Space Between Lenses

[E. von Hoegh]

Not saying it's easy but, if you're interested in finding the "correct" spacing for infinity, there are only two parameters: (1) spacing between the two cells and (2) the back focal length.

The range of values for these can be further narrowed down by using domain knowledge. For example, intuition says the spacing is most likely not going to be massive like 10 inches. So you can put it between 0-10" (or say 1-250mm). For the second parameter, BFL, again some educated guess about focal length can narrow the range down. Let's say 1000mm (which is most likely grossly over in reality) so that puts the range between 1-1000mm.

Assuming an initial precision of 1mm, we have 250*1000=250,000 combinations to try out. This is where a heuristic is going to help. If properly done, a Monte Carlo search should allow the OP to estimate the spacing (and BFL) within a hundred or so attempts. But I think it's going to be even quicker in practice since this is not a very stochastic problem and the parameter ranges are continuous.

After finding a rough (1mm precision) configuration, fine-tuning can be done by hand easily to whatever the possible precision is.

And how does one know when the spacing is correct? Your exercise is based on two guessed parameters. Unless you have one known, say the original measured EFL, you're throwing darts in a darkened room, and you don't know where the target is.

[E. von Hoegh]

Measure the entire assembly = distance A
Measure the front cell from filter ring to the area that touches the shutter (not including the threads) = distance B
Measure the rear cell same as above = distance C

The space between the cells is then = A- (B + C)

The OP is asking how to determine the spacing when he doesn't know what the shutter dimensions were. If you have the lens in shutter, all you need to do is unscrew the cells and measure the distance between seating surfaces on the shutter, one measurement, not three.

[genotypewriter]

And how does one know when the spacing is correct?

Well it's a photographic lens... the objective is to get something in focus, right?

Your exercise is based on two guessed parameters. Unless you have one known, say the original measured EFL, you're throwing darts in a darkened room, and you don't know where the target is.

Not guessed... empirically calculated. Also, there is no dart throwing of any sort. Monte Carlo methods are very successful in just this sort of thing.

There's going to be at least one configuration in which sharp focus will be obtained. If there's more, one can pick which ever suits them the most. Can't guarantee that the methods brings the lens to the original design/configuration. Can't guarantee that it's going to be worse either.

[E. von Hoegh]

Well it's a photographic lens... the objective is to get something in focus, right?




Not guessed... empirically calculated. Also, there is no dart throwing of any sort. Monte Carlo methods are very successful in just this sort of thing.

There's going to be at least one configuration in which sharp focus will be obtained. If there's more, one can pick which ever suits them the most. Can't guarantee that the methods brings the lens to the original design/configuration. Can't guarantee that it's going to be worse either.

Please explain how one "empirically calculates" these unknowns....when a lens can easily be + or- 5% from the marked focal length. Your best bet would be to set the cells up on a simple optical bench and set the spacing at the distance that gives you the flattest field. Monte Carlo methods....... try the experimental method, it's even more succesful in this sort of thing.

[ic-racer]

The OP is asking how to determine the spacing when he doesn't know what the shutter dimensions were. If you have the lens in shutter, all you need to do is unscrew the cells and measure the distance between seating surfaces on the shutter, one measurement, not three.

That is not a parameter that is "computed" it is established by trial and error.

[seawolf66]

Thank you all for your time and effort here to answer my crazy question<
One being it came as just a pair lenses marked 175mm at F4.7 so its going to be send it to
Grimes or find a shutter they will and play with till the image is sharp: Thank you All

[E. von Hoegh]

That is not a parameter that is "computed" it is established by trial and error.

Sorry, but it's part of the design process of the lens, no trial or error about it. One resorts to trial and error when one has lost the original number.

[genotypewriter]

Please explain how one "empirically calculates" these unknowns....when a lens can easily be + or- 5% from the marked focal length.

I have already explained most of it in my previous post. For the rest, please refer to:
http://en.wikipedia.org/wiki/Search_algorithm
http://en.wikipedia.org/wiki/Monte_Carlo_method
http://en.wikipedia.org/wiki/Evolutionary_algorithm

Sorry, but it's part of the design process of the lens, no trial or error about it. One resorts to trial and error when one has lost the original number.

Lens design also uses these exact techniques:

http://spiedigitallibrary.org/oe/res...sAuthorized=no
http://www.opticsinfobase.org/ao/abstract.cfm?id=131885
http://www.sciencedirect.com/science...45782599003989
http://ieeexplore.ieee.org/xpl/login...umber%3D823279
http://onlinelibrary.wiley.com/doi/1...20577/abstract
http://spiedigitallibrary.org/procee...sAuthorized=no
http://www.springerlink.com/content/rvcx1uaewuqnlqb7/


In addition to having the possibility of finding the exact original spacing when using these techniques, there might even be chance of finding something even better than what the manufacturer did

[Leigh]

...empirically calculated. ... Monte Carlo methods are very successful in just this sort of thing.

Empirically calculated from what? What parameters do you propose to use?

As to MC analysis, how would you set the goal and evaluate the results? What dataset would you analyze?

Buy a copy of Zemax, plug your numbers in, and let us know what you find.

starting point...
What values will you use for the optical characteristics of the glass, for the radii of the lens surfaces, and lens thickness?

- Leigh

[genotypewriter]

Empirically calculated from what? What numbers do you propose to use?

As to MC analysis, how would you set the goal and evaluate the results? What dataset would you analyze?

Again with the give an inch, obliged to complete it with a mile thing. I suggest you find out how focus can be evaluated. It's not that hard to come up with.

Also there is no "data set" here in a finite sense. We're optimising focus as a function of two variables:
http://en.wikipedia.org/wiki/Mathema..._of_a_function

Buy a copy of Zemax, plug your numbers in, and let us know what you find.

- Leigh

The method I suggested doesn't need Zemax Remember how I said one only needs to know only two parameters? If you were to use Zemax you'd need more information on the optical element properties of the lens cells too. Good luck measuring those.