From what I can tell about the zulus hd 5-20x, there is NO mention of optical zoom anywhere. It appears to be a 1080p camera that is zero optical zoom and 5-20 digital zoom. That's it.
With cameras (still or video), normally they would say something like "20x (12x optical)" for example. This would mean that once you have zoomed in to 12x, you have maxed out the optical zoom. When you zoom beyond 12x, it is a digital zoom being applied to the already maxed out 12x optical zoom. So in this example 20x would be full 12x optical then digitally scaled to get the total of 20x. In digital cameras I used to totally dismiss the digital zoom part of any specifications. I assumed it would just be trying to make silk out of a sow's ear -- no real gain in resolution -- and usually that was true. But then I got a higher-end panasonic video camera that was 20x (12x optical). To this day I am still amazed that the digital zoom part of it is actually giving more resolution -- as if the camera sensors had more pixels than needed at 12x optical and then started providing that additional "info" once I exceeded 12x.
My point is, unfortunately, a mixed bag of "digital zoom sucks", but sometimes it doesn't. The camera sensors used can make a big difference. Some cameras brag about using sensors that have a ton of pixels -- more than enough, but the size of the sensor is so small that light from one pixel bleeds over to adjacent pixels, which kind of defeats the purpose of lots of extra pixels on the sensor. Larger sensors get more light and avoid the bleeding if they don't cram too many pixels onto it.
In the case of optical zoom in darkness, the more "optically" zoomed in you are, the less light you are letting in which means less light for the sensor to "sense". Yikes -- we don't want to starve the sensor(s), but we want zoom too. This makes it really difficult, as a consumer, to judge a night-vision rig by only optical vs digital zoom specs.
stovepipe
through the scope and you are on like 6 or 7x. Lol.
