old shutter speed conversions

[rich caramadre]

I have a lens in a compur shutter with 5-10-25-etc. speeds. What compensation do I have to do to these to match up with a copal shutter?

[Shen45]

Short answer -- none. The actual variation in real speed will more than make up for any differences. If you see a huge difference between the 2 lenses one or the other is not firing at it's correct speed.

Long answer is too long and will depend on how anal you are about what you consider precision.

The differences are only fractions of f-stops and even on colour tranny you may be very hard pressed to see a difference.

[Donald Miller]

1/4, 1/15, and 1/30 second...most shutters are no nearer than that anyway.

[Doremus Scudder]

I'm sure the posts above are correct, but I have a tendency to want to be a little more accurate. IF your shutter is dead accurate (unlikely), the differences between all the non-standard speeds and the modern speeds is approximately 1/3 stop.

I had a Kodak Flash Supramatic shutter CLAd by Grimes & Co. not long ago and asked them to test the shutter and provide me with actual speeds. Surprisingly, for all but the highest two or three settings, the speeds were all very close to the indicated speed.

For this, and a couple of my other shutter that are consistent but not accurate, I have taped the actual shutter speeds to the lensboard. I have my shutters tested often and get a list of the actual shutter speeds from the tech. I made a table of shutter speeds in thousands of a second and compare the values for each shutter tested with that and round at the closest 1/3 stop. (I use "+" and "-" signs to indicate the differences from standard speeds, e.g. 1/30- = about 1/25).

Here is the list (apologies for formatting inadequacies...):

Shutter Speeds
in milliseconds with corresponding fractions in approximately 1/3-stop intervals

milliseconds fraction
2000 2 sec
1600 1.7 sec
1250 1.3 sec
1000 1 sec 1
800 1/1.25 (4/5) sec
630 1/1.6 (2/3) sec
500 1/2 sec 6
400 1/2.5 (2/5) sec
320 1/3 sec 4
250 1/4 sec 3.2
200 1/5 sec 2.5
160 1/6 sec 2
125 1/8 sec 1.5
100 1/10 sec
80 1/12 sec
63 1/16 sec
50 1/20 sec
40 1/25 sec
32 1/31 sec
25 1/40 sec
20 1/50 sec
16 1/63 sec
12.5 1/80 sec
10 1/100 sec
8 1/125 sec
6 1/167 sec
5 1/200 sec
4 1/250 sec
3.2 1/333 sec
2.5 1/400 sec
2 1/500 sec
1.5 1/667 sec
1.25 1/833 sec
1 1/1000 sec
0.8 1/1250 sec
0.6 1/1667 sec
0.5 1/2000 sec

Hope this helps a bit,

Best,

Doremus Scudder

[Emmanuel BIGLER]

You'll find here a bar-chart giving at a glance the correspondence with the modern series of speeds.
The difference with the modern series never exceeds 1/3 of f-stop.
So depending on your application you can (or cannot if you are very strict) ignore it and take the nearest reading on the modern series.

[rich caramadre]

Thanks for all the replys. Yeah, I've always used the closest shutter speed and fudged the f-stop a little. But the doubt always creeps in as I'm doing so. Things usually work out pretty well.

Rich