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Thread: Leaf Shutter High Speed

  1. #11

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    Sep 2011
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    Re: Leaf Shutter High Speed

    That's cool information to read, thanks!
    Quote Originally Posted by Bob Salomon View Post
    American Traffic Systems needed a medium format camera with a leaf shutter that could operate at its highest speeds so it would synch with strobe for their traffic cameras. They chose the Hasselblad EL cameras with their latest Prontor shutters. Once in service they found that after a couple of shots at 1/500 the shutters broke down.
    So they contacted us and bought Rollei 6008 cameras with the PQS linear motor leaf shutters with carbon fiber blades that could reach a true 1/1000. They installed the first system in the backwoods of Australia and contacted us to let us know that after 100,000 exposures at 1/1000 they had no failures. A mechanical shutter could not reach 1/500 and could not reliably operate continuously at that speed. The spring would break.
    The Rollei shutter had no springs.

  2. #12

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    Jan 2019
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    Re: Leaf Shutter High Speed

    Quote Originally Posted by gnd2 View Post
    Long post for anyone interested in geeking out for a bit. If you think high shutter speed are irrelevant because we should always stop down to maximize DOF and only use slow shutter speeds, this may not be of interest to you. No need to comment, just enjoy the scenery I would actually like to use the faster speeds, and as I've been checking out the used lenses I've been buying, the phenomenon of the fast speeds measuring much slower than their labelled speed piqued my curiosity.

    I've been doing a lot of searching to understand this, but my tests don't quite match up with what I've been reading.

    Let's start with the basic shutter speed tester: phototransistor to detect the opening and closing of the shutter and measuring the elapsed time. Works great at slow shutter speeds where the time it takes for the shutters to transition is insignificant compared to the time the shutter is open, so the shutter is effectively fully open for the entire shutter duration. But at faster speeds (say about 1/125 and faster), the shutter transition time becomes significant.

    At the fastest speed, using the basic shutter tester, people will typically measure a speed roughly half of the labelled speed. The claim is that this is because the tester measures from when the shutter first starts opening to when it's completely closed, but since it takes time for the shutter blades to transition from fully open to fully closed, they start closing again as soon as they're fully open. So if you integrate the light intensity over the entire time, it's actually 1/2 what it would be if the shutter transition where instantaneous. Therefore, if the shutter takes 1/200 from start to finish, the effective exposure is equivalent to 1/400.
    This gets discussed in roughly every thread about shutter testing. If one puts the phototransistor directly behind the lens on-axis, or if one puts the phototransistor on axis behind a naked shutter w/o lens elements, it "sees" light from the first instant the shutter opens a little bit at the center, to the last instant it closes. So it doesn't compensate for the "shutter efficiency" where the shutter is open across the outer parts of the pupil for a much shorter time.

    Putting the sensor well behind the lens at the focal plane and illuminating the lens uniformly from the front may show a slower ramp up, but you do still have to worry about whether the phototransistor is linear or a switch as someone else mentioned.

  3. #13

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    Nov 2017
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    Re: Leaf Shutter High Speed

    I wonder what the result would be if you compare a normal Compur with an electronically controlled one like in a Fuji GX680 or Bronica.
    Expert in non-working solutions.

  4. #14

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    Aug 2005
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    Illinois
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    44

    Re: Leaf Shutter High Speed

    You might also test at various apertures. I recently read a paper that claimed that leaf shutters were more 'efficient' at smaller apertures. The reasoning being (I think) that the shutter clears/occludes the smaller aperture faster. There is also a ramp up/dwell/ramp down cycle in the shutter action that might make faster speeds somewhat more challenging to measure- the dwell portion may be quite short compared to the open/close times. I think this is one reason a high speed video would be interesting...

  5. #15

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    southeast Idaho, Teton Valley
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    Re: Leaf Shutter High Speed

    Quote Originally Posted by Robert Tilden View Post
    Depending on the phototransistor it may be acting more like a switch than providing an analog signal proportional to light falling on it. For fast shutter speeds the 'plateau' of a fully opened shutter may be quite short, so the opening and closing 'ramp' will be a significant portion of the total exposure. If the phototransistor triggers early in the shutter cycle the graph would not be showing an accurate representation of the effective shutter speed.
    In the shutter tester I built, I use a phototransistor (a Vishay BPW85B) and it detects pulsed light through the lens at 10K Hz. There is a dramatic difference in amplitude in the signal representing this pulsing during the time the shutter is open. Here is an example of the signal for 1/100 sec. The opening and closing parts of the shutter action are visible. The entire opening time is 1/111 s. Without the ramping up and down, it is 1/114 s. The stop was f8.

