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 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 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