I found this page interesting: https://www.cambridgeincolour.com/tutorials/diffraction-photography.htm#calculator?utm_source=newsletter&utm_medium=email&utm_campaign=fup4

(Edit: I now see it was posted on p. 3. Missed it then.)

The thing bothering me up to this point is that the topic is about landscape photography, which generally means maximizing the DOF, and much of the discussion on diffraction does not address DOF. Enjoy playing with the calculator.

My conclusion: f22 for full frame and f16 for APS-C are OK. But yes, if you want to maximize sharpness at the focus then more like f16 and f11.

Both diffraction and AF micro-adjusting your autofocus lens seem to have the same issues among photographers. Many say neither is a problem, and other swear by paying attention to both. I use all expensive L Canon lenses with the Canon 5D Mark IV and Canon 1DX Mark II. While sharpness certainly isn't everything, it really does not make any sense to me to shoot images that aren't as sharp as they could have been had I paid more attention to my shooting procedure. In both cases - diffraction and AF microadjustment - if you just shoot at f/22 or don't microadjust your lens and camera, then most likely the results will be suitable for you and many will say look how sharp they are. Indeed, they do look fairly sharp and you assume that is the best your gear can do. But, unless you run a test and evaluate, you won't realize how good the image results really can be. Decades ago I tested diffraction by shooting a flat target at f/8 and f/22, and the difference in sharpness due to a loss of sharpness from diffraction is significant to me. Therefore, I try to avoid stopping down much, and never past f/16. I usually focus stack when I can as it is so easy to do and process. Three years ago when I tried a new Canon lens and the results weren't quite as crisp as I expected (they still looked fairly sharp), I decided to AF micro adjust it (first time I tried it) and discovered the lens needed a -9 adjustment to correct for a little backfocus. Since then, I have tested at least 20 lenses (mine and clients) and none have had best focus at the zero default setting, and I don't expect them to. These adjustments aren't huge, but they do give your sharper images. By using the sharper apertures around f/8 and using lenses with proper AF microadjustment, it truly is amazing at the sharp photos you get. If you never see how well your gear can really do, then you don't miss it. But if you don't want to give up quality, and sharpness matters to you, then don't neglect those sharpness factors. It truly is up to what you hope to desire. Diffraction is real and you can see it when properly compared, and lenses most likely benefit from AF microadjusting - all of mine did with two different cameras. Just saying.....

Some of the other posts have taken my comments, but I'll repeat:

1. Sell your FD lenses and by EF lenses, or keep for when you have a mirrorless body and a glass-less adapter. Don't bother with using them on the EOS body.

2. Download a PDF copy of the 5DII manual from the Canon site.

3. Download the latest copy of the DPP v4 software, again from Canon site. Shooting in RAW and processing the results, via DPP or other tools, will maximize the quality of the images from this body and to process it's older noise capabilities.

4. Download the latest version of the Canon EOS Utility and then download and install lens profiles for your EF lenses into both the Camera and your DPP software. Here's a link for the camera aspect steps: https://support-th.canon-asia.com/contents/TH/EN/8202118200.html

5. Although the 5DII is limited in AF points, if not doing so already, learn to move and use the individual AF points for off-center selective focus.

6. Assuming your 5DII will be your workhorse camera for the next few years, get yourself at least 3 x 32GB class-10 CF cards. Bodies are moving away from CF cards so you may find them a bit cheaper now than in the somewhat recent past. You also might find them a bit harder to find, particularly if you wait.

7. Assuming your 5DII will be your workhorse camera for the next few years, assure you have two new, genuine Canon LP-E6 batteries. These were Canon standard for a few generations of Canon models, but againm things are moving on. Purchase them now before they get harder to find, as well as get a back-up charger.