For playback, I like to download the clip that you are interested in, to a computer and then use the advanced features of VLC to enable interactive zoom.
VLC lets you do some handy enhancements to the area of interest.
I only use this method when I know there is some detail that requires further inspection, not for general reviewing.
I use the mouse scroll in Blue Iris which allows digital zoom. My cams are 3mp.
The quality goes down significantly when you zoom. I would be interested if the quality stayed say on a 6mp cam
