites.

    I also noticed slight motion blur on some portraits when shooting with the strobes. Even at 1/160 with 50mm lens. I didnt noticed any motion blur on similar photos when I used speedlites before I bought strobes and I didnt noticed anything like this when I shoot events without any flash/strobe. Any ideas where could be the problem?
     
  2. The black bar could be the result of a delay with the transmitter.
     
  3. What Bebu said with respect to the black bar. WIth studio flash, I generally set my shutter speed at 1/3+ stops less than my maximum sync speed, since that is usually the quoted number for using dedicated (Nikon) flash units. For motion blur, the only I could think of is that there is enough ambient light blended in your exposure to cause that. Although it seems unlikely that your model moved that much to blur with a shutter speed of 1/160. Make sure your camera is set to manual exposure.
     
  4. AJG

    AJG

    I don't know what the flash duration of your studio flash but most studio flash units have a much longer duration than most speed lights which are usually at least 1/1000 of faster. This could explain the motion blur that you're seeing. I always set my Pentax cameras 1/3 stop below the sync speed because of the transmitter/receiver delay that you're seeing with your Nikon.
     
  5. You're using wireless (RF) remotes, right?
    These introduce a slight delay (a few hundred microseconds) and require a slower shutter speed to be set than when a flash is fitted directly in the camera hotshoe.

    FWIW. The delay is caused by the decoding of channel identifier encoding, and has nothing to do with the propagation delay of the radio signal.

    The adjustment of shutter speed needed usually amounts to 1/3rd of a stop.

    As AJG said, and contrary to popular belief, the duration of a flash on full 'power' is closer to 1/250th of a second rather than 1/1000th.

    Here's a storage 'scope trace of a typical full energy flash output vs time (milliseconds).
    SB-25_flash_curve.jpg
    If you need a shorter duration you need to set half 'power' or lower on the flash(es).

    A few thyristor-controlled flash durations at minimum output:
    Flash-durations.jpg
     
  6. Thank you for your answers.
     
  7. Flash duration of my strobes is: 1/800 - 1/2000.
     
  8. Bebu and Rodeo Joe are right. I do hummingbird photography in the spring and summer and I wondered why I missed shots I was sure I had. The cheapo RF flash trigger I'd gotten off Ebay has a 1/140th of a second processing delay (as per the makers specs). As a hummingbird flies at a max speed of around 30 MPH, it means the hummer could fly 3-4 inches before the flash triggered. That's just enough for a hummingbird to get all or part of the way out of the frame.

    After researching the speed of electricity thru wires ( never could find an exact figure!) I made myself a 25 foot sync cord out of 16 gauge wire as the speed of electricity thru a wire is about 80% of the speed of light. My number of good shots climbed a bit once I started using the sync cord.
     
  9. What is interesting is that this didnt happend to me when I was using speedlites with cheapo RF flash trigger.
     
  10. BTW, do you thing this delay could cause aforementioned slight motion blur on some pictures?
     
  11. I measured my cheapo RF flash triggers as having between 600 and 700 microseconds delay, dependent on the channel chosen. That's about ten times faster than 1/140th s.

    Are you sure the makers didn't state 1/1400th of a second?
    No.
    It's more likely to be a high ambient light level, or just using too high an output setting on your strobes.
    Makers have two standards for declaring flash duration, 50% and 10%, t0.5 and t0.1 durations. Meaning they measure the time until the light output falls to 50% or 10% of its peak. Obviously the 0.5/50% figure gives an illusory shorter duration than the 0.1/10% figure.

    The t0.1 figure is more realistic.
     
    Last edited: Jan 8, 2021
  12. Joe the instruction sheet said 1/140th. Now it could have been a misprint...
     
  13. A delay of 1/140th of a second (7 milliseconds) would make a trigger only usable at a shutter speed of 1/60th or longer. Hardly likely.

    I own a variety of RF flash triggers, and none of them need more than a slight adjustment of the maximum synch speed on the camera, say from 1/250th to 1/200th or 1/160th at most.

    If the delay on those triggers really is 7ms, then you should buy better ones!
     
  14. Well, considering I paid a princely $14 for it.... My best guess it that the 1/140 is a misprint on the instruction sheet. I gave up on it anyway and now use a homemade 30 foot sync cord.
     

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