Lens cells and their measurments,question.

JMC1969 · Jul 10, 2008

In another thread I had asked about a particular lens and a member (John Kasaian) answered to hold the lens against a light colored wall with the a sunny window behind it and move the lens until it was in focus.

It got me to playing around with a ton of old enlarger lenses we have sitting around. Example is a 105mm Rodenstock Rodagon. When using this method, lens in right hand, ruler in left, it seems as though the 105mm focus plane falls Approx. to the dead space between the two elements. So I then wanted to know what each element measured. Rear element approx 130mm and double that for the front element at 260mm. I don't understand the calculations. I had been hoping that since the rear element screwed directly into my Prontor press #0 that the front would fit as well. Not so lucky. Then hoping maybe two rear elements might be something to try, being that all these lenses are here. But if I am measuring right, I'm not going to get much out of doing such a thing.

Maybe someone could help me understand or point me in the direction of an explanation your average dope could understand.

Thanks

Ed Sukach · Jul 10, 2008

I've read this six times....

Just what are you trying to do - what is your ultimate goal - here?

Curt · Jul 10, 2008

I read it only once and a half and got a headache but I'd like to know what's going on with the measurements.

Dan Fromm · Jul 10, 2008

Guys, the OP is telling us that he's discovered (a) that plasmats are separable and (b) that the two cells of a separable lens have focal lengths that don't have to be equal to each other or to that of the entire lens. Yawn.

And he told us that a plasmat's rear node is near the diaphragm. Yawn again, this is true for most, not all, lenses.

He also told us that not all lenses in barrel have cells that go straight into standard shutters. Yawn again, again.

Apo-Gerogon, anyone?

Deckled Edge · Jul 10, 2008

I read it once and immediately agreed that I am stupid when it comes to optical mathmatics. I have looked to buy various plasmat sets, but faced with the prospect of combining front cell A with rear cell B and getting the square root of the cotangent of W, then imagining trying to do this in the field while the light was deteriorating on the crosses at Hernandez, I invariably give up.
What the sarcastic Jerk and I really need is a pointer to optical theory, a topic obviously mastered by those who yawn.

BrianShaw · Jul 10, 2008

Deckled Edge said:
What the sarcastic Jerk and I really need is a pointer to optical theory, a topic obviously mastered by those who yawn.

There are books and web sites on this topic. I once read them and quickly decided to forget the notion of understanding photo-optics and be satisfied with just taking pictures.

Paul. · Jul 10, 2008

I feel that some of the replies to the OP have been unhelpful and not to put too fine a point on it Bloody Rude. This may be a cultral differance my being on another continent, but I thought the idea here was that we helped each other, if this is not the case please tell me so that I can bugger off to some other site and not bore anyone here with questions.

The attitude displayed by some of the responces makes me hesitate to ask any questions of fellow LF users here. As unfortunatly I was not born knowing all things as some here seem to of been I shall just have to buy the book and manage as best I can.

Regards Paul.

BrianShaw · Jul 10, 2008

http://www.google.com/search?hl=en&q=photographic+optics

Ed Sukach · Jul 10, 2008

Paul. said:
I feel that some of the replies to the OP have been unhelpful and not to put too fine a point on it Bloody Rude...

Regards Paul.

At first glance, I would agree ... but!!

The writer signs him(?)self as "Jerk151". and somewhere, there is a reference to sarcasm. While indelicate - decidedly - directing the answers to "jerk" might be expected - at least that is something I would prepare myself for.

I only called for a clarification - I did not understand the question (note there was *no* mention of "Plasmat"), and truthfully, I still don't.

Steve Smith · Jul 10, 2008

Some basic optical theory as I understand it:

To find out a lenses power in dioptres, divide 1 by the focal length. e.g. a 100mm lens = 1/0.1 +10 dioptres.

When you put two lenses (or elements) together you just add the dioptres together*

So if you have two lenses of 100mm focal length the new power of the lens is 10D +10D = 20D.

The new focal length in metres is 1/Dioptre value. In this case, 1/20 = 0.05 or 50mm.