    The capacitance for the phototransistor at the voltage I run it at (9 VDC, C to E)) is 2.5 pF -- very small. The bandwdith is about 180 KHz

    I read the voltage with a sound card, which only goes to 20K Hz. I set the 555 timer to 10K Hz so that I would have double the bandwidth to better capture the signal.

    Click image for larger version. 

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    The signal is more than precise enough to capture the shutter action.

  6. #16

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    Re: Leaf Shutter High Speed

    Quote Originally Posted by Havoc View Post
    I wonder what the result would be if you compare a normal Compur with an electronically controlled one like in a Fuji GX680 or Bronica.
    If it is spring driven with conventional blades it will be essentially the same.

  7. #17

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    Re: Leaf Shutter High Speed

    Mike, I could be mistaken but I don't believe that a Supermatic opens and closes as fast -- in 0.000237079 seconds combined -- as your measurements show.

  8. #18

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    Re: Leaf Shutter High Speed

    Quote Originally Posted by reddesert View Post
    This gets discussed in roughly every thread about shutter testing. If one puts the phototransistor directly behind the lens on-axis, or if one puts the phototransistor on axis behind a naked shutter w/o lens elements, it "sees" light from the first instant the shutter opens a little bit at the center, to the last instant it closes. So it doesn't compensate for the "shutter efficiency" where the shutter is open across the outer parts of the pupil for a much shorter time.

    Putting the sensor well behind the lens at the focal plane and illuminating the lens uniformly from the front may show a slower ramp up, but you do still have to worry about whether the phototransistor is linear or a switch as someone else mentioned.
    Thanks for this. I've read quite a few threads but don't recall ever seeing this bit of information.

    I diffused the light going into the lens and placed the sensor near the focal distance of the lens and got this:

    Click image for larger version. 

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    Not perfect, but much closer to expected. Close enough for me to chalk up the difference to experimental error (a lot of eyeballing going on) and satisfy my curiosity.

  9. #19

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    southeast Idaho, Teton Valley
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    Re: Leaf Shutter High Speed

    Quote Originally Posted by Dan Fromm View Post
    Mike, I could be mistaken but I don't believe that a Supermatic opens and closes as fast -- in 0.000237079 seconds combined -- as your measurements show.
    I agree, the time difference is very small may probably wrong, but I am not sure why. It could easily be 2X the 0.0002 measure, as 10 KHz cycle is 0.0001 s.

    My measurements could be biased to the len's central axis, as reddesert mentions in post #12. The lens elements remain on the shutter, but the pulsing LED is about an inch from the front of the lens, and the phototransistor is within a 1/4 inch of the rear element. I do it this way to make sure I get a high signal-to-noise ratio. The light is so out of focus though, that the on-axis effect is probably negligible. The light circle on a surface very near the rear lens element (as close as I can get it and still see it) is about the same size as the rear element, so we are not dealing with a narrow light source at the sensor location.

    What I was mainly trying to show is that the phototransistor response is fast enough to capture a 10 KHz rise and fall, and using pulsed light gives a more intuitive sense of the shutter opening and closing, and finally, amplitude linearity is not that important when interpreting a pulsed-light signal.

    Measuring the phototransistor's voltage amplitude linearity to light amplitude would be worthwhile (the device's datasheet does not show this), and may not be that hard to do, so I may try it. The other aspect is the angle of sensitivity -- it's +- 25 degrees for 50% sensitivity for the device I am using, so off-center light is detected (but the angle is reduced by the entrance hole in the transistor mount). I could get a sense of the on-axis effect by moving the sensor off-axis and see what happens.

    I get good exposures using my corrected shutter speeds, and I mostly use slow speeds and f-stops of 16 to 22, so never was that concerned with shutter efficiency.

  10. #20

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    Nov 2017
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    Re: Leaf Shutter High Speed

    Quote Originally Posted by gnd2 View Post
    Thanks for this. I've read quite a few threads but don't recall ever seeing this bit of information.

    I diffused the light going into the lens and placed the sensor near the focal distance of the lens and got this:

    Click image for larger version. 

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    Not perfect, but much closer to expected. Close enough for me to chalk up the difference to experimental error (a lot of eyeballing going on) and satisfy my curiosity.
    I find this an interesting curve. It is indeed more what you would expect. Could you please give the horizontal axis and what setting the shutter was set at?
    Expert in non-working solutions.

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