Lens cells and their measurments,question.

[JMC1969]

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]

I've read this six times....

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

[Curt]

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]

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]

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]

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.]

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]

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

[Ed Sukach]

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]

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