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Testing Leaf Shutter Speeds

 
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bartbob



Joined: 30 Oct 2010
Posts: 102

PostPosted: Sun Nov 07, 2010 1:23 pm    Post subject: Testing Leaf Shutter Speeds Reply with quote

I came up with my own shutter speed tester that measures what counts; to me, that is. Shutters are supposed to let light through them for a given amount of time. Leaf shutters inside lenses have a transit time between their closed and full-open positions. For a given shutter speed, I think a lens with a smaller f/stop will have a longer "open" time than what a larger f/stop has. So I wanted a method to show the actual amount of light that gets through the shutter during its cycle time. Here's what I came up with.

Most electric motors turn at about 1750 rpm. They'll make one turn in about 1/30th second. If you put a black round thing (pulley?) on it with a white spot at its edge then take a picture of it spinning, that spot will make a circular blur on the image plane while the shutter's open. Each end will have light fall off as the shutter opens and closes. To see exactly what your motor turns that pulley in 1/60th of a second, use a decent digital camera to take a picture of it. Here's what my pulley looks like; a reflective white square's at the edge spinning at about 1750 rpm using a digital SLR's speedlight:



Here's the pulley picture taken at 1/60th second with the same camera; note the grey blur starting at the top then ending about 162 degrees clockwise:



The spot's arc should have been about 175 degrees at 1750 rpm. As the arc's only about 162 degrees, that translates to about 1890 rpm.

This is how I calibrated the motor speed using a very accurate 1/60th shutter. Comparing a test picture using the shutter under test to this one will show how accurate it is. A drill press spindle spinning at a lower speed will work for shutter speeds below 1/60 second. Plans are to get a really flat black paint (or something else) so there's better contrast between the white spot and the pulley.

I'm working on the setup to test my Graflex 1000 shutter. Plans are to use the DSLR to photograph the Super Speed Graphic's ground glass in a dark environment showing the bright-light illuminated reflector making traces. Grade school math can be used to determint acutal shutter speed by how many degrees rotation the spot has.

Comments, suggestions are welcomed.

Bart
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Henry



Joined: 09 May 2001
Posts: 1442
Location: Allentown, Pennsylvania

PostPosted: Sun Nov 07, 2010 2:28 pm    Post subject: Re: Testing Leaf Shutter Speeds Reply with quote

bartbob wrote:


Most electric motors turn at about 1750 rpm.

Bart


Seems to me that one cannot make such an assumption without knowing the voltage and motor characteristics (size, windings, etc.). The best source would be a manufacturer's label on the motor: "X" r.p.m. @ "Y" volts [AC or DC, as applicable].
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bartbob



Joined: 30 Oct 2010
Posts: 102

PostPosted: Sun Nov 07, 2010 2:37 pm    Post subject: Re: Testing Leaf Shutter Speeds Reply with quote

Henry wrote:
Seems to me that one cannot make such an assumption without knowing the voltage and motor characteristics (size, windings, etc.). The best source would be a manufacturer's label on the motor: "X" r.p.m. @ "Y" volts [AC or DC, as applicable.
I agree; name plate on this old motor says "1750" rpm, 110 VAC." Voltage at the plug used was 119.1 (+/- .1 or so) VAC on a digital meter when tested. 119.1 is about the same percentage higher from 110 VAC as 1890 rpm's is from 1750.

I'm reasonably sure Nikon's electronic shutter in their D40 DSLR's are accurate to very few percentage points. 'Tis easy to figure out to the same accuracy how fast that pully's turning by seeing how many degrees of arc that white blur has. Most photo editing software allows 1 degree or finer rotations of digital images. I'll do the same tests with my D700 to see if there's any difference.

Editing this response later. Seems I forgot something that may be crucial to the issue.

If Nikon's shutter electronics in their D40 is exact double/half from a given full stop setting starting from 1 second, setting the shutter to 1/60th would be 1/64 second. At 1750 rpm, that dot will travel about 164 degrees. Which now convinces me that my reasoning was a bit flawed before. It's probably the same thing as f/stops where only f/2, f/4, f/8.... and doubles thereafter are exact. Apertures of f/4.5, f/5.6, f/11, f/22.... are not exact, but just easy things to remember for settings inbetween the exact ones.

