The Canon Canon Macro Photo Lens 20mm 1:3.5 is a tiny special lens for macrophotography in the range of 4:1 to 10:1 (fourfold to tenfold enlargements). Is is intended for use with a dedicated macro bellows such as the Canon Auto Bellows. For attaching the lens - which has a RMS thread - to the bellows one needs the Canon Macro Photo Lens Adapter. Ideally one has also the Canon Macro Stage which allows you to place your objects on a suitable glass plate. These four parts (or a similar other bellows) are necesssary to take good images with the 3.5/20mm Macro lens.

A few years ago I got both the Canon Macro Stage as well as the Canon Auto Bellows for the FD system. Some months later I had the chance to buy a Canon Macro Photo Lens 20mm 1:3.5 with the dedicated Canon Macro Photo Lens Adapter.

For years the setup was laying around, litteraly collecting dust. Yesterday - on the attic, while looking for something else - I found a small piece of rock I had collected as a teen in the Val Nalps (Swiss Alps). Apart from some rutile crystals (titanium dioxide) it had also a tiny (< 0.5mm) yellow crystal from a rare elements mineral (maybe monazite). So finally I took out the Canon 3.5/20mm Macro lens ... and here it is:

Both images shown above were taken at about 10:1 (the second one was slightly cropped, thus the seemingly larger crystal). Nominal aperture was f8, effective aperture (due to the increased distance between sensor and lens) was about f80. Of course this results in a lot of diffraction, and the original 24 MP image is by no means sharp. This is not a fault of the lens, but simply an effect of the tiny aperture through which the light enters the lens. Below is a 100% crop from the original image (24 MP FF):

If we reduce the original image to 50% (resulting in a 6 MP image), details become reasonably sharp. For enlargements in the 10fold range such a setup may be a good compromise between a reasonable depth-of-field and a useful resolution: