Copal shutter speeds B, 1, 2, 5, 10, 25, 50, 100, 250

[blevblev]

I have seen Copal 0 shutters with shutter speeds marked as B, 1, 2, 5, 10, 25, 50, 100, 250, which I assume represent 1, 1/2, 1/5, 1/10, 1/25, 1/50, 1/100 and 1/250 seconds. Why are these speeds not the normal divide the previous shutter speed by 2 kind of thing? Seems like they would be pretty off as far as being one stop from one to the next.

[Leigh]

The 5/10/25/50 shutters must be pretty old. And yes, those stray a bit from the 2x progression.

That's why modern shutters use the markings shown below.

The proper 2x progression (with marked speeds in parens) is:
1
1/2
1/4
1/8
1/16 (1/15)
1/32 (1/30)
1/64 (1/60)
1/128 (1/125)
1/256 (1/250)
1/512 (1/500)

The error is pretty insignificant, given that the shutter speed tolerance on modern Copals is +/- 30%.

- Leigh

[Emmanuel BIGLER]

The following visual chart explains where old speed values take place in between new speed series.
The difference is never larger than 1/3 of a f-stop.
https://www.flickr.com/photos/43175600@N00/412475261/

[ic-racer]

Unless the speeds are linked in the EV system, deviations from the current series would not affect its use.
However, any shutter old enough to show those numbers is unlikely to give the speeds indicated on the dial.

[blevblev]

I'm having it cleaned, the timing checked, and adjusted.

[siuling337]

Thanks for the sharing.