A lense you didn't know you had

[AgX]

As the Schneider company did refer do the convertibilty of their Symmars (three out of one) only decades ago, I corresponded with them about it years ago. And as far as I remember they definitely did not recommended this for their current Symmars. (Have to look for that letter.)

Anyway as far as one would take the inconvenience/danger of deassembling a lens in the field this procedure would give you an interesting lens for portreture (where aberrations would be of less importance), given the fact that the (complete) Symmar is rather a wider angle lens.

 

[Sparky]

'far as I know' (as I heard) they stopped doing this - even though the lens design is identical, despite what marketing hype says, years ago because of complaints that the 'converted' lens was of 'poor quality'. In order to save their reputation - they stopped advertising that feature. But I imagine the Sironars and Nikkor-W's are just as good at this trick, as is any lens of the general (planar, I believe) design.

 

[David A. Goldfarb]

Early Sironars were marketed as convertibles with the bonus of being designed so you could use the front cell in front, protecting the shutter blades and having access to the filter thread.

Before learning that, I tested my modern 135/5.6 Caltar II-N (Sironar-N) using only the rear cell behind the shutter. I don't have my notes handy, but this is probably f:22 on Type 55 (if you google it, I'm sure I've posted this somewhere before with all the data). From the image it doesn't look like I used a filter for the test, but I would normally use one in practice with a converted lens for B&W--

The attraction of this is that my usual backpacking setup would be a 90mm/f:8 Angulon and this lens, and getting a lens of around 180-200mm as a convertible would be a real bonus.

 

[jstraw]

I'm probably going to test all of my lenses with front only and rea only configurations. If any of those look just worthless on the ground glass, I'll skip exposing a sheet of film. I'm not that concerned with exact aperture for this so I'll probably just try to do the spot-meter the ground glass approach.

I was thinking about the unprotected shutter and perhaps front elements can be replaced with a clear filter.

 

[Curt]

I'm probably going to test all of my lenses with front only and rea only configurations. If any of those look just worthless on the ground glass, I'll skip exposing a sheet of film. I'm not that concerned with exact aperture for this so I'll probably just try to do the spot-meter the ground glass approach.

I was thinking about the unprotected shutter and perhaps front elements can be replaced with a clear filter.
__________________

Michael, I hope you didn't stay up all night working on them! I couldn't sleep because of the mail deliver who didn't knock the door but drove off with my package leaving only a note saying that I couldn't be reached. On a Friday too, I need those filters and adapters.
It was probably the Kona coffee I drank and the lenses I went through also. I checked a few but nothing new. None of the Ektars and all of the Turners of course. Hope you find something of interest. I keep an old 8x10 ground glass on the book shelf for spot checks.

 

[PHOTOTONE]

I did try this with my 180mm Nikkor W, using just the rear element, and I could not bring the image into sharp focus, even stopping completely down. I had plenty of bellows draw, I could go beyond "best" focus, but "best" focus was not sharp. So perhaps the Nikkor W series will not do this.

 

[jstraw]

Well, I let you all know what I find out but I'm pretty sure the 210 will be a really nice, soft, 370mm, f-12.5 portrait lens...give or take.

 

[PHOTOTONE]

I think (just my opinion) that the most recent modern versions of these "symmetrical" lens designs are more and more unable to function as "sharp" convertable lenses, as they are becomming more "un-symmetrical".

 

[jstraw]

I think (just my opinion) that the most recent modern versions of these "symmetrical" lens designs are more and more unable to function as "sharp" convertable lenses, as they are becomming more "un-symmetrical".

Yeah, ain't it great!

 

[PHOTOTONE]

Well, for my use (as I have soft portrait lenses), I would prefer a lens that is convertable to be able to produce a sharp negative from either group, as this would help me cut down on weight of my location kit, shooting subjects that demand sharpness.

 

[jstraw]

That would be cool too. But as someone with no portrait lens, finding out I have a good one just by removing the front of a lens is great news.

 

[David A. Goldfarb]

Usually it's best in principle if the diaphragm is in front of the single cell, and it might turn out that the front element works better behind the shutter than it does in front of the shutter or than the rear cell does behind the shutter.

 

[jstraw]

Do both elements on a lens always have the same threading? I guess shutters are the same size on both sides, eh?

 

[David A. Goldfarb]

Unfortunately, sometimes the two sides of the shutter don't have the same thread, so it's not always possible.

 

[Ole]

The sad thing about the most standard of all shutter sizes - the Compur/Prontor/Copal #1 - is that it has different front and rear threads.

I really like the sadly defunct #2 size, a compact size which fits on most lensboards, yet perfectly symmetrical threads. I believe my 240mm f:5.6 is my "most converted" lens. Fortunately I also own a 165mm f:6.8 Angulon in exactly the same size shutter, which saves me getting four different lensboards for only one lens...

 

[Murray@uptowngallery]

JimG - nice to hear you have a mutant G-Claron and it STILL works.

This is interesting to me because proper cell spacing and symmetry is usually demanded by most people typing about it, usually in the context of mounting in a shutter.

