Remember that to be useful, the recorded video must be usable. That sounds stupid, but if you can't see what you need to, when you view the recorded video, it is useless. I was a security consultant and can't remember how many times a client said "we have CCTV" but on viewing the recorded video, they couldn't tell the difference between me, and their own mother. Don't put a lot of faith on what you see when viewing live images, what counts is what you can see when viewing recorded video.
The combination of the camera lens and the imaging chip makes a tremendous difference, make sure that you know what you want to see before you purchase. Measure the distance from the camera lens to the target, and the width of the spot where your target is. The wider the field of view is, and the farther away from the camera it is, the better a camera/lens you will need. Know ahead of time whether you want to see clearly enough to identify who a person is, or merely know that someone is there. Do you want to know that a vehicle pulled into your drive, do you want to know what year/make/model car pulled into your driveway, or do you want to be able to read the license plate. Once you know this info, you can begin shopping.
Another real killer for video, and for cameras, is to have them positioned so that the rising, or setting, sun falls into the lens. First, that huge amount of light, called "backlighting" will wash out your image, and can make it unusable. Secondly, while the newer cameras are better at enduring the sun shining into them than the older ones were, the bright light will shorten the life of the image chip. I've seen cameras which "looked" at the same image for so long that the image was burnt into the chip, even with the power off, the image was still present. If your placement requires that the camera look into the sun, try to place it higher, above the ground, and tilt the camera downward to exclude the sun.
Storing video consumes a lot of disk space. How much space is controlled by the resolution of the video, the number of images stored and how long you want to keep the video. Realtime video is 30 images ( 60 frames ) per second, but that is huge. Dropping the image rate, saves space, but makes the video appear jumpy. If it is slow enough, you can entirely miss a fast moving object. If you plan to store your own video, buy something that can have additional drives added. If your combination of resolution and images/second doesn't allow you to store enough video, add a drive, or two. Also, if you add cameras, down the road, you'll be able to add storage space too. Remember, if something happens that you want to preserve the video of, you can copy it to a flash drive, or another disk drive. Some video recorders will let you mark video that you want to save, then they won't overwrite it, but generally, those aren't the models you can buy at Best Buy or Costco.
Lots of considerations, but I've rattled on too long, consult the web sites of the manufacturers of better cameras, ( AXIS, Pelco ) for tools to help you determine things like the lens size, focal length, image chip quality, and video retention, then shop for something with those parameters. Many of them also have storage drive size calculators too.
Good luck,
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