300mm Symmar-S transplant

[Dan Fromm]

The lens aperture is the ratio of the lens focal length to the diaphragm diameter.

Not quite right. The right diameter to use is the diameter of the entrance pupil.

The aperture scale will not be the same for a 300mm lens used in a shutter with aperture scale for a 360mm lens.

Right.

[Daniel Unkefer]

I use a metal rule metric mm's, back up a couple of feet to minimize parallax (for precisely measuring and marking entrance pupil size), use a fine point sharpie and masking tape for making f/stop bands. Or of course you can print one up with computer if you know how to do it. Mark a fine point dot at half f/stop intervals.

It works.

[Dugan]

As Dan stated above, the f-stop is calculated by measuring the apparent aperture as viewed through the front element...

[Peter De Smidt]

I do this a bit differently. Put the lens in db mount on camera. Put a light meter with Bluetooth on the ground glass inside the camera. Mine is accurate to 1/10th of a stop. Point the camera at a constant light source. Take lumen readings at the various aperture settings. Put lens cells in 'new' shutter. Adjust the aperture until it matches the various lumen levels. Mark the corresponding aperture in some way.

[abruzzi]

Sorry to have opened a can of worms here. Several presumably knowledgeable people are stating it’s the actual physical aperture, and others are stating it’s the optical aperture as seen through the elements in front of lens. The latter is what I’ve always heard. Is also jibes with my original thought, specifically how can a Copal 0 (which based on specs I looked up has a 24mm max aperture) house a 150mm lens any faster than f6.25, since 150/24=6.25. My assumption has always been that with the front optics it makes the size of the opening look larger by bending the light. I could understand the limitation of the 90 (which should be able to get to f3.75 on a 24mm aperture) since the lens elements themselves might occlude the outer circumference of the shutter’s aperture.

Anyway, it’s neither here nor there for my original issue, since the accuracy of the aperture on a mismatched shutter is still 100x more accurate than my shutter speed if I do the lens cap thing.

[Paul Ewins]

If your existing mount has the aperture scale marked on it all you need to do is measure the aperture diameter and transfer it to the new shutter. You don't need to derive the f/stop from first principles (in which case apparent vs actual aperture does matter), if f8 is a 10mm aperture in the DB mount then f8 will be a 10mm aperture in a shutter.