Hey David,

I’ve experienced the same thing with my Basic Shapes of Imaging Systems series. The optimizer reduces the merit function: that’s all it can do. It doesn’t know a Cooke triplet from a Tessar from a double Gauss.

I think this is A Good Thing. Many of the classic designs resulted from tracing a handful of rays, and they are not necessarily optimal when you can trace many rays. In particular I have noticed a tendency to add a field flattener to almost any design with sufficient field unless you constrain the design to have a large back focal length.

The constraint for large back focal lengths in imaging systems was of course to accommodate the shutter and viewfinder of classic photographic lenses. Without that constraint, it’s hard to get the classic shapes, and you’ll almost always get a lens close to the image surface as this controls field curvature so well.

• Mark

I think that’s a good point, Mark. It’s probably not good to assume a classic form is the best we can do. 

I have to admit that most of the time when I want to constrain a design to a particular form is when I am trying to figure out what someone else has done.

I recently came across a very low cost 50mm F2 lens designed for the new mirrorless cameras.  (I have one on order for my Nikon Z6ii.) The vendor literature on the lens provided the usual advertising graphics. An online review of the lens says that it has acceptable image quality, except for being a bit soft wide open, but suffers from both flare and veiling glare when a bright source is anywhere near the image area.

I thought I might like to investigate the design, but of course I do not have a prescription. What I have is this from the vendors site:

And this from an online review:

So what I have is a basic layout without dimensions, and evidence that significant vignetting is being used to reduce aberrations at the edge of the field. We know the lens is 50mm and F2, and it’s for a full frame camera so we can assume the extreme field in image space is at about 21mm. Since it’s for a mirrorless camera the back focal distance is likely about 20mm. Given the price and reasonably good bokeh it seems unlikely there is an aspheric element.

I crafted a rough layout for this in OpticStudio:

This starting point is 70mm F3.8, and needs to become 50mm F2. It also needs to achieve good imaging. What I am asking then is this:

Is there a good way to use OpticStudio to optimize this lens by determining curvatures, thicknesses, and glass types while preserving the form of the design such that the final layout still looks like the vendor’s illustration.

I stress that I am NOT trying to reverse engineer this lens for any nefarious purpose. This is just an exercise. However, if this makes the moderator’s uncomfortable, please feel free to delete this post!

I have attached my “starting point” as a ZAR.