There's easily a 2 degree error in my measuring techniques for that arc of grey shown in the picture. It's a fluke that the difference I thought was caused by an outlet voltage to spec'd on the motor would cause the same error.

My reasoning's got a lot of scar tissue on it. It's been flawed many times over the years. What's another one or two...........
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1banjo



Joined: 16 Nov 2008
Posts: 478
Location: kansas

PostPosted: Sun Nov 07, 2010 8:02 pm    Post subject: Reply with quote

hey all
I know that some people have used old LP turntable to test shutter speeds
banjo
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Henry



Joined: 09 May 2001
Posts: 1442
Location: Allentown, Pennsylvania

PostPosted: Sun Nov 07, 2010 8:23 pm    Post subject: Reply with quote

At 33-1/3 rpm, a phonograph tt's degrees of rotation would barely be detectable at higher shutter speeds, although perhaps meaningful at slow speeds. Do the math: 33-1/3 revolutions per minute = .555 revolutions per second (33-1/3 60), or say 200 degrees of rotation (.555 x 360 degrees = 199.8 ), easy enough to measure visually. But at 1/100 sec., the platter would move through only 2 degrees of rotation (200 100). That's why you really need a *rapidly* rotating disc of known speed, as bartbob's posts indicate. (NB: check my math reasoning, since I am not an engineer or mathematician; as a musician I usually had to count only to four
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bartbob



Joined: 30 Oct 2010
Posts: 102

PostPosted: Sun Nov 07, 2010 10:30 pm    Post subject: Reply with quote

Henry wrote:
(NB: check my math reasoning, since I am not an engineer or mathematician; as a musician I usually had to count only to four
I almost pursued a musical career after high school, I thought I could count to four. But as things worked out, I didn't make it as a musician. I ended up working on computers 'cause I could only count to 2.

Bart
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C. Henry



Joined: 13 Dec 2005
Posts: 358
Location: North East Georgia, USA

PostPosted: Mon Nov 08, 2010 10:04 pm    Post subject: Reply with quote

If we are looking for an accurate repeatable motor speed a synchronous motor such as used in the older electric clocks (those with a dial & hands) had speeds that were either 3600 rpm or even fractions of that speed. One that turned at either 3600 RPM (1 full rotation in 1/60 sec) or 1800 RPM (180 degrees in 1/60 sec.) would be useful with this method of testing faster shutter speeds. BTW a synchronous motor's speed is governed by the AC frequency not the voltage supplied. My statement applies only to those motors designed for and run on 60Hz AC current!

C. Henry
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bartbob



Joined: 30 Oct 2010
Posts: 102

PostPosted: Tue Nov 09, 2010 12:01 am    Post subject: Reply with quote

C. Henry wrote:
BTW a synchronous motor's speed is governed by the AC frequency not the voltage supplied. My statement applies only to those motors designed for and run on 60Hz AC current!
Right. I think the old one I've got had a rheostat controlling the voltage level to it for speed. It's label says nothing about being synchronous.

And yes, a faster rpm would be best for the faster shutter speeds. I'm planning on using my drill press spindle with that pulley on it. It'll go up to 3000 rpm.

Bart
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Henry



Joined: 09 May 2001
Posts: 1442
Location: Allentown, Pennsylvania

PostPosted: Tue Nov 09, 2010 2:03 am    Post subject: Reply with quote

There's a dandy website for Telechron clocks: http://www.telechron.net/main.htm
Highly recommended! See especially the Helpboard; there are some very knowledgeable folks who can address just about any topic having to do with electric clocks, and Telechrons in particular.

My 1H1308 Telechron Commercial wall clock is watching over me as I type this.
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hartwell_a_m



Joined: 04 Jun 2001
Posts: 84
Location: Northern California

PostPosted: Tue Nov 09, 2010 6:32 pm    Post subject: Reply with quote

The turntable Idea seems to be the best way to test Iris shutter speeds, especially if you can find an older turn table that runs at 33-1/3, 45, & 78 rpm. In order to Make seeing the timing mark easier paint a radius line on the turntable mat. When doing the tests make sure that the lighting is diffused enough to avoid hot spots, and photograph the turntable in black and white.
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