Your lens must be something else with the transplant...not symmetrical, but positive with either or both cells...triple-convertible G-Mutantagon?

I don't want to call it an Illegitimagon - I registered that one with the Ministry of Silly Lenses for one of my upcoming unspeakably mutant projects.

Mu-ray

 

[Sparky]

Since someone resuscitated this thread... I really have to ask (I'm sure nobody knows for SURE... but maybe someone heard something) - I dunno - Mr. Goldfarb or Ole want to take this on...?

What PRECISELY (or roughly I guess) is the nature of the aberrations experienced when using rear cell only? I'm wondering this - because I'm considering that there may be possibility of salvaging an actually sharp image when using a single cell and a deep red or other filter...!

Of course - for this to work- the aberrations would have to be largely chromatic. If it's coma from hell... well that's a whole other thing.

I recall doing this about 10 million years ago on my first lens - a convertible symmar (probably 60s vintage with green & white printing)... but I found the rear-cell only configuration too soft even to really pull a good 8x10 print.

 

[jstraw]

I still haven't processed the test sheet I exposed using my 210 Componon-S with just the rear element but I can tell you that what I percieved on the ground glass was more or less a nice 360 (give or take) with a soft focus and a discernable glow on the highlights.

 

[David A. Goldfarb]

Since chromatic aberration is one of the things that can be corrected by making a symmetrical lens, yes, it's usually a good idea to use a strong monochromatic filter when shooting b&w with a single cell.

There's also some focus shift associated with putting the stop in front of the cell, so it's a good idea to check the focus at or close to the shooting aperture.

I'm not sure about coma, but if you're not using the whole image circle, it's probably small enough not to worry about.

In my experience, single cells are never as sharp as more sophisticated lenses of the same focal length, but sometimes they're sharp enough stopped down, or they can be handy when you don't actually want a sharp lens.

 

[Ole]

Just off the top of my head, and based on one exposure on colour film with half a 150mm Symmar...

There's a little chromatic aberration, but not enough to be the main problem. Mostly it's field curvature and/or astigmatism!

This is the picture - with two small sections scanned at 600dpi:

 

[Sparky]

Hey Ole - wasn't that the same image you'd posted on the LF forum?? I wasn't aware that it was from a half-lens. Wow. That's an encouraging result! I think I'll re-explore it.

Actually - I just tried something with your tree detail, Ole - this is kind of fascinating. I opened it in photoshop - just to look at the different channels to see if there would be a big improvement with the use of a filter. Seems to be. What's even MORE interesting about this... is that if you click on each of the R G and B channels consecutively, you can really SEE the movement between them... you can see how the lens is projecting each of those spectra at a totally different place on the film!

I was going to try to make an animate gif to illustrate this in photoshop.. but alas.. I don't know how!

J

 

[Ole]

Sparky,

I hadn't thought of that! You're absolutely right - there's a definite shift from B-G-R. Red seens sharper too, but that's usually the case. There's still quite a bit of astigmatism to bee seen, but a red filter would certainly make a difference. Unless you shoot colour film, of course...

 

[Murray@uptowngallery]

Types of aberrations when using half a lens:

Symmetry and stopping down correct a significant number of aberrations, so it might be hard to state from observation which of these two affects which.

Hopefully someone more confident will specifically answer your question, but I think symmetry improves all aberrations but transverse chromatic.

I also think that stating what effects result from amputating a lens to a single cell will depend upon the design.

For non-symmetrical lenses, you (obviously) have to have the half with a positive diopter value. Tessars and derivatives have a positive half and a negative half. In some of those derivatives, there is some order of lens element reversal. With some designs the function of each variation is for some goal of the designer, so I don't think one can be too specific.

I just read something (a patent; can't tell you which right now but I DID print it out so I could find it again) that compared three design methods of correcting spherical aberration, and cited examples (specific lens designs) with respect to each methods predominance in that design. It stated spacing, bending and index of refraction/dispersion difference.

Is spherical aberration the most common? Or is it the easiest to correct?

 

[Sparky]

I wouldn't say spherical is the most COMMON aberration, so much as the one that gets talked about most, as more people understand it than they understand others, such as chromatic, or coma (point distortion), astigmatism, etc, etc... but yes, I did know that symmetrical designs were an attempt to cancel aberrations. I did NOT, however know that about tessars. Kind of fascinating. I'd like to learn more about primitive lenses and the specific genealogy leading up to the tessar. I've got some pretty good books that may cover it. I'll have to check it out or google it maybe...!

 

[Ole]

... but yes, I did know that symmetrical designs were an attempt to cancel aberrations. ...

Symmetrical lenses are free from coma, distortion and lateral colour aberrations when used at unit magnification. So each half needs only be corrected for spherical, chromatic, astigmatism and field curvature.

That fits with the observed reults using converted lenses - the "automatic corrections" from symmetry are lost, and only partly corrected in the lens design.

If I had owned a "letter Symmar" (i.e. anything newer than a convertible) I could have compared them to see if the old one is any better than the newer ones.