I do it every day and have for years on my 70D, 80D, and recently, a 90D. I have two L lenses and other EF lenses and so far no problem. I cannot see any problem with www or printed images. I have an EF 28-135mm that won't work on my 90D. Don't know why, but it won't connect the electronics. I didn't push it, bought the 90D its own 28-135 EF-S. I have shot 10s of thousands of photos on the 70D, 80D with EF lenses with no problems with exposure or focusing. Also a long Tamron and Sigma.

Excellent explanation, very clear. For focus stacking it appears that there is a limit to the number of bracketed images to use as you noted. I never thought of that although I have noticed that 5 or 7 images for HDR often leads to a disappointing result. Three is enough. It appears that the exposure interval would be a factor and would not be linear. For focus stacking the depth of field must be an important factor in making the pixel overlay process work. Your explanation for the noise generated by the iso signal may explain why I get fewer issues with noise when I am doing winter photography. More please...

and I have a bottle of Dawn... thanks!

Yes, I definitely got some useful info that I would never have thought of. Thank you!

Can someone suggest an objective protocol for testing a used lens? Used lens buying advice would be a big help.