This wa an easy example as it had two equal values.

The same equations can be used to work out how, for example, focal length is changed by adding a 2 dioptre close up lens to a standard lens.

50mm lens = 1/0.05 = 20D. Add the 2D supplementary lens and you get 22D.
New focal length = 1/22 = 0.04545m or 45.45mm

So you can easily work out the overall focal length by adding the powers in dioptres of all the parts then converting back to distance.

* If there are negative (concave) elements involved, they are subtracted.

A lens of 1 dioptre will focus parallel beams of light to a spot 1 metre away. 2 dioptres; 0.5 metre, 4 dioptres; 0.25 metre, etc.

I think that's exhausted my optical knowledge but may be of some help (or further confusion!).

Steve.

* If there is

Dan Fromm · Jul 10, 2008

Ed, Rodagons are plasmats. Didn't you know that?

So are Apo Gerogons, which are process lenses and also outstanding taking lenses. Unfortunately the wretched things' cells won't go into standard shutters; this makes them pretty useless as taking lenses, except for those of us with Speed Graphics. As the Packard car advertisements used to say, ask the man who owns one.

Paul, you're typical of the high-minded right-thinking people who infest photography bulletin boards. You add nothing to the discussion, abuse those who do. Small wonder that fewer and fewer people here know anything. Bad people drive out good.

Dan Fromm · Jul 10, 2008

Steve, please check my arithmetic. I just looked up some convertible lenses in a Boyer catalog from the 1930s.

It shows front cell focal length = 235 mm (4.26 D), rear cell 195 (5.13 D), combined 130 mm (7.69D). Boyer's calculations were done by Suzanne Levy-Bloch, a student of Henri Chretien and one of the first, if not the very first, French female optical engineers. Similarly, she has two 315 mm cells combining to make a 180 mm lens; by your arithmetic they should make a 157.5.

Rats! I may have to go to the bookshelf m'self.

Cheers,

Dan

Dan Fromm · Jul 10, 2008

Steve, see S. F. Ray, Applied Photographic Optics, 3d Edition, p. 310, where he says "A single element lens or a single group consisting of two or more cemented elements has a single value of effective focal length (EFL) other than slight variation due to residual aberrations. But when it is combined with another element or group, a different EFL results, depending on the powers and separations of the components, as given by equation 8.3."

Equation 8.3 is on p. 47,

"For the elements separated by a distance d,

F = 1/f = 1/fa + 1/fb - d/fafb"

Your equation is missing the last term; it drops out for elements in contact, but that's not the case with the lenses we're discussing.

The italics are Ray's.

What any of this has to do with actually taking pictures escapes me.

Cheers,

Dan

Sparky · Jul 10, 2008

Ed Sukach said:
At first glance, I would agree ... but!!

The writer signs him(?)self as "Jerk151". and somewhere, there is a reference to sarcasm. While indelicate - decidedly - directing the answers to "jerk" might be expected - at least that is something I would prepare myself for.

I only called for a clarification - I did not understand the question (note there was *no* mention of "Plasmat"), and truthfully, I still don't.

Ed!!! You're a smart guy... I'm more than a little taken aback to learn that you might read into the intent of a post according to the poster's fictitious name...!! I'm flabbergasted. Perhaps he's referring to jerk chicken... or perhaps simply being 'modest'...!

At any rate - it seems (somewhat) clear to me that maybe he's trying to fit the cells on an existing shutter and notes that the cell distances are different than hoped for. So he's looking for a little clarification, maybe an answer along the lines of...

"actually most lens/shutter assemblies are somewhat custom in their setup... and you can't simply swap out cells even if they screw directly in... since you'll find they can't effectively focus. If you want to try mounting it in a shutter - you'll have to go to someone qualified who can do the work for you like SK Grimes... as for learning about optical assemblies and optics in general - there's a steep learning curve involved. Just understand that each group of elements function like a separate, whole discrete element. You should pick up a good book on the subject if you would like to know more."

