Graflex.org

AaronC · Joined: 16 Jan 2004 Posts: 4 Location: Texas

I have seen references that direct measurement of focal plane shutters (using a centrally located photo diode) requires some sort of mathematical correction to get an accurate shutter speed.
Does anyone know if this is true.
I built a shutter speed tester, and have measurements for 2 anniversary's and 1 pacemaker.
I know the tester is accurate, and I have been surprised with my results.
Thanks for your responses
aaron

t.r.sanford · Posted: Tue Apr 13, 2004 8:40 pm Post subject:

The difficulty about measuring focal plane shutter speeds is that the speed of the curtain tends to vary as the slit travels over the film. It usually speeds up, as the spring gets everything moving and overcomes the rest inertia. Self-capping shutters can be designed to compensate for this; it's tougher with single-curtain designs.

In the old days, Graflex shutters were known for this acceleration. The saving grace was that the speed increased (and so exposure decreased) as the top of the subject (bottom of the frame) was being imaged, which often produced better printing density in the (usually overexposed) sky area of the negative.

If your tester is based on a photosensor, have you tried checking the apparent speed at the top, middle, and bottom of the film gate?

I think your results would be of considerable interest here. They certainly would be to me.

AaronC · Joined: 16 Jan 2004 Posts: 4 Location: Texas

Thanks for the info. The counter we built is very accurate, so it should record very subtle differences. I have a lot of things going on right now, but I will test the three cameras as suggested and post information here.
Two problems I've noticed so far is that there is little variation in shutter speeds within some settings on the Anniversary's. I don't have the data here, but for example B1-6 are all effectively one shutter speed. But A and C show a greater range of change through 1-6.
Also, my experience suggests that adjusting spring tension is trickier than manuals report- to much/to little may get you close to the shutter speed you want in one range, but put you way off on the top and/or bottom ends of the scale. So you have to fiddle around and get a compromise tension setting that gets you close to desired settings for all the speed ranges.
Romney states that a lot of the old leaf shutter high speeds were never as high as indicated. But reports that graflex focal plane shutters are accurate- particularly pacemakers. The fastest focal plane shutter I have is about 1/700th. I can adjust my pacemaker to get relatively close to target shutter speeds, but it seems to require 2x more tension than my manual suggests, and thumps noticeably when set at 1/1000.

Les · Joined: 09 May 2001 Posts: 2682 Location: Detroit, MI

"I can adjust my pacemaker to get relatively close to target shutter speeds, but it seems to require 2x more tension than my manual suggests, and thumps noticeably when set at 1/1000. "

Well it takes about 2x the effort to get me up to the same speed I was 40 years ago so that doesn't seem all that far out of reason.

glennfromwy · Joined: 29 Nov 2001 Posts: 903 Location: S.W. Wyoming

Photographers who did a lot of high speed, stop action work used to give the spring an extra wind to get a more true 1/1000 out of these things. I seem to have one with an overly wound spring, as it is difficult to get it to the "D" setting. I'm here to tell you, that shutter just about flies out of the box at 1/1000. I don't use speeds that fast, so I can't attest to it's accuracy.

_________________
Glenn

"Wyoming - Where everybody is somebody else's weirdo"

AaronC · Joined: 16 Jan 2004 Posts: 4 Location: Texas

Thanks for all the responses.
I have posted the actual shutter speeds of my 2 anniversarys at
http://www.graflex.org/helpboard/viewtopic.php?topic=2505&forum=1&0

t.r.sanford · Posted: Sat Apr 24, 2004 7:11 pm Post subject:

It might be interesting to take either camera and run a series of tests, first with the photocell positioned about a quarter of the way down from the top of the filmgate, and the second, with the cell moved down to a position about a quarter of the way up from the bottom. This would provide a gauge of the efficiency of the shutter's speed governor, and could be helpful in determining what speed to set when confronted with a specific lighting situation.

Dan Fromm · Joined: 14 May 2001 Posts: 2148 Location: New Jersey

t.r.sanford · Posted: Sat Apr 24, 2004 9:47 pm Post subject:

That is my usual recourse, but it won't detect acceleration of the shutter curtain.

The whole trick with focal-plane shutters is that they do things locally at a rather low speed, with the cumulative effect on the film of something done globally at a rather high one.

As far as I can see, only something that can measure speed (exposure) at specified points in the curtain's travel will identify acceleration (or deceleration).

I've tried to think of a method of photographing a mechanical stroboscope that will provide this local information, but to date, the only things that have come to mind are far too complex to be workable.

Sjixxxy · Joined: 27 Apr 2004 Posts: 109 Location: Midwest US

A1 1/28

Interesting. I took some outdoors stuff this summer at the A1 setting that came out quite thin. I figured an incorrect filter factors would have accounted for some of it, but they seemed a lot thinner then a slight FF error would account for. If my camera's A1 has shifted to a similar speed, it would easily make up the rest of the thinness.

How well do the El-cheapo homemade shutter testers work on the focal plane shutters? Would the single photocell device be sufficient? Or would a two cell device be better?

_________________
K. Praslowicz Photo | Watershed - Photos from the Lake Superior basin

glennfromwy · Joined: 29 Nov 2001 Posts: 903 Location: S.W. Wyoming

What I have found, trying to test these shutters with my little Calumet tester, is that extraneous light from the overly wide curtain aperture cause consistently erratic readings. So, what I did was take an old dark slide, drill a 3/16 inch hole in the center and let the photo cell see through only the hole, with the dark slide placed almost in contact with the curtain. The one and only camera I bothered to test was almost dead on, up to about 1/200, where I quit. I mean, who uses speeds that high for 4X5 with classic lenses that must be stopped down? Not moi.

_________________
Glenn

"Wyoming - Where everybody is somebody else's weirdo"

Dan Fromm · Joined: 14 May 2001 Posts: 2148 Location: New Jersey

Sjixxxy · Joined: 27 Apr 2004 Posts: 109 Location: Midwest US

Jim23 · Posted: Sat Dec 11, 2004 3:43 pm Post subject:

I use a very odd method to check exposure on a Graphic FP shutter. I have a Compur leaf shutter on another Graphic that I had a camera repair guy "certify" the speeds, and exposures are spot-on when I measure using my Gossen digital meter and a gray card using Polapan 3000 film (not much lattitude). I make a test shot with the Compur (Xenar lens) on Polapan (Poloroid) film. I then open the leaf shutter and make the same exposure (at the same speed) with the FP shutter and compare the exposures. If the FP exposure is comparable, I assume the FP shutter is reasonably accurate. If the exposure is over/under then the shutter is slow/fast.