Tool to assess LoCA of stereomicroscopes and macro lenses

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jim271828
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Joined: Wed Feb 22, 2017 5:35 am

Tool to assess LoCA of stereomicroscopes and macro lenses

Post by jim271828 »

I made a web-based tool to assess longitudinal chromatic aberration (‘LoCA’), aimed at stereomicroscopes and macro lenses. It’s at http://web.ncf.ca/jim/photomicrography/loCAte/

Here’s the longer story:

I noticed my stereomicroscope was coming to best-focus at focal distances dependent upon colour. To investigate that further, I came up with a method involving analysing a photo of an inclined straight-edge target. It worked out well, so I created a web-based tool to make the analysis more convenient and shareable.

The tool works by measuring the degree of edge-spread of an image of an abrupt edge, which is inclined. Because the target edge is inclined, it will be in sharp focus at only one point along the edge. Ideally, the point of best-focus should be the same for all colours, but if there is longitudinal (or ‘axial’) chromatic aberration (‘LoCA’), the point of best-focus will be different for each colour channel. This tool measures and charts the degree of edge-spread vs position along the inclined target, which, because of the incline, maps to focal distance.

This isn’t as information-rich as what could theoretically be obtained using something like Frans van den Bergh’s MTF Mapper, for example, if only there were a way to create the grid of slanted edges required by MTF Mapper that would be small enough to fit in the field of view of a stereomicroscope or macro lens. Absent that, I found a way to obtain quite a bit of information just using a single straight edge.

If you’d like to have a look as sample analyses, I’ve put four analyses online (two stereomicroscopes, two macro lenses), at http://web.ncf.ca/jim/photomicrography/loCAte/gallery/

The tool itself, with a link to ‘how to’ information, is at http://web.ncf.ca/jim/photomicrography/loCAte/

For example, below is a result from an Olympus OM-D EM-5 II with a M.Zuiko Digital ED 60mm macro lens, at f/5. The horizontal axis is field of view (across the camera frame) and, as mentioned, because the target was inclined, the horizontal axis is also focal distance. The vertical axis is amount of edge-spread; less is better. The chart shows that the green and blue channels came to best-focus at nearly the same horizontal point, but the red channel is offset (and not quite as sharp). I think that’s actually a pretty good lens and result (but I don’t have much data).

Image

Sadly, my stereomicroscope doesn’t perform as well (below). But I think it’s fairly typical of performance at its price level. Here it is at max zoom (worst case):

Image

All this is ‘beta’, and I’m not a physicist or microscopist, so no one should rely upon the tool for anything serious. It’s a Creative Commons contribution. I’d welcome suggestions or bug reports. Enjoy!

Jim

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