Rodenstock Eurygonal f4.2

[Struan Gray]

I have just acquired a Rodenstock Erygonal 220 mm f4.2 and am looking for more information about what its intended use might have been. I have seen the spot diagrams and other curves on the dioptrique site, and have confirmed that my lens is a 6-3 as shown in the diagram here (http://dioptrique.info/OBJECTIFS7/00302/00302.HTM).

I have the wherewithal to test the thing for myself (albeit on 4x5), so what I'm looking for is what the lens' original purpose might have been. It looks like a Dagor/Kollinear with the inner element of the rear cell split off, and dioptrique says it has a similar angle of view - 60°. Was it a Kollinear successor, or a cheaper version, or was it optimised for some other use? It opens beyond a marked f4 to a marked f3, despite saying f4.2 on the front rim, so perhaps this was meant to be a portrait dagor.

Mine is in a sunken mount (no focussing) in black-painted brass. It has an iris diaphragm, so was presumably a taking lens for on-camera use. It wouldn't have stuck out far beyond the front standard, so I'm wondering if it was meant to be used on the popular travel cameras.

Any ideas or old catalogue descriptions welcomed.


PS: series number is 7857, which seems a bit Spartan.

[Dan Fromm]

The VM says very little about the Eurygonal, suggests that it was intended to be used as a portrait lens, also that the air-spaced version may have been (VM vagueness, the antecedent is open to interpretation) sold as a casket set. More to try ...

[Struan Gray]

Perhaps it was just Rodenstock's way of getting round the Dagor patent. The rim text says 'Doppel Anastigmat "EURYGONAL"', which suggests they were playing the here-is-our-version game.

The single cells are pretty awful. The front cell is hard to remove from the mount (needs a lens spanner), and although the rear cell does unscrew as a unit, that leaves a front element acting as a mensicus the wrong way round and in front of the aperture. It doesn't *look* like a convertible setup.

They don't seem to have been made for very long. I suppose I can sell it as 'RARE'. It's not 'MINTY', as someone's been mucking about with the rear cell.

Thanks for the info. Grateful for any more.

[Steven Tribe]

dioptrique is a very reliable source.
Looking at it, I thought immediately of a modified Petzval - extra correction of the front achromat and of the final lens. A bit like the work done on the Cooke triplet by Voigtländer to make the Heliar. An anastigmatic Petzval, no less!
Or perhaps, another "tweaking" of the Cooke triplet. It was a period when extra lenses, to avoid patents, were popular.

[Struan Gray]

The front cell is a Dagor, the rear cell a Plasmat.

The pessimist in me says it'll have the residual aberrations and uncertain centering of a Dagor, *and* the stupendous flare of an uncoated Plasmat :-)

Dioptrique is great. The spot diagrams are v. useful when thinking about bokeh. The only caveat is that patent literature doesn't always correspond exactly to the manufactured lens. But for trying to decide what an early lens might be able to do, and how well, the information there is unbeatable.

[ridax]

According to the aberrations curves, at f/6,3 and smaller stops the lens should make its background blur no worse then a Dagor's one (from f/10 on). Wide open, it may (or may not - this really has to be verified practically) make its foreground blur really beautiful. But astigmatism is awful - 3.3 mm difference at 45° (~22° from the center) for your 220mm focal length, and so is the barrel distortion (except within the central ~35°).

That means the glass is perhaps pretty usable for portraits and maybe certain still lifes, too (think same subjects RRs and Petzvals are nice for) but I certainly would not use it for much else. In fact, most probably I'd just sell the rare piece of nonsense to some crazy collector and keep using my trusted Dagors :)... But if you need that extra speed for portraits, perhaps you'd value the Eurygonal really high.

Pictures taken with the beast are welcome BTW. (And please never forget to specify the f-stops used!)

[Struan Gray]

Ridax, I agree with your reading of the data at Dioptrique. My only caveat would be that I personally don't place so much weight on the difference in the saggittal and tangential astigmatism. For judging the visual impact I prefer to look at the spot diagrams ('Mise au point') and do a mental convolution. When reading the aberration-vs-aperture curves I'm more worried by an asymmetry in the two astigmatism values than - as here - roughly equal values of opposite sign.

That said, it's pretty clear from the spot diagrams that astigmatism (and coma) are going to make for some pretty funky bokeh wide open. Contemporary Dagors are pretty blurred wide open, but it's a nice blur. This looks more edgy.

Wide open, at portrait distances, this will probably be an interesting swirly lens on 8x10. At f11 at infinity on 4x5 it should be sharp. I don't do many portraits, but when I do I aim for f8 or thereabouts, which allows the lens character to show through without getting into wild-and-stormy territory.

