Originally posted by Kiwizinho but I've figured out what the issue is with the Tok 28-200. It's a 3.5-5.3 lens, but it's not transmitting changes in zoom to the body, so wide open at 200, the body is reading 3.5 when in fact it's 5.3 so to get a true 11, I guess I'll maybe need 8 showing in the viewfinder?
OK had a look in the reviews and it looks like your lens is like an "A" series. And you are using it on "A".
Take it off "A" and set it to the aperture you want on the ring. When set like this the lens still stays open while you are focusing and stops down when you take a shot. Take a selfie and see if it is working correctly.
The other advantage of using it at this setting is that Live view is more effective - it autobrightens so that your target is always bright. It won't do this set to "A"
This should help your focusing.
If you set your camera to Manual and set your own same exposure for all the lenses then any significant variations in exposure (apart from the light changing) will indicate faulty aperture.
And above all don't overexpose - that may be mostly what is wrong with the F 70 - 210.
And by the way my attempts are by no means "scientific". What I am trying to achieve is a practical in the field comparison. The trick to that is to reduce the variables. That is why be in manual mode and aperture on the ring. But hard to get around the lighting variation unless I go inside.
The reason for using the Tak lens caps is to try and give an international standardised target but I suspect not as many people as I thought have one!