[jimduk]

The back to back infinity-focussed objective set up works nicely with 2 sets of cheap infinity microscope objectives 2x or 4x back to back in a thorlabs tube. Paired lenses cancel out a lot of aberrations (but not all of them). I use this setup regularly for fairly close up, highly planar projective work. For 'low magnifications' say 2:1 and up the Edmunds 2X, 0.15 NA, Ultra Compact Objective has decent price/performance, but this is for sensors in the 1/1.8 - 1" range. If you come up with a nice, planar, cheapish 1:1 design for large sensors, drop me a line. (Disclaimer - not an optics person, but peered into the fascinating rabbit hole)

[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