Speed accuracy?

[Paul Coppin]

At the risk of setting off another 150 post thread, why are you surprised at the results obtained using pre-WWII inexpensive technology? Until I mention the D word, you're playing with hardware designed in the 30's, in which the process is self-calibrated, less so the equipment. For most, LF remains a craft, rather than a technology, wherein the results are driven by the skill of the operator to understand and learn to use his tools, not the tools themselves. Exactly the same methodology you use in your workshop. As has been mentioned, repeatibility is the holy grail. The numbers on the dials really mean squat. Your job is to find the right combination of click time over the right sized hole that works in your process to give you the end result you want. LF is a craft of first principles, not technology (unless of course you have enough money to afford the D word .

As has been pointed out, even on new lens-shutters, the manufacturers generally quote +/- 25-30%, because they're still building 1930s shutters.

[Paul Coppin]

I did note the set speed v.s. the actual speed in my notebook for each shutter. Maybe I should put this info on a sticky label and attach it to each lens board....

You're on the right track. I made up a table of corrections for various things, printed it out in a text size I can just make out and laminated it into a small card that clips to the tripod for ready reference when I'm setting up a shot.

[Calamity Jane]

Paul: Thru my 56 years I have worked ALOT with antique machinery. I have a great appreciation for the craftsmanship, quality of materials, and ingenuity of old designs. Though I have restored/rebuilt many antiques, from steam engines to clocks, I had not worked on shutters until recently.

Certainly antique clocks are capeable of delivering good accuracy and I guess I was expecting shutters to be of similar quality. Non-pneumatic shutters certainly do well on slow speeds, there more sophisticated timing delays are used. Pneumatic shutters, of course, are effected by wear of the pneumatic delay cylinders, so I expected to find some shift with age.

It was while working on a Wollesak pneumatic from about 1901 that I began to understand the higher speeds are controlled by spring tension v.s. inerta and are therefore subject to changes due to wear, fatigue, dirt, and numerous other factors.

I posed the original question simply to find out if my group of shutters was typical or if they all needed work. It was not ment as a criticism of old shutters or their manufacturers. I have always been in awe of the complexity and design of shutters!

I refuse to even mention the "D" word!

[Paul Coppin]

And I wasn't trying to be critical in my response either. Its just that in this age there is a tendency to expect higher precision from the tool and less from the tool user... Within the scope of practice of "ordinary" LF, there is enough controllable variabilility in the process to accommodate the variability of the shutter, and it was pretty much always so... Clock precision is another matter. There was a necessity for a high degree of accuracy from the beginning, and so the mechanics evolved to reflect that accuracy. Shutters are not clocks, they are short timing devices that need to be far more robust than clocks, in order to drive a mechanism, that in and of itself has significant restraints due to a need for compactness and affordability. An interesting parallel would be to measure the variability in the precision of modest small mechanical alarm clocks - how precise do they work when you set them to go off at 8 o'clock... Wouldn't surprise me that you would see similar variability. Again, consistency is the higher goal in shutters, rather that accuracy. Accuracy is nice, but at what cost? In the trade-off, consistency carries much more weight than accuracy. Like when you're running your lathe (unless you are blessed with a multi-stage NC mill, it matters little that the speeds it turns are precise, only that they are adequate for the job, and repeatable, for consistent performance.

[Neal Shields]

Last time that I had S K Grimes service a shutter it came back dead on at all speeds.

I checked against my tester and against a 35mm camera with an electronic microprocessor controled shutter.

However, I keep a plastic lamanited card in my camera case for the rest of my lenses with marked and actual.

Note if you ever take an old shutter apart you will realize why some speeds can be slow and others fast. There are two gear mechanisms and a high speed spring. At normal speeds you use one gear drive and the main spring, at slow speeds an additional retarding gear train is shifted in, and at the highest speed there is an additional spring engaged.

On many shutters nothng but the front cover holds the high speed spring in place and it loves to jump out and run off and hide in dark corners.

[Bill_1856]

Mein Gott, you're 56 years old? How did you get...? Oh, never mind.

[Calamity Jane]

"Bill", as in "Wild Bill" ?? !! I thought you waz dead!

Guess, in a way, I'm older than 56, seein as how I was born in 1852 the first time ;-)

[Michael S. Briggs]

Understanding shutter efficiency is important in measuring fast shutter speeds. Calamity, specifically what time did you measure? The total time opening time, from when the shutter blades just admit some light to just before they block all light, is not the correct time to measure. You want to allow for shutter efficiency, which allows for the transient time of the blades. If the lens is set at f16, as the blades just start opening, they will admit light at f64, so you have to treat that delta-t of the blades at that position as counting for 1/16 of delta-t of exposure time.





Chapter V of the Handbook of Photography by Henney and Dudley (1939) explains this with figures.





Simpliest is to realize that LF photographers rarely use the fastest speeds.





You did find your shutters to be very consistent.





Previous discusssions:
http://largeformatphotography.info/l...ic/287113.html

and http://largeformatphotography.info/l...ic/113993.html.

[Calamity Jane]

Thanks for the reference to those old threads Michael - VERY interesting.

Indeed, I was measuring the time from when the light started to pass until the light was cut off which, as you state, is not accurate. I don't know the proper measurement but it seems logical that the exact exposure would be "the area under the curve". The waveshape, at high speeds, begins to look like an inverted V, where the shutter transition times are a large part (most) of the waveshape.

When I have time, I should go back and look at the area under the curve - it is likely that the shutters are more acccurate than my first impression.

(When it comes to "digital capture device", I LOVE my digital storage scope!)

[John Cook]

Jane, I believe your results are typical. The instruction sheet for Ethol T.E.C. begins with the statement: “Photography is not an exact science”.

The actual exposure is, of course, further distorted by the slop in the f-stop lever. The diameter of the aperture is often quite different between opening up to a setting versus closing down to that setting.

Mechanical shutters also get slow and sticky with age and inactivity. Including those in a Hasselblad. Part of my first job as assistant/studio manager in Hollywood in the old days was to get out all the lenses and work the shutters every Friday afternoon.

At the risk of sounding like a heretic, this is why I do my film exposure/development tests (as much as possible) on the EOS with the electronically-set shutter and f-stops. Try exposing a gray card over seven stops in perfect third-stop intervals on a view camera.

Also why I shy away from emulsions which do not come in both 35mm and sheet sizes.

But don't tell anybody my secret...