Hi David!

Nice to see a familiar face hope you're doing well!

As far as I know, I have seen this method of 'floating the STOP' used to find the potential best starting point for this surface at initial stages in the design, though applying this technique to locating 'best' position for a patent certainly makes sense to me. I suppose since patents like these are meant to be demonstrative, the priority in your case seems to be placing the STOP so that you receive the same performance as the literature describes.

Of course, the only 'downfall' I could see in this case is that the STOP surface gets too close to a lens edge (if you're trying to make it a separate aperture from a lens), that the STOP becomes embedded inside a lens element, things of that nature. At this stage, though, you're trying to ballpark it anyway, so you could then use the optimized result to decide whether the STOP should be a separate aperture or an actual lens surface.

I don't have much to add in terms of other 'reverse engineering' tricks, but I'd be curious to hear from other members of our community! It would certainly be interesting to see what other ideas folks have had :)

~ Angel

Hi Angel,

I am well thank you. I hope the same for you too :)

That is absolutely correct. if I could reproduce the results demonstrated in the patent, then I would be confident about the STOP position.

This is what happened. After optimisation, the STOP was right in the middle of an actual lens, which I'm not sure is physical actually.

Yes, I'm hoping to get good input from this discussion with other engineers.

Take good care of yourself Angel,

David

Hi David,

Your method sounds like it is really finding the pupil, not the stop. Pupils are usually virtual, so having them appear to be in the middele of a lens is not unusual.

Try creating the file as a non-sequential file, and fire rays that overfill the first lens into the lens from a couple of field points as well as on-axis. Look at where the ray bundles overlap or have their closest approach. Doing this from the front and back of the lens is probably a good idea.

Also, sometimes a lens aperture of mount is the stop, so don't be surprised to find a stop on or near an optical surface.

HTH,

- Mark

Hi Mark,

Thanks for your answers.

It does make sense. Depending on the lens orientation, I am probably more likely to find the entrance, or exit pupil. I completely overlooked this issue.

Do you think there still is a chance for this method to actually find the correct STOP position considering the pupils are images of the STOP?

I'll definitely try the non-sequential method.

It does help a lot Mark, thank you, and take care,

David

Hi all,

Mark, that's a great point you brought up I totally overlooked also. I suppose if your pupil aberrations aren't too bad, you could still 'eyeball' the approximate STOP position based on where your dummy pupil surface lands. I used the Cooke Triplet as a quick test -- I did a quick local optimization for spot size in the original file first, then I added a 'floating' dummy surface, made it the STOP, and did another optimization just varying that thickness, which made the following trace:

Showing just the physical surfaces:

It looks like a good candidate for the STOP position is at the second lens (which is where it is placed in the sample file), since this is where all the field points are overlapping. Of course, it's not perfect, as it will be obfuscated by any pupil aberrations for your system. But I suppose it at least gives you an idea, which you can then physically define and optimize a bit further to see if any higher performance can be achieved.

I've also attached the file here for reference!

~ Angel

Hi Angel,

Great work, and it makes sense.

On my end, I had a go at the non-sequential approach proposed by Mark (see figure below).

One can see where the fields overlap, and it also probably a good STOP candidate (note how the fields overlap on a curved surface, could it be pupil aberrations?).

On the other hand, in the sequential file. The optimised STOP surface, and the surface where the fields meet seem to nearly coincide (see figure below).

While I still haven't wrapped my head around both methods. It gives me a good idea where to look for the real STOP position.

Thanks a lot for your help, and take care,

David