Ray Optics Simulation of Swarovski NL, SLW (1 Viewer)

[kimmik]

To help me understand the optical design of the NL series, I have approximated the design using the online simulator linked below.

The design file is attached so you may do your own modifications, including changes to the field flattener lenses, to see its effect on field curvature, field of view, and field distortion. Just unzip, then use the web simulator to open.

All cemented doublets are drawn as thick singlets for simplicity. The app allows you to adjust the refractive index of each element to see its effect on the full system.

To modify the field curvature, adjust the curvature of the first element after prism.
To modify the pincushion distortion, adjust the curvature of the second element after prism.

Based on this simulation, the distortion and field curvature are adjustable independently, suggesting that current crop of flattened binoculars have deliberately chosen low pincushion distortion, rather than unavoidable side effect.

Why this is chosen? Perhaps to increase the true FOV. Or to enable better eye panning. The downside is less comfortable bino panning.

Simulator - Ray Optics Simulation

A free, open-source web app for creating and simulating 2D geometric optical scenes.

ricktu288.github.io

Binocular Evolution II: Curvature and Distortion

What I found interesting in holger's article (amongst many) was the way the SLC 56's were spread so widely accross their tendencies for different distortions. I assume that's because the eye lenses are the same most of the way through with the very last lens being changed to cater for the...

www.birdforum.net

 

[tenex]

That is truly cool. But can it reproduce any of the peculiar second-order effects? And what happens when you try to build the nonexistent bin with traditional pincushioning and a fully flat field?

 

[kimmik]

Thanks tenex, your very precise questions have lead me to investigate a lot further than I would have otherwise.

Pincushion with 0 field curvature can be achieved by removing the positive meniscus and you gives higher magnification smaller FOV (keeping eye relief constant), but without field curvature. Then to reduce the magnification you need faster objective lens, which is probably not possible, or increase eye relief which is what Noctivid has done.




Add back the meniscus, this is my reproduction of EL flattener, showing the central rays being minimally divergent coming out of the flattener, and outer rays being barrel distorted inwards, ie higher order image distortion.



As for an SLW, this next diagram maintains the true FOV but gives higher AFOV (ie magnification) and shorter eye relief. Can't get larger true FOV this way - needs either bigger prism/eyepiece, or faster objective lenses.

 

[elkcub]

Many thanks for introducing us to the ray-optics simulator.

Ed

 

[GrampaTom]

Love to hear what the folks at Swarovski are saying, "Oh my God, they've cracked the code!" or "He he, those dudes at Birdforum are at it again, if they only knew what we go through....."

 

[limonabe]

I am loving all your thought provoking posts of late, Kimmik.

 

[tenex]

Noctivid doesn't have a fully flat field, or the strong pincushioning typical of other Leicas... I think your first image (in #3) illustrates the challenge of that combination, and why flatter designs resort to lower distortion. But what happens if you just make the initial converging lens of the eyepiece weaker and move it back to the flattener?

Regarding eyepieces, this simulator must be fairly crude compared to what's used in Absam. I doubt it can handle aspheric surfaces, which surely occur in EL/NL. But it's a great way to get a basic sense of what's going on. Thanks kimmik.

 

[kimmik]

what happens if you just make the initial converging lens of the eyepiece weaker and move it back to the flattener?

Just tried this, if I make it stronger (not weaker) when moving it back to the flattener, it catches more FOV and reduces magnification as you suspected.

Absam most likely uses Zemax, whom I emailed for a trial version and received instead a newsletter subscription haha. They probably saw me write "hobby" under company name and clicked delete.

 

[tenex]

Just tried this, if I make it stronger (not weaker) when moving it back to the flattener, it catches more FOV and reduces magnification as you suspected.

So can you tell from this simulation how well that would really work out in practice? Surely there's a reason why it isn't done.

 

[kimmik]

So can you tell from this simulation how well that would really work out in practice? Surely there's a reason why it isn't done.

Going by conventional thinking, it is better to use two weaker elements than one stronger element in order to suffer less aberration. A field flattener is kind of the reverse, where you want the strongest curvature possible to un-aberrate the objective lens.

The above reason is how I came to simulate the SLW in post 3, by adding a positive element halfway between flattener and existing 8x eyepiece, making 10x42. It would either need more glass than SLC, or more aspheric surface.

 

[Maljunulo]

Fascinating.

Thank you.