Dan Fromm · Jul 10, 2008

Um, Sparky, I mentioned Apo Gerogons because I have some experience with them, also with Steve Grimes and his successors. Not only that, I've taken pictures at distance with enlarging lenses.

Short answer, enlarging lenses can be extremely useful for closeup photography. Some lenses sold for enlarging are simply rebadged taking lenses (same prescription, possibly different engraving and mechanical details); these are jes' fine as taking lenses. But most modern enlarging lenses are pretty bad at distance.

I mentioned Apo Gerogons because they're outstanding at distance. At least as good as G-Clarons. But the wretched things' cells won't go into a standard shutter without expensive machining.

The machining is so expensive that a roughly equivalent taking lens already in shutter, e.g., Sironar instead of Rodagon or Apo Gerogon, is almost always a better deal. Shutters aren't cheap, machine shop charges are high, so even a free, e.g., Apo Gerogon is somewhat of a poisoned gift.

One always has to check cell spacing when swapping cells from a barrel into a shutter, but I've never had a problem swapping post-WWII cells into a post-war #0 or #1. #00, yes, absolutely; #00 plain Compurs and Compur Rapids from old folders are also somewhat poisoned gifts. But in my limited experience, #00 Synchro Compurs and Prontor Press are fine.

Cheers,

Dan

Sparky · Jul 10, 2008

Dan Fromm said:
Um, Sparky, I mentioned Apo Gerogons because I have some experience with them, also with Steve Grimes and his successors. Not only that, I've taken pictures at distance with enlarging lenses.

Dan - I never questioned the wisdom of doing that. I only glossed over your post, but didn't take anything from it in terms of my response. I was mostly reacting to ed - and then giving a response that I was assuming/hoping the OP was looking for. misunderstanding I guess?

Curt · Jul 10, 2008

Um, Sparky, I mentioned Apo Gerogons because I have some experience with them, also with Steve Grimes and his successors. Not only that, I've taken pictures at distance with enlarging lenses.

Short answer, enlarging lenses can be extremely useful for closeup photography. Some lenses sold for enlarging are simply re badged taking lenses (same prescription, possibly different engraving and mechanical details); these are jes' fine as taking lenses. But most modern enlarging lenses are pretty bad at distance.

I mentioned Apo Gerogons because they're outstanding at distance. At least as good as G-Clarons. But the wretched things' cells won't go into a standard shutter without expensive machining.

The machining is so expensive that a roughly equivalent taking lens already in shutter, e.g., Sironar instead of Rodagon or Apo Gerogon, is almost always a better deal. Shutters aren't cheap, machine shop charges are high, so even a free, e.g., Apo Gerogon is somewhat of a poisoned gift.

My 16 1/2 inch RD Artar in barrel would be $700.00 mounted in a Copal shutter with spacers and an engraved iris. $$$$$$$$$$$$$$$$$ It's nice to have the same lens in a shutter!

I know what you mean by poisoned gift, I have a 210mm Apo Gerogon that's like new, no wonder, no one found out how to use it. It has a flange and in checking I found that it's not really a very good enlarging lens, so "they" say. What were these lense originally designed for?

I have a 240mm Rodagon that I am going to try out for enlarging, the Apo Gerogon might go on a board for the hat trick.

Sparky · Jul 10, 2008

Curt said:
It has a flange and in checking I found that it's not really a very good enlarging lens, so "they" say. What were these lense originally designed for?

They're supposed to be 'budget' or 'student' lenses. Not sure what the deal was with those - wonder if they were made in taiwan and then stamped in germany or what. They're kind of renowned for their lack of usefulness..! I always wondered if the name wasn't supposed to indicate a line of lenses for geriatrics with failed eyesight...(?)

Curt · Jul 10, 2008

(there was a url link here which no longer exists)

http://cgi.ebay.at/Objektiv-Rodenst...5897450QQihZ010QQcategoryZ15424QQcmdZViewItem

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=330250427376&ssPageName=STRK:MEWA:IT&ih=014

They're everywhere, pages and pages of discussion on the Net., I think I'm lucky to have a lens that has such a cult following. There must be something about them that's useful.

k_jupiter · Jul 10, 2008

Curt said:
My 16 1/2 inch RD Artar in barrel would be $700.00 mounted in a Copal shutter with spacers and an engraved iris. $$$$$$$$$$$$$$$$$ It's nice to have the same lens in a shutter!

