@rickmp001 wrote:
Thank you for your reply. I realize the lens has many deficiencies, but until I can justify spending on a more suitable replacement, I'll live with it.
Not that much editing is needed to compensate for the extender. The extender will magnify any flaws in the lens. An old lens might work better for black and white instead of color. The editing will be unsharp mask and if the aperture is too small then Richardson/Lucy deconvolution. I usually do those for any photo.
I suggest using the lens in manual mode and take advantage of the manual focus assist in the EOS R. The Kenko TELEPLUS HD C-AF 2X DGX is much less expensive than the Canon extender and will work with many EF and EF-S lenses. If cropping, the image is reasonably good for the price. There will be some vignetting. Small aperture diffraction blur will be a problem unless one does Richardson/Lucy deconvolution. Rawtherapee free software does Richardson/Lucy deconvolution in "capture sharpening".
https://www.cambridgeincolour.com/tutorials/diffraction-photography.htm has a "diffraction limit calculator". When using it, use the F Number set manually on the lens multiplied by 2 for the extender. Select "Full Frame" for the EOS R. This will give "diameter of Airy disk". Check the spacing of photo sites on your sensor in the camera specifications at Canon. Divide the number for Airy disk by 3 and divide by photosite spacing. This is the "radius" to use in Rawtherapee capture sharpening. The number of iterations can be larger if low ISO and little noise, but must be smaller if there is noise or non-Gaussian blur and distortion present. The Richarson/Lucy deconvolution does very well at removing small aperture diffraction blur if the distortion in the image does not deviate too far from the Gaussian approximation or if one does not do too many iterations. A wrong radius will result in ringing at edges (halo) when one does many iterations.
https://en.wikipedia.org/wiki/Airy_disk#Approximation_using_a_Gaussian_profile has the calculation. A wavelength for green light is a good approximation because there are twice as many green pixels than either red or blue.
I made this table for myself for my EOS R5. I used 0.52 as the wavelength, but 0.55 is more common as an approximation for green light. Instead of the 0.84 and 1.22 mentioned in the Wikipedia article, I used 2/3 of 2.44.
In rawtherapee, capture sharpening, contrast 1, 20 to 60 iterations, Richarson/Lucy deconvolution
For EOS R5, 4.39µm pixel spacing on sensor, wavelength 0.52
Airy disk size is 2.44 * 520 *F Number
F Airy pixel radius for rawtherapee
F/8 10.1 / 4.39 / 2 / 3 == 0.38
F/9 11.4 / 4.39 / 2 / 3 == 0.43
F/10 12.7 /4.39 / 2 / 3 == 0.48
F/11 14.0 / 4.39 / 2 / 3 == 0.53
F/13 16.5 /4.39 / 2 /3 == 0.63
F/16 20.3 / 4.39 / 2 /3 == 0.77
F/22 27.9 / 4.39 / 2 / 3 == 1.01
https://en.wikipedia.org/wiki/Unsharp_masking
