Infinity focus blurry for fireworks video

<p>I have a Sony A7S camera and using a 28-70mm lens. I have been practicing filming fireworks looking for sharp HD type footage. <br>

When I tried with the following settings and focus on Infinity the results were slightly out of focus. What can I do to get the crisp HD type video?<br>

Settings:<br>

MF set to infinity<br>

ISO 200<br>

F8<br>

Shutter Speed - 1/80<br>

Thanks for any help you are able to provide.</p>

<p>I have a Sony fdr ax 100 - I've had pretty good success letting the autofocus handle it. If you find the auto focus doesn't hold the focus, you might let it focus then switch to manual while it is in focus. Or possibly make a manual setting then leave it. Good luck!<br>

<br /> <a href="https://youtu.be/1pfdaPZYv-Y">2014 Fireworks</a></p>