Just a thought - Does the diopter adjustment on your camera ensure that you get a sharp image in the viewfinder correctly adjusted for your sight ?

To have diopter adjustment correct for your vision you need to set it and then check periodically that it has not accidently been moved from your setting.  The performance of your eyes will very throughout the day, and from day to day depending upon tiredness, if you spent a night on the tiles, etc. etc..... so for critical shots worth checking the adjustment at the beginning of a session.

 

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html