Leaf Shutter - Speed testing
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
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.
5x7 Eastman-Kodak kit / B&L 135mm Zeiss Tessar + Compur Deckel
4x5 Graphic View / Schneider 180 5.6 Symmar in a Synchro Compur
RB67 Pro S /50 4.5 / 90 3.8 / 180 4.5 / WLF / prism finder / polaback
Random 35mm stuff
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.
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...
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%...
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.
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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.
Originally Posted by bernard_L
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.
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.
“Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”
Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2
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.
Originally Posted by MattKing
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...
Originally Posted by Tom1956
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.