Quote Originally Posted by tandeejay View Post
Yep, I think your right... time to do some standardized testing using a tripod and exposure delay and anything else to eliminate external factors, rather than comparing two random shots taken at different times of days with different conditions......
If you do this and want to test it at infinity focus and end up doing it outdoors, be mindful of any wind/breeze on the day too. ie. do it on a nice still day.
Even tho you use a tripod and exposure delay/mirror lockup etc .. it's all of no consequence of it's a slightly breezy day!

You can account for any tripod inadequacies if the camera has the ability to counter those inadequacies. And it's very rare for most of use to have access to a tripod and tripod head that is literally rock solidly stable!
From about 300 or so mm you can clearly see this .. and by about 500mm you can easily visualise an inadequate tripod/head setup.

If you're curious to see it for yourself: take said tripod setup outdoors on a slightly breezy day, even if the breeze is only mild.
If your camera has liveview and or video ability, use this. set lens to 300mm and I suggest use video mode. take a 30sec video of a fixed subject that you KNOW doesn't move.
What happens in a slightly breezy day is that you will see some vibrations in the video at random moments in the timeline.
So for some part of the video it can be rock steady, then (due to the breeze) it will have this shimmy effect, which is simply vibration.
The problem is the randomness of it. Even with mirror up/exposure delay to counter mirrorslap, the random nature of what a breeze does to image sharpness is the issue. You just have no idea that's it's causing the vibration.
And don't assume that just because you're shooting with a high enough shutter speed that you are safe too.
That's the purpose of testing all this .. to know the limitations of your gear.