Leaf Shutter - Speed testing

Discussion in 'Exposure Discussion' started by Bill Burk, May 4, 2014.

  1. Bill Burk

    Bill Burk Subscriber

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    The record player part of this trick likely would not work on a focal-plane shutter. But the exposure part will work.

    I'd done an electronic shutter speed test on my Kodak Retina I and doubted my results because they "felt" wrong.

    Low speeds felt right, so I trust the speeds I measured for 1, 2, 5, 10 (shutter speeds in fractions of second)

    But for the higher speeds, 25, 50, 100, 250 and 500, where my tester told me I was really getting 13, 22, 40, 80 and 160, I started to have my doubts.

    I used the electronically-tested numbers last weekend as-is, and when I came home with doubts I tried setting up my electronic tester to count pulses instead of time. In pulse mode (light interrupted by a fan), my electronic counter acted like there was a dwell, as if a capacitor was holding a charge... a behavior that has me concerned that 250 might not be 80, and I might have underexposed nearly 2 stops.

    So I thought of another test. One involving actual photography with the camera...

    I setup two targets. One is the exposure target from Sekonic with a set of gray patches 1/3 stop apart surrounding 18% gray. The other is an old record player running at 78 RPM, with a black sheet of construction paper with a white bar. The idea... to measure the angle of the bar recorded on film. I did a series of tests trying to get exposure as close to nominal as possible (by incident meter). I tried both Tungsten and Sunlight. It was harder to hit nominal with Tungsten because the f/3.5 lens and 100 ISO film couldn't be properly exposed with two 500 watt photofloods. So some tests are a couple stops under nominal.

    I also added a sensitometric exposure to the film.

    When it's dry I'll measure the sensitometric test and graph the result. Then I'll photocopy that graph and plot densities from each camera test shot on the graph. For that graphing, I only mark the density recorded where the sensitometric curve hits that density. Because the entire roll's characteristics are defined by that sensitometric test.

    Then following down from the actual densities to the LogE axis... And noting the deviation in exposure from nominal... I'll be able to deduce the effective shutter speeds. There will be flare included in this but I'm measuring mid-tone densities where I expect flare is less of an influence.

    The angle of the record player rotation will be used to confirm or contradict the exposure test. I expect some "greater" angles than expected due to the "bell curve" way a leaf shutter exposes, but I don't think I that my test is able to measure the bell curve because of the simplistic design.

    Tests are hanging to dry next to the film from last weekend. At first glance, it seems all my real photographs are well-exposed. So that makes me think my shutter really IS a bit slow. Since I rated the film at 50, (assuming the shutter is one stop slow instead of my originally measured two stops slow at 1/250) I may actually have "exactly correctly" exposed all the photographs.

    Here's the simple math I did to figure out the angle to look for:

    Using 78 RPM turntable as shutter speed tester

    1/100 second = x degrees of rotation:

    1/100 second = 0.010 seconds of time.

    78 RPM = 1.3 Revolutions per second of time.

    Sanity check: Expect three tenths more than 1/100th Revolution.

    0.010 S times 1.3 R/S = 0.013 R

    Revolution is 360 degrees

    360 D/R times 0.013 R = 4.68 D

    So 1/100 should be 4.68 Degrees.

    ---

    1/500 second should be 0.936 Degrees.

    1/250 second should be 1.872 Degrees.

    1/100 second should be 4.68 Degrees.

    1/50 second = 9.36 Degrees

    1/25 second = 18.72 Degrees

    1/10 second = 46.8 Degrees

    1/5 second = 93.6 Degrees

    1/2 second = 234 Degrees
     
  2. j-dogg

    j-dogg Subscriber

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    I used a very lo-tech (and accurate) way to test the shutter on my Compur-Deckel for 5x7

    I rigged up an external microphone to the side of the shutter and fired it while a program called GoldWave (free download, use 4.26) listened and recorded it.

    I zoomed in on the waves it made in the file and measured the distance between the peaks in seconds. It will measure to the millisecond I believe......but the negatives it made were perfectly exposed.
     
