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How to choose stop location for Ernostar lens?

  • 13 April 2023
  • 5 replies
  • 226 views
Yadong.Wang
Userlevel 1

Hello Zemax community,

My question might be very simple for Lens Designer Pros, but it has been bothering me for while. 

Background: I have been referencing the famous Lugwig Bertele’s Ernostar lens patent and try to recreate it in ZEMAX to study aberrations. 

Problem: There is no relevant info about the aperture stop in the original patent sheet (See blow). Reference link: Espacenet – search results

 

I have see some example that the choice of the stop location is between the 3rd and 4th lenses (about 15mm in the middle). I also know that by definition the stop should be located where the chief ray crosses the optical axis.

But somehow, I failed to actually find a logic way to define this stop location. Can someone please share your thought process on how to find the proper stop location in this case? Also, how will you choose the stop size? thanks 

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Best answer by Mark.Nicholson 16 April 2023, 19:10

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Mark.Nicholson
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Userlevel 7
Badge +3

I’d make the thickness from the third lens variable, and put a position solve on the thickness from the stop to the fourth lens that keeps the total inter-lens distance fixed. The optimize for best RMS set or wavefront or whatever. That will give you the optimum stop position.

Yadong.Wang
Userlevel 1

I’d make the thickness from the third lens variable, and put a position solve on the thickness from the stop to the fourth lens that keeps the total inter-lens distance fixed. The optimize for best RMS set or wavefront or whatever. That will give you the optimum stop position.

Thanks Mark for the suggestion. I used your method and got an optimal stop location!

 

Mark.Nicholson
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Userlevel 7
Badge +3

😁

Do you have ray-aiming turned on?

  • Mark
Yadong.Wang
Userlevel 1

😁

Do you have ray-aiming turned on?

  • Mark

Hello Mark,

Good question. I have actually played around before. But with the ray aiming on or off the high fields (green and red) do not seem to change much (please see Fig 1). I also check a Sonnar lens (Fig 2) that I wanted to benchmark the performance with, also appear to have the same issue with/without the ray-aiming.  

One thing I guess for the Sonnar is the ray bundle for the high field is smaller and the focal lengths is shorter for Sonnar by nature, the edge of the image will have less illumination? But for the Ernostar design, it is better but still might have the similar issue?

 

Mark.Nicholson
Rank
Userlevel 7
Badge +3

Yes, you’re seeing a lot of vignetting by the apertures of the first and last lenses, and it will make the edges less bright as a result. Not much you can do about it, as vignetting also improves image quality by clipping out the most aberrated rays.

Always have ray-aiming on, and recompute the vignetting factors with it on. Then, for the highest accuracy, convert all apertures to hard apertures, and then increase the entrance pupil diameter so that it is overfilled with rays. That will give you the most accurate illumination of the pupil.

Looks like you have a good handle on this design: well done.

  • Mark

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