Converted focal length; 360mm

[2F/2F]

Hello,

I am wondering what a good set of converted apertures would be, and an approximate converted focal length, for my 360mm f/6.8 Schneider Symmar-S.

I know this is not "technically" a convertible lens, but I have done it before with a 210 Symmar-S and it worked very well for my purposes.

I have found reference to a first gen. Symmar that was a 360/620. If my lens were the same, by my quick calculations, f/6.8 on a 360mm is 53mm in diameter. 53mm in diameter is f/11.something on a 620mm. However, this math can't be working properly, as a 210 5.6 becomes an f/12.5 when converted, if I remember properly. If this same formula applied to an f/6.8 lens, you'd have an f/14 lens when converted. I really don't know.

I also have a Symmar-S 240mm and have the same questions about it.

If it is actually a 620 converted, next problem is getting enough rail and bellows for it...

Thank you.

2F/2F

 

[freygr]

If both elements are the same focal length then you would have two 720mm elements. As the focal distance doubled all your F-stops will be one stop off. F 6.8 becomes F 14.4 {Formula F stop = (Focal length)/ (Iris opening); Opening = (Focal length)/(F Stop)}

 

[Ole]

The Symmar, despite the name, is not entirely symmetrical. The two cells have different focal lengths, the rear one being a bit shorter than the front cell.

So 360/620 should be about correct - I doubt they changed that bit by much.

Your assumption that the aperture opening is 360/6.8 mm is where the problems start: The actual physical opening is a little smaller than the entry pupil which is what gives the effective aperture.

But when the lens is converted by removing the front cell, the entry pupil is identical to the physical aperture - there's no glass in front of the aperture which could mess up the mathematics.

So aperture = F/d, where F is focal length and d is aperture diameter. The Copal 3 shutter has an opening of 45mm. Assuming 620mm focal length, we get 620/45 = f:13.8 - which is your answer.

The old convertibles were mounted in Compound shutters, which had different aperture diameters.

 

[Ian Grant]

There's a 1937 Symmar for sale on Ebay at the moment, it's marked as - Schneider Doppel Anastigmat Symmar, it is in an Ilex shutter that is unlikely to be the original.

Schneider themselves state the Symmar is "a Symmetrical design". My convertible Symmar 240ml was in a Compur shutter not a Compound (it stll is if I ever find it).

All the information is on Schneider's website.

Ian

 

[Ole]

Ian - the 1937 "Doppel Anastigmat Symmar" is a Dagor-type triple convertible.

The Symmars are symmetrical in that the two cells have the same construction, but they do not have the same focal length.

The Symmar 240mm was sold in a Compur #2 shutter, at least while production of that lasted.

Most of the information is on Schneider's website - but on www.schneiderkreuznach.com, not www.schneideroptics.com.

 

[epatsellis]

And you will be pleasantly surprise at how well the converted 360 looks, as well.


erie

 

[2F/2F]

Thanks for responses.

Forgot to mention that my lenses are in DB mounts. There are always the edges of the aperture blades just visible on this lens, and a few of my other DB-mounted lenses. I am not sure if this affects max aperture or not. I can understand why on the lenses that are faster than 5.6 and use the red screw-in 5.6 stop for the aperture control lever, like my 90mm 4.5 Grandagon. However, I could never quite understand why on the 5.6 and slower lenses. My best assumption is that this might correct for the DB mount having a slightly larger diameter than the intended Copal size diameter.

At any rate, I am going to assume f/14 max. aperture and make changes if it appears that f/14 is not quite "spot on".

Right now I have what amounts to a "reverse" C. I started with an F1, but added a P front end. This leaves me with a multipurpose standard and an extra bellows. All I have is 24" of total rail. I think I need another multipurpose standard, another bellows, and another rail extension before I can give the 620 a try. I definitely need two bellows to focus the 360 close, and imagine I would need yet another to use the 620 at infinity, let alone closer in studio. Maybe the school will let me combine their lone F2 camera with mine for each shoot where I want to use the 620 in their studio, but probably not, as they love to say no to those kinds of sensible requests. Their Sinar was a recent donation and is marked for studio use only; not for checkout.

Good thing the school at least has a camera stand so I don't have to trust this crazy getup on my Bogen 3051.

I am looking into this for use as a long lens on 4x5, not for ULF. I love the 360 on 4x5, but often want longer. Also for an 11x14 box camera I am considering building.

2F/2F

 

[Ole]

Yes - the flange focal length of the rear cell of a 360/620 Symmar is considerably longer than 620mm. I believe it's closer to 720mm - or 29 inches. Call it 30", and you'll have enough for infinity...

I might check this in a while by mounting my 360 Symmar f:5.6 (620/12) on my longest camera - the Russian 30x40cm plate camera. But not soon, since I'm at work for at least some days more.

 

[Ian Grant]

Ian - the 1937 "Doppel Anastigmat Symmar" is a Dagor-type triple convertible.

The Symmars are symmetrical in that the two cells have the same construction, but they do not have the same focal length.

Not sure where Schneider have ever stated that the usual Symmar is a Triple convertible.

It's amazing what you can find quickly on the internet though. There are references to a Schneider triple convertible Symmar f6.8 180-285-355 with a Compur shutter without the triple f/stop scale. (Ole of course took part in the thread)

But run of the mill Symmars were originally true symmetrical lenses, although as Ole says later models weren't perfectly Symmetrical however the differences in focal length were only very slight so insufficient to be triple convertibles.

You'd need two Symmars of different focal lengths but same shutter to try something like this for yourself.

Ian

 

[Ole]

Not sure where Schneider have ever stated that the usual Symmar is a Triple convertible

No, they haven't claimed that. The "triple" bit was abandoned when they changed from Dagor-type to Plasmat - not because the focal lengths were insufficiently different, but because the two cells had different levels of correction so that only the rear cell was deemed useable alone. Somewhere there is actually a list of front cell focal lengths, in case of insufficient bellows for the rear cell.

One thing Schneider have never claimed is that Symmars are symmetrical! Even the original Dagor-type Symmars were unsymmetrical enough to be triple convertible.

 

[Ian Grant]

Ole, Schneider do claim that the Symmar's are Symmetrical, I think it was a War time Patent where Schneider first mooted an un-Symmetrical Symmar.

The two Schneider sites you posted links to post different data, but they don't contradict themselves. But they do claim the Symmar is a symmetrical lens.

The Symmar S series must be the plasmat series, according to what you are saying, which makes sense this is when they stopped being convertible.

Isn't the difference in focal lenght of the front & rear cells in a symmetrical pair used singly actually also to do with the position of the aperture.

Ian

 

[Ole]

The pre-WWII Symmars were Dagor-type f:6.8 (3 cemented elements in each cell), and were sold as triple convertibles. Post-WWII Symmars were Plasmats, (2 cemented and one air-spaced element in each cell), f:5.6, and still (!) unsymmetrical. The Symmar-S is also a plasmat, but the overall corrections have been improved by dropping the idea of the rear cell being moderately well-corrected alone (or so they claim).

If you put the front cell of a 240/420 Symmar convertible on the back of the shutter, you will not have a 420 f:12. You'll get something like a 500mm f:14. Don't confuse flange focal distance with focal length - I don't.

Symmetrical in lens design doesn't mean that the lens is absolutely symmetrical in the strict sense of the word, only that the two halves are of the same basic design. "Unsymmetrical" lenses are things like Tessars, where one cell is cemented and the other one not.

Fully symmetrical lenses with two cells of the same focal length can be found too - but that implies that they are optimised for 1:1 and not infinity. Repro and macro lenses are often fully symmetrical, with two identical cells.