I got this lens because it was going cheap from a seller who was also selling a pair of Apo-Germinar-W's. I really need a few more leaves to fall of the local trees before my standard test scene will be in suitable condition, but it'll be fun to compare the character of the Eurygonal to the 210 A-G-W, whose curves are close to clinical perfection.

[Steven Tribe]

Sunken mount usually means boxy focal plane cameras like the Mentor, later used as press cameras.
If you something "odd" to test, Struan, there is an early periscope (pre-1858) on loan a few miles away from you which will "test" your tests. You know where.

[Struan Gray]

Thanks Steven. The last thing I need is to expand the parameter space for testing any more.

I'll play with the lens - as Dan likes to say, I have it, and can ask it directly instead of speculating.

Still interested in any info on how Rodenstock positioned this lens in its original market.

[ridax]

I personally don't place so much weight on the difference in the saggittal and tangential astigmatism. For judging the visual impact I prefer to look at the spot diagrams ('Mise au point') and do a mental convolution. When reading the aberration-vs-aperture curves I'm more worried by an asymmetry in the two astigmatism values than - as here - roughly equal values of opposite sign.

That said, it's pretty clear from the spot diagrams that astigmatism (and coma) are going to make for some pretty funky bokeh wide open. Contemporary Dagors are pretty blurred wide open, but it's a nice blur. This looks more edgy.

Wide open, at portrait distances, this will probably be an interesting swirly lens on 8x10. At f11 at infinity on 4x5 it should be sharp. I don't do many portraits, but when I do I aim for f/8 or thereabouts, which allows the lens character to show through without getting into wild-and-stormy territory.

...it'll be fun to compare the character of the Eurygonal to the 210 A-G-W, whose curves are close to clinical perfection.

At f/11 the astigmatic difference would still be 1.5 times more then the depth of focus (assuming 0.1 mm circle of confusion usually said to be OK for contact printing). To make the depth of focus just covering (without any surplus for focusing errors etc.) for the residual astigmatism at 40° to 45° (which at infinity means 4x5" with a little bit of movements), you'd have to stop down this lens at least to f/16.5 - just for contact printing. (But if you don't mind the lack of sharpness too badly, that certainly might not be important at all.)

I personally don't think Dagor's foreground blur to be any good at any f-stop (the Eurygonal should certainly be much better at this wide open). I use Dagors for background blur stopped down to f/10 or more but no wider. That's what the spherical aberration curves say - and my own practice confirm. But the Eurygonal should show no worse background blur at about f/6 already (though at the edges the out of focus rendition would most probably get ruined by vignetting as well as too prominent field aberrations). That's 1.5 stops faster compared to a Dagor, and I think that's the main (and maybe the only) advantage of this lens - though that could be a great advantage sometimes.

P.S.: Not sure what you mean by "difference in the saggittal and tangential astigmatism". Astigmatism is the difference between the saggittal and tangential 'planes' (actually, curved surfaces) of focus. And any derivation on the saggittal and the tangential surfaces themselves from perfect planes is actually curvature of field. For example, when the saggittal and tangential surfaces are very curved but curved the same way and thus coincide, that means zero astigmatism but a lot of field curvature.

P.P.S.: I also don't see what is there to compare in this terribly unsharp but beautifully rendering its unsharpness glass specimen to any lens close to clinical perfection as the latter means tack sharpness combined with unavoidably ugly and disgusting out of focus picture parts...

[Struan Gray]

I hear you ridax, and appreciate the comparison to the depth of focus as a useful metric, but can't help feeling that you're - perhaps - reading the curves a bit too closely. This lens has a rear element that someone has buggered about with (taken the rear pair out of a spun-in mount, loosened the single element so that it moves a bit) so it's certainly not going to be as well centered as the patent prescription.

My point about the astigmatism is that it is especially ugly when asymmetric. If the values of saggittal and tangential are radically different - ignoring sign - the effect on the image is to smear it outwards, or into concentric circles. If they are roughly the same, the blur is more symmetric, and generally more pleasing once convolved with the image. It's true that if they are the same, and the same sign, that is formally equivalent to field curvature, but that's why you don't bother to plot field curvature as well - the astigmatism curve gives you that information already.

As I said, I find the spot diagrams are more useful. They show the actual blur shape, rather than characterising it by some average or maximum width, which is what the astigmatism value at a particular angle gives you. They also include the effect of coma, which for me at least is harder to envisage as a shape given only the value from the curves. The only downside of the spot diagrams is that they are not greyscale or 2D histogrammed, so the visual effect of the tails of the distribution is over-emphasised.


I like clinical sharpness. And funky blur. It's nice to have both in the toolbox.