conor0460,
I had a similar question regarding the 100-400L MkI on my 90D which as you may know, has a 32.5MP sensor like the R7. I don't have an R7 and don't know if same information applies to that camera/sensor or mirrorless cameras in general or quite frankly, even if my conclusion is valid. I may have chased the facts down a rabbit hole to an invalid conclusion. However, it seems to make sense to me at least with regard to the sensor on the 90D and possibly may apply to the sensor on the R7 or other cameras with high density sensors.
Case: I rented a 100-400L ver. 1 push/pull zoom (Mk1) and took it to the zoo and some of the places I like to shoot. For the most part, I was satisfied with my results had a great time with the lens. Most shots were tack sharp and well exposed. I'm more or less obsessed with nailing focus (or at least trying to do so) so when a few shots were soft, it puzzled me and I tried to figure out the reason(s) they turned out soft.
I came across a discussion by Michael the Maven on a topic called “Pixel Pitch” that I understand to be the distance between the center of each pixel on the sensor. What I took away from the article, the list of lenses in the article and Michael’s video was that the pixel pitch of the 90D is 32.5 microns and some older lenses (particularly zoom lenses) made for film cameras in the 1990's like the 100-400L MkI may not have enough resolving power across the sensor to produce "tack sharp" results. The 90D and other cameras with high density sensors including some full frame high megapixel cameras have a smaller distance (measured in microns) between the center of each megapixel on the sensor and have a higher Pixel Pitch. Other factors include the characteristic of the actual active sensor (cross-type, vertical, etc.) and AF Mode. For me it was pretty complicated, but not totally incomprehensible.
What I think I learned above made me rethink the wisdom of buying "vintage" EF lenses made and sold for EOS film cameras - particularly zoom lenses. To an extent, the "market" and the price of those vintage lenses vs. their newer variants is pretty much telling us the same thing. You more or less get what you pay for.
Again, I don't know if the R7's sensor and it's "pixel pitch" behaves similarly to the 90D's when using older zoom lenses designed for EOS film cameras or even if the conclusion I reached above is valid. After all, most of my results with the 100-400 Mk1 were in-focus and well exposed and there was only a small number of soft shots that I couldn't blame on poor technique, etc. However given what I think I know, I wouldn't purchase a 100-400 Mk1 unless of course, the price, condition, etc. was way too good to pass up even given the limitations of that lens.
Again, I am extrapolating data from the 90D to R7 and there may be reasons that shouldn't be done. But if you in someway find the above helpful and/or my assumptions are not totally off-base, then perhaps it will be useful to you in making your decision.
Regards,
LZ