  3. bernard_L

    bernard_L Member

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    Could you please be more specific (setup, schematic) about your electronic tester, in both modes: time and pulses, and what precisely you saw or measured that made you think of a "capacitor holding a charge". Reason I'm asking is most simple circuits for shutter timing that are floating around require some caution in using the results, at least IMO.

    BTW, I have a Retina I with the shutter part dis-assembled since x months, when I gave up adjusting all speeds; but maybe I should have settled for +/-20%...
     
  4. Tom1956

    Tom1956 Inactive

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    Bill, since I've been restoring these old 78 radio/phonographs for the last 17 years, I can tell you one of the standard items I have to address is to send the rubber drive tire off for re-rubbering. These things are not running at correct speed. I doubt they're running a true 78rpm even after I'm finished with the restoration.
     
  5. RalphLambrecht

    RalphLambrecht Subscriber

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    try this:smile::smile:
     

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  6. silveror0

    silveror0 Member

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    +1
    I assembled a sensor, based on Ralph's tester, and then made a jig out of MDB to hold both a lens-on-lensboard and a 75W flood in position, carefully aligning the sensor to the lens axis. Then used free download of the Audacity program to record the waveform on my PC screen. Then the program is able to measure the time from halfway up the rising wave (shutter opening) to halfway down the falling wave (shutter closing). All six of my large format lenses in leaf shutters checked out within tolerance, except for a couple of speeds, which I then labeled on the lensboard so I could make exposure corrections when using those speeds. I did, however, see a frequency of 120 Hz superimposed on the Audacity wave (which did not affect the ability of determine the shutter speed), but I was curious to learn the cause. So I contacted Chris Woodhouse (co-author of Way Beyond Monochrome 2nd Ed., WBM2) who is the electrical guru of this checker. His response to my query:

    "It is the mains - but you commonly get 2x the mains frequency (100Hz in the UK and 120 in the US). It normally comes from the fact that any power supply that has a bridge rectifier will cause current bumps 120 x per second."

    I've not tried my setup with a battery-driven light source yet to see if this oscillation goes away, because I'm satisfied with the results I got.
     
  7. Bill Burk

    Bill Burk Subscriber

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    I use a battery powered unit... Black output terminal of unit connects to a toggle switch and the negative side of battery. The battery plus (26.5v) -> Emitter of ECG3038 light detector NPN photo-transistor Collector is connected to -> 460 ohms resistance -> Red output terminal of unit.

    Wired across the two outputs is a VU meter.

    I use a Frequency Counter where I jump A-B and set the unit to Time A->B, Slope A is set to .4 v + and Slope B is set to 6.4v - the Gate is set to Auto.
    For pulse, I set to Count A and set the Gate to Open. Then I put a small fan from a computer on top of the unit so the blades of the fan cross the light. This was doing a good job counting "pulses" except when I got to the shorter times, when it counted only 1 pulse.
     
  8. MattKing

    MattKing Subscriber

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    What are you using your shutter speed check to accomplish?

    I ask, because if you are using it to check the accuracy of your exposure calculations, the way that leaf shutters work may screw up the calculation.

    They take time to open, and take time to close. And that affects the resulting total exposure in a different way than with focal plane shutters.

    I would expect certain parts of the image forms as a part of your "vinyl" shutter speed tester to show those effects.
     
  9. Bill Burk

    Bill Burk Subscriber

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    The time to open/time to close problem could have affected my electronic time test. But my Sekonic exposure target will tell me the effective shutter speed (exposure base) deviation from nominal. That's more important to me. The vinyl test is a confirming check and a reminder about the real-time shutter time involved - so it will tell me how much motion blur I might expect from a particular shutter speed selection.
     
  10. Bill Burk

    Bill Burk Subscriber

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    The diameter of the idler wheel does not enter the calculation, it merely provides an interface between the motor, which has an accurate speed due to AC line frequency, and the rim of the turntable...

    But to double-check, I took out Bix Beiderbecke's Rhythm King and timed it on the record player I used (3:15) compared to my computer-controlled Dual CS-5000 (3:18). I'll adjust for the difference in rotational accuracy only at 1/5 second (where I add 1 degree) and 1/2 second (where I add 3 degrees). Not too shabby for an old record player.
     
