R5 lunar photography with stacked EF extenders

A very interesting thread!

I too have played around with stacking teleconverters on telephoto lenses. As a challenge, on a budget, i tried a number of inexpensive vintage telephoto and modern telephoto lenses, and found that for an inexpensive lens the Canon EF-S 55-250mm IS STM is quite sharp.

I also found a couple of teleconverters that are relatively sharp (although admittedly not as sharp as the Canon extenders), a Kenko 1.5x 5-element Teleplus SHQ which is very sharp, and optics from a vintage Kiron MC7 2x teleconverter in a Kenko 36mm Canon EF extension tube, which is slightly less sharp but still quite good.

The total number of optical elements in my case is 27, 15 for the lens, 7 for the MC7 2x, and 5 for the 1.5x. So many elements takes its toll on the image quality, of course. I then shot the Moon with a 90mm aperture Meade ETX-90 (1250mm f13.8, got for $100 off Craig's list) for comparison. For its size, the Meade is a good scope and showed more detail, as expected, as well as being the less expensive option (used or new).

Processed using DxO PL5 with Deep Prime de-noise, sharpening, chromatic aberration correction, tone, contrast, and micro-contrast applied (more extreme for the stacked TC shot), shown at full pixel resolution.

Canon M6ii, Canon EF-S 55-250mm IS STM, Kenko 1.5x Teleplus SHQ teleconverter, vintage Kiron MC7 2x teleconverter, 675mm, f16, 1/125s, ISO 800

Canon M6ii, Meade ETX-90 at prime focus, 1250mm, f13.8, 1/125s, ISO 400