I have the Canon 500D lens on my Canon 28-300L and I highly recommend it.
Yes, it is a lens and not a diopter.
Bryan Peterson uses it in his videos, on his Nikon lens, gasp!
https://www.adorama.com/ca52cu500d.html?origterm=canon+500d+lens&searchredirect=true
I also use Xume filter adaptors that facilitate fast and easy on-off for times.
Please do yourself a favor and look at the short 1-min. 45-sec video in the second link below.
My Xume (pronounced “Zoom”) filter adapters take away my fears of cross-threading, they make the filters easy to remove, replace or change, and to easily clean your front element. When you use Xume Filter Adapters you may throw away those pesky "filter wrenches".
Watch the 1:45 video at - https://www.youtube.com/watch?v=nUPvEgFItt4 Purchasing info at https://www.adorama.com/searchsite/default.aspx?searchinfo=xume+77mm+3.03"+lens+adapter
No, I don’t sell this stuff, but I do use these adapters and they do make life a LOT EASIER!
Smile,
JimmyT Sends

Hey, all, I am a newbie on this forum -- been enjoying the digest for a while. My question is whether I am the only one who has persistent focus issues with long 'big glass' (in my case 500mm f4, Nikon, but I have a friend who complained of the same kind of issue with his Canon 500mm f4). The problem is that if the subject (usually an animal) is around 1/10th of the frame, the single-point autofocus yields a frustrating fraction of images focused on the background behind the animal. Often this renders the image useless. This is especially the case when the background is more contrasty or complex than the subject (e.g., mammals against grass, ducks against water). It happens no matter how careful I try to be with the placement of the focus point (avoiding, if possible, having ANY part of the indicated square fall outside the main subject). It is usually NOT a problem if the background is less complex than the subject (e.g., flying birds against the sky). The problem is not a fine-adjustment issue, as the focus is perfect when the subject is considerably larger (like 1/4 of the image). I have tried to use the 'face detect' feature (although the Nikon version does not serve particularly well for animals) to encourage focus on the head, to no obvious benefit. It has occurred on at least five different models of Nikon DSLR (D3s, D4, D5, D500, D850). Can any of you explain what aspect of the autofocus system causes this behavior? Is there anything to be done about it other than checking the shot and retaking it if possible? Many thanks for your input!