  11. Tom1956

    Tom1956 Inactive

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    Too bad ole' Bix left us so early. You sound like you know what you're doing. Just for edification about phonographs--no the diameter of the idler wheel doesn't factor in, but its condition does. It either gets hard and slick, or it gets gooey. So I have to send off idler wheels and Rochelle salt cartridges for rebuilding. I re-cap the chassis, check and/or replace tubes, and put it up on the Hickok for re-alignment. Typical procedure.
     
  12. Jim Jones

    Jim Jones Subscriber

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    In the days of analog television, we did quick checks of high shutter speeds by watching the TV screen through the shutter with the lens removed to observe how much of the image appeared bright. The full TV screen equaled 1/30 second; half a screen, 1/60 second, and so on. It was certainly accurate enough for most B&W photography.

    As for the 60- or 50-cycle ripple from AC lamps, using low voltage high current lamps reduces this. 12 Volt automotive lamps powered by an old computer power supply eliminate the problem entirely.
     
  13. wombat2go

    wombat2go Subscriber

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    I use a photodiode press fitted in a block of acetal. A very small drilled hole forms a tube to limit the angle of view. The block is pressed against the ground glass.

    The light source is a floodlamp, its 120 Hz ripple allows a handy way of double checking the scope calibration.
    The light is adjusted so the photodiode stays in its linear region when it comes off dark current.
    The thing is sourced with a 9V battery and a storage oscilloscope measures the bias resistor voltage.

    I have tested a few old shutters and I think the tester is quite accurate, showing penumbra too.
    It also works on the curtain shutter of the Speed graphic.


    Curiously, although the Speed Graphic rear shutter is completely clockwork and the camera has no batteries, I have observed the rear shutter emit a solid electromagnetic pulse when it starts to move. That sometimes triggers the scope early. I have not figured out what causes that, maybe some part in the clockwork is magnetic.
     

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  15. paul ron

    paul ron Member

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    Funny you're using an old record player. Once upon a time we used the tv scans to time shutters.

    I converted to an ociliscope n triggered a freeze tracer i built from popular electronics in the 70s. Still works fine but its such a pita to drag out my techtronics scope that weighs a ton n browns out the neighborhood when i fire it up!

    Ive since built the photo diode used through my computer's sound input n recording program. Its as accurate as my scope n has been just as reliable as my old calumet shutter tester now collecting dust on top of my osciliscope.
     
  16. omaha

    omaha Member

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    I'd be interested in seeing the test shots.
     
  17. Bill Burk

    Bill Burk Subscriber

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    I've finished crunching the numbers and have come to the conclusion that I will keep the numbers from the electronic test for the slow speeds. The turntable test was amusing but not precise. The exposure test (18% gray patch) gives me new shutter speed values for the high speeds.

    [​IMG]

    What I've noted for my camera:

    Setting: 500
    Degrees: 2.5
    Speed by Degrees: 1/184
    Speed by Exposure: 1/125
    Speed by Electronic: 1/160

    Setting: 250
    Degrees: 3
    Speed by Degrees: 1/154
    Speed by Exposure: 1/80
    Speed by Electronic: 1/81

    Setting: 100
    Degrees: 6
    Speed by Degrees: 1/77
    Speed by Exposure: 1/50
    Speed by Electronic: 1/40

    Setting: 50
    Degrees: 13
    Speed by Degrees: :1/35
    Speed by Exposure: 1/25
    Speed by Electronic: 1/22

    Setting: 25
    Degrees: 23
    Speed by Degrees: 1/20
    Speed by Electronic: 1/13

    Setting: 10
    Degrees: 69
    Speed by Degrees: 1/7
    Speed by Electronic: 1/4

    Setting: 5
    Degrees: 139
    Speed by Degrees: 1/3
    Speed by Electronic: 1/3
     
  18. Xmas

    Xmas Member

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    taking some of the Retinas apart can be difficult...
     
  19. Bill Burk

    Bill Burk Subscriber

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    So I had a hunch the exposure method would throw in a wrench...

    1.0 LogE from metered point to speed point (0.10) is for approximately 12% but my target was 18% gray.

    So I shifted my shutter speed estimates 1/2 stop towards being faster speeds.

    And you thought there was no practical use for knowing the trivia about 18% gray cards...

