300mm Symmar-S transplant

[abruzzi]

I have a 300mm Symmar-S in what I think is a DB lens mount. Actually to be clear (since I know little about the Sinar system) it is a shutterless board that I can adjust the aperture from the front of the lens. I know I've seen DB mounts that don't seem to have that capability and some that do, but I assume they are both DB mounts?

Anyway, I don't have a Sinar shutter, and I'm a little financially tight at the moment, trying to get a properly shuttered lens for testing my new 8x10 camera. I'd rather not test it while trying to simulate a shutter with a lens cap, given that my inexperience would spoil most tests. In my 4x5 lens collection, I have a 360 Tele-Arton in a recently CLA'd Copal #3 shutter. The shuttered versions of the Symmar-S 300mm I see on eBay also use the Copal 3. Can I assume that the elements from the DB board will work correctly in a Copal #3 shutter, or did the DB versions of these lenses have different mount/spacing? The Tele-Arton is a ƒ5.5 so the aperture scale should be close enough.

(I don't use the Tele-Arton very often, and probably will less now that I have the Nikkor Tele 360/500, so I can put the tele-arton's elements on a shelf for a while, if it will help me spread out the cost of kitting up the 8x10.)

thanks.

[Keith Pitman]

Pictures of the "DB mount" would help someone identify it for you and tell you what you've got.

[abruzzi]

I don't have photos of mine, but it looks pretty identical to this item on eBay:

https://www.ebay.com/itm/284340748189?hash=item4234077f9d:g:RcIAAOSwWztgz-e7

EDIT: And I don't know that this is a DB, mount, but that seems to be what they are generally called. I apologise if my terminology is incorrect as the Sinar system is pretty alien to me.

[Dan Fromm]

Well, you can always unscrew the Symmar-S elements from their Sinar board, unscrew the Tele-Arton elements from their shutter, and put the Symmar-S elements in the shutter. If the Symmar-S cells fit the shutter and the lens' total length -- from end of front cell to end of rear cell -- is the same in shutter as it was on the board then all's correct.

[Peter De Smidt]

I have the 300mm Symmar -S in a Sinar DB mount. You should be able to swap lens elements into a Copal 3, but note that the aperture settings will be a bit off.

[abruzzi]

Thanks, that’s what I figured, but I’ve read enough of odd variations, that I didn’t know if these Sinar mounted lenses had odd variations.

A tangential thought—the aperture scale—is the fact that the tele-Arton is a tele, does that affect the scale of the aperture, compared to a non-tele lens?

[Peter De Smidt]

I don't think so, but I don't know for sure. My guess is you'll be about 1/3 stop off from the 360.

[sharktooth]

The lens aperture is the ratio of the lens focal length to the diaphragm diameter. The aperture scale will not be the same for a 300mm lens used in a shutter with aperture scale for a 360mm lens.

For a 360mm lens set to f8, the diameter of the diaphragm will be 45mm. If you screw in a 300mm lens, a 45mm diameter diaphragm will give an aperture of f6.7. You would need to get a new aperture scale, or tape over the existing one and mark up some new numbers. It's roughly a half stop difference.

The shutter needs to be able to accommodate many different focal lengths and max lens apertures. This is done by only changing the aperture scale to suit the lens. This is why you can open the lens wider than the maximum aperture in some cases, because the shutter can provide a larger diaphragm diameter than the lens can use.

[abruzzi]

The lens aperture is the ratio of the lens focal length to the diaphragm diameter. The aperture scale will not be the same for a 300mm lens used in a shutter with aperture scale for a 360mm lens.


Is this true though? For instance I have two lenses installed in a Copal #0--a 150/5.6 and a 90/5.6. When I set both lenses to 5.6, the aperture blades are fully retracted, and the actual physical opening, 24mm is identical even though one lens is almost double the focal length of the other:

90/24 = 3.75
150/24 = 6.25

If the ƒ number is based on the actual physical opening, the 90 would need an opening of 16.07mm, and the 150 would need an opening of 28.79mm.

I'm not particularly knowledgable about optics, but I thought that the front elements distort the apparent size of the opening by bending the light more or less to achieve ƒ5.6 in both cases?

[Bob Salomon]

Is this true though? For instance I have two lenses installed in a Copal #0--a 150/5.6 and a 90/5.6. When I set both lenses to 5.6, the aperture blades are fully retracted, and the actual physical opening, 24mm is identical even though one lens is almost double the focal length of the other:

90/24 = 3.75
150/24 = 6.25

If the ƒ number is based on the actual physical opening, the 90 would need an opening of 16.07mm, and the 150 would need an opening of 28.79mm.

I'm not particularly knowledgable about optics, but I thought that the front elements distort the apparent size of the opening by bending the light more or less to achieve ƒ5.6 in both cases?

No,, f stop is based on the actual useable opening.

[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.