Ray tracing programs only really work well when you know a lot of info about the types of glass involved and there specific curvatures.
The refractive index of glass varies with wavelength - I play with NIR increasing the range of wavelengths beyond most lenses design so chromatic aberration could be more of an issue than for purely visual uses. Standard historical achromats use competing lenses of crown & flint glasses to make a combination where all wavelengths of visual light focus much closer to the same point than we'd get with a single simple element.
Picking available elements at random I wouldn't even recognize which were crown glass & which were flint glass. In lenses there's actually a much wider range of glass types used . With the aid of kit at work I could possible identify a few of them Calcium fluoride (from it's mid IR transmission) & pure quartz (UV transmission) the bulk of the others I can spot are technical glasses of more use in filters than lenses. I don't have the means to determine the curvature of the surfaces to any useful level of accuracy either.
Combining achromatic doublet of known focal lengths to get basic telephoto or retrofocus designs is something I don't need ray tracing for. Simple application of formula available on-line (IIRC they're on the homemade lenses group on Flickr and added into a spread sheet I have) will calculate combined focal length & back focal length, from the individual doublets focal lengths & their relative spacing. Of course the aberrations from such simple designs will be significant, but that's half the fun of experimenting. The only trouble is finding the time to build it - I may still be years away from building my own experimental lenses from scratch.
Quick modifications to existing lenses are much easier. I have two in use where the rear group has been removed;
The 50mm I did this to became a soft focus lens of about 100mm focal length (I stick it on bellows to focus, the soft glow decreases rapidly as the aperture is closed down). Here's an example fully open
soft focus mod fully open by
Mike Kanssen, on Flickr
Another lens (a soligor 105) had a badly fogged rear group that proved more difficult to clean up than I'd hoped. Trying it without the fogged elements left me with a lens of roughly 200mm focal length & much higher contrast (again needing extension to focus). I was surprised that aberrations in the remaining lens are not particularly noticeable.
an example of that :
Soligor 105 without rear group by
Mike Kanssen, on Flickr
From what I hear flipping elements on some normal lenses (the Helios 44 is the usual subject) also creates a soft focus effect but with 'crazy bokeh' too. The mod, is simple & reversible & I have several spare 50mm lenses so I'll probably get round to trying it with both front & rear elements (both reportedly work but give different effects) sometime before I retire.