    Also I said the rotational speed was a sanity check. You can see from the photo... There's no denying... at 1/25 the rotation in degrees is extremely clear. Shutter speed is really 1/20.

    I made up a new cheat sheet for the camera where I use Exposure-based high speeds, rotational-based medium speeds, and electronic-based slow speeds.

    As you can see, no one method (as I've implemented) works effectively for all speeds - but the entire set work together well.

    500 = 1/160 (exposure-based)
    250 = 1/100 (exposure-based)
    100 = 1/70 (exposure-based)
    50 = 1/35 (exposure and rotation agree)
    25 = 1/20 (rotation-based)
    10 = 1/7 (rotation-based)
    5 = 1/3 (rotation and electronic agree)
    2 = 3/4 (electronic)
    1 = 1 (electronic)
     
  20. Bill Burk

    Bill Burk Subscriber

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    I'm fortunate to be working with a late-model Retina I, and have two of them (thanks momus)... I did not go deep... Just cleaned, lubed and adjusted the easy-to-access shutter parts and made sure the shutter blades were about as free as they could be without disassembly.

    My wife told me I can't get the "index finger as rangefinder" tattoo I wanted. So I'll be using a sharpie.
     
  21. Tom1956

    Tom1956 Inactive

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    As upsetting as it is to accept, Bill Burke's readings are something I can believe. But all my testing has been on an IR diode, robbed from an Apple mouse, and the typical sound-card method. And I've come up with equally dismal results. So I can [sort of] concur. Evidence the shutter is in bad need of CLA. And even then I doubt the speeds could come up to actual marked speeds and stay that way for more than a year. The more I study and work on shutters, the more I believe that accuracy is fleeting. I've also learned that 1/25 is usually about the only one that's anywhere near, by percentage.
    I know Bill is disappointed at these numbers, because I've been there. But I also believe his numbers are the sad truth.
     
  22. Doremus Scudder

    Doremus Scudder Member

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    It is simply wishful thinking to expect older clockwork leaf shutters to give full speed; even newer ones are notoriously slow at higher marked speeds.

    That does not necessarily mean that they are inaccurate, however. Once you know that your 1/500 is really 1/160 or whatever, then just use that. If you really need reliable speeds faster than about 1/125, get a camera with a focal-plane shutter.

    FWIW I test my shutters by sound (ms between opening and closing peaks) and have very consistent results through the range of speeds. I use mostly smaller apertures, and thereby eliminate much of the opening-closing light loss. When I get my shutters CLAd, I always ask the tech to check the shutter speeds optically at f/22 and give me a list. This goes on the lensboard so I can use it in the field.

    Best,

    Doremus
     
  23. E. von Hoegh

    E. von Hoegh Member

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    The old Kodak Professional Dataguides used to have efficiency charts for leaf shutters used at different apertures and speeds.
     
  24. Xmas

    Xmas Member

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    I need an Avery label on the back of my focus hoods.

    f 16 22 32
    /125 x x -1/3
    /250 x -1/3 -2/3
    /400 -1/3 -2/3 -1

    from my RB67 hood & 90mm

    I think this means I need to reduce exposure by a stop at /400 f/32 i'd need to be using fast film though.

    For a TLR 11 16 22
    etc.

    without the Avery Id bracket.
     
  25. Chan Tran

    Chan Tran Member

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    The tester I build is great for focal plane shutter but with leaf shutter I think I need a redesign. I use a thru beam sensor that uses a very narrow beam of laser. It detect the shutter curtain edges very well and I see the signal on an oscilloscope. I would need a larger light source and sensor with an analog output to check the leaf shutter taken into account of the opening and closing.
     
  26. Bill Burk

    Bill Burk Subscriber

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    Thanks Tom1956, I think you'll understand I'm not disappointed at all... I've got shutter speeds I can use.

    It's interesting how far the speeds are from nominal, which illustrates the often-heard expression: your light meter is probably the most accurate piece of equipment you have.

    It's also a good learning experience for me... Now I know how to verify shutters of any camera, and this will help me when it comes to interpreting other people's Zone System tests... Because the Zone System calibration is designed to incorporate shutter speed variations in the personal EI. I now understand some people may "correctly" arrive at an EI that is higher than rated speed.