I know what you mean by poisoned gift, I have a 210mm Apo Gerogon that's like new, no wonder, no one found out how to use it. It has a flange and in checking I found that it's not really a very good enlarging lens, so "they" say. What were these lense originally designed for?

I have a 240mm Rodagon that I am going to try out for enlarging, the Apo Gerogon might go on a board for the hat trick.

Ask the man who owns a Packard (Shutter). Send me your unwanted process lens. I'll fit them for either my Speeds or make an adapter for one of the Packards that seem to lie around here.

As for the tone of the conversation. 1.) some responses seemed rude. 2.) we are all big boys and girls, just get over it.

tim in san jose

Ed Sukach · Jul 10, 2008

Try to be a "Peacekeeper...."

Lately, I've been trying to enhance - more honestly, ESTABLISH communications between two Hospitals ... with me in the middle. In a past life (I should select another term...) I worked a LOT in communications as a sort of "osmosised" area vital to Quality Engineering. Hospitals are now sensitive - VERY sensitive ... make that PARANOID ... as far as protecting patient information - to the point where patient CARE is negatively affected. One way out of this mess is to "paste the information on the side of the elephant as it goes by." In this case, the "elephant" is me. I am the agent revealing the information, so it would be very difficult to sue myself. I regularly carry and deliver interesting test results, Operating room Records.., to BOTH hospitals.

What I saw when I read "Paul's" response was an overreaction to something of a failed communication ... where Paul was inordinately "bent out of shape".
I tried, in my own feeble, inefficient way, to pour a few ml of oil over troubled waters. For that ... I've been accused of NOT KNOWING that a "Rodagon" enlarging lens WAS a "Plasmat"!!!
Oh - the HUMILIATION ... NOT!! I might have read something of that fact (?)
somewhere - but even considering the effect on my reputation and image - Frankly, Scarlet, (or anyone else) I don't give a damn.

I've been caught in the fringes of these "ultimate lens design" discussions, and I try to make my involvement as minimal - as short and sweet as possible. They draw too much of my energy from the real question that has been haunting my life for a long time, now; "Why does one photograph`work' and the next, FAIL". As near as I've been able to tell, "Plasmat, Curtagon, Super Catadioptic Reversed Huygens - NOTHING (pinhole) - has very little correlation with the final appreciation of the photograph.

I have some small amount of experience - Quality Assurance Specialist - in a wildly "cutting edge" Optical manufacturery. If I can help, I would be glad to lend whatever small amount of intelligence I may.

sun of sand · Jul 10, 2008

this has to be in the running for thread which has the most confused posters posting award

Curt · Jul 11, 2008

I was glad to have participated.

k_jupiter · Jul 11, 2008

Curt said:
I was glad to have participated.

You were in this thread?

tim in san jose

Ole · Jul 11, 2008

Dan Fromm said:
Steve, see S. F. Ray, Applied Photographic Optics, 3d Edition, p. 310, where he says "A single element lens or a single group consisting of two or more cemented elements has a single value of effective focal length (EFL) other than slight variation due to residual aberrations. But when it is combined with another element or group, a different EFL results, depending on the powers and separations of the components, as given by equation 8.3."

Equation 8.3 is on p. 47,

"For the elements separated by a distance d,

F = 1/f = 1/fa + 1/fb - d/fafb"

Your equation is missing the last term; it drops out for elements in contact, but that's not the case with the lenses we're discussing.

The italics are Ray's.

What any of this has to do with actually taking pictures escapes me.

Cheers,

Dan

Once you start using casket sets, it matters. Also if you start combining unmatched cells, "converting" lenses et cetera.

The instructions sold with most casket sets just ignore the "d/fafb" term, even if it had been known for a very long time before the sets were made.

Lens cells and their measurments,question.

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