There is a large difference between optical and digital zoom. Although both methods allow for zooming in on an image, an optical zoom retains quality as it zooms whereas a digital zoom degrades the more you zoom in. It does that because of the way they actually zoom.
In an optical zoom security camera, often called a vari-focal (variable focus) camera, there is a physical lens that is adjustable. Think of the way binoculars work. This adjustment to the lens will zoom in or out which will produce either a wider or more narrow shot just depending on your desired viewing angle. Since the camera is using optics to adjust the field of view the quality of the image does not degrade. This means that you would be able to have cameras very far away from your target and not lose quality. This is assuming your cameras have a clear line of sight, of course.
On the other hand, a digital zoom takes the image as it's produced and stretches the pixels to get an artificially zoomed image. This is incredibly important to understand because depending on what resolution your camera is producing will dictate exactly how much you'll be able to stretch that image before it starts to pixelate, or blur. If you were using a 4K Ultra High Definition Camera that would be able to digitally zoom in a lot more than say, a legacy analog system would be able to before the picture became unusable.
With that being said you would never want to rely on a digital zoom to take care of a high risk area. If you have an area that you need to monitor closely, then you're going to want to either mount the camera closer to that area or install a camera that has a vari-focal lens and optically zoom in for coverage. When you try to stretch a camera to do both general coverage with a wide angle and then cover specific areas using a digital zoom, you end up with a camera that can't really do either job very well and you will lose detail when it matters most.
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