Blur control

Hi there!

Can anyone help me?

I want to make an old pannel with a screen, but you have to adjust your “focus”. This has to be done by a knob or slider almost like you would a pair of binoculars. I want to use the blur filter for this effect.

Any help would be appreciated, ive searched as good as i could on google but couldnt find anything that covers that…

Thanks in advance!