# Thread: Wista Copal Press shutter #1, how to calibrate the f/stop for differents focal length

1. ## Wista Copal Press shutter #1, how to calibrate the f/stop for differents focal length

I have a very nice Wista Copal Press shutter #1. It came with an aperture scale. Are some data are available for this shutter as an f scale is already affix to? At the top of the scale I can see this indication W180.
Same as this one. Mine doesn't have the black tube, don't know what it is? See mine below.

For calculate the good f-stop for a specific focal lengh I think that I need to take exact diameter of each f-stop. Focal Length / diameter = new f-stop Right??

EX: Actual 5.6 on the aperture scale = diameter of the blades 25mm. So for a 300mm it will become 300 / 25 = f12 for a 150mm lens it will be 210 /25 = f8,4

Is this a simple way to calculate an f-stop correction that can be enter in an application like the Reciprocity Timer in terms of a final correction of the time. ex: the calculated 8.4 vs the original 5.6, what is the mathematically coorect operation 8.4/5.6 = 1.5 stop less??

2. ## Re: Wista Copal Press shutter #1, how to calibrate the f/stop for differents focal le

If the scale indication is calibrated for a 180 lens, can I calculate an f-stop factor this way?
for a 360mm, it will be -2 stops
210mm cells will be -1stop 1/6
150mm will be + 1/6 stop

3. ## Re: Wista Copal Press shutter #1, how to calibrate the f/stop for differents focal le

Can someone help me with theses f-stop calculations?

4. ## Re: Wista Copal Press shutter #1, how to calibrate the f/stop for differents focal le

The second paragraph in the 1st paragraph is correct.......But..........the diameter is measured through the front element then calculated.
It may be easier to just make up a card with the apertures for each focal length.

5. ## Re: Wista Copal Press shutter #1, how to calibrate the f/stop for differents focal le

And is the second post good too?
On the actual scale I have the indication W180. Is that means that the f-stop scale is calibrated for a 180mm lens??
And then 210mm cells will need 1stop 1/6 compensation (210/180) and a 150mm will be + 1/6 stop more luminous (150/180).
I have Graphic-Kowa 210mm and 150mm cells that will fit parfectly on this shutter.

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