[namibj]

What numerical aperture and image circle are we talking about? 0.15 NA leaves a lot on the table (at least 0.6 NA objectives can be had for still-decent prices).

But I'd probably look at a Zeiss Planar style design.

In general, a 3/5-element design (achromat/apochromat) should do the trick I think, provided you have access to arbitrary aspherics. And in case you can accept a shift in focal length with wavelength (like monochromatic photolithography would), a single symmetric element should be up to the task, unless I'm missing something.

[jimduk]

Thanks, we are pretty low budget. Image circles are in the 1/3", 2/3" range and we are looking at patterns with say 5 to 50 um pitch on a flat planar object, monochromatic, we control illumination, standard 3.45 um pixels (so 1k to 5kish). We have a decent small design with two elements (good planarity, ok resolving and ok ish depth of field are what we care about). But we also sometimes look at larger frame (aps-c and up) and are also interested in smaller (eg VGA Omnivision type tiny module but with a decent planar macro lens e.g. f= 4 or 6mm, for 0.5 or 1x mag would be interesting). We do imager based metrology at the sub um and nm level

[namibj]

Yeah, sounds like a normal https://en.wikipedia.org/wiki/Double-Gauss_lens from BK7 or so should do the trick. You'd have significant chromatic abberation, but with monochromatic illumination that's not a problem.

This would just be 4 normal lenses, afaik. And you should be able to reach around 100 mm image circle I guess with the low NA you get away with at that relatively low resolution.