View Full Version : Copal 1 minimum iris opening

Gene M
30-Sep-2003, 17:21
I'm confused.....I see some lenses mounted in Copal 1 shutters that have minimum iris openings of f64 and other lenses mounted in the same shutter that have some different minimum opening. Too confuse me further, I have a fairly new Copal 1 shutter (it has no iris scale)that appears to close down to about f45 or so. How can the same size shutter have different minimum iris openings ?

Ernest Purdum
30-Sep-2003, 18:00
The minimum iris opening depends on the lens rather than the shutter. The same size opening for one lens might be f16, but f32 on another. An f stop is the result of the focal length divided by the effective aperture. This is not the actual aperture of the iris, but the size the opening appears to be when viewed from the front of the lens. When you are looking at it like this, the actual circle is magnified, and the amount of magnification differs from one lens to another.

Also, the minimum marked stop may be chosen to indicate that the lens should not be stopped down further so as to avoid bad results due to diffraction. When light scrapes by an edge like that of a diaphragm blade, some gets diverted away from where it belongs. This is diffraction. The shorter the lens, the greater the problem. Big lenses may stop down to f90, while small lenses might be marked only to f32. Sometimes a mechanical stop will be provided to ensure that you are not stopping down so far that diffraction becomes a major problem.

Ralph Barker
30-Sep-2003, 18:03
Don't forget, Gene, that the f-stop is relative to the focal length and design of the lens. As such, the same size physical opening of the iris in the shutter would likely have different f-stop values for different lenses. That's why the f-stop scale needs to be calibrated for the specific lens that is mounted in the shutter.

Gene M
30-Sep-2003, 18:09
I see. Thanks guys.

Edward (Halifax,NS)
16-Mar-2004, 12:06
As a slight aside I have also found out that the Copal Press shutters used for the 3 inch Tominon CU-5 camera has a smaller max iris diameter than the regular Copal 1 or Copal 1 press. I don't know about minimum iris diameter.

15 or 16mm vs 30mm.

First diameters are from Jim Galli and the second are from SK Grimes website.