How Far Can a PTZ See?By John Honovich, Published Sep 22, 2012, 12:00am EDT
Knowing how far a PTZ can see is a critical but tricky question for PTZ use. On the one hand, seeing very far away is a common reason for choosing PTZs. On the other hand, we regularly hear people radically overestimate how far one can see with a PTZs (hundreds of meters and miles are commonly claimed).
In this note, we break down four key issues in determining how far a PTZ can see:
- Why you have to be very specific what you want to see
- Why optical zoom ratios are very misleading
- Why PPF will mislead significantly with PTZs
- Why an on site test is so important
What You Want To See
Time and again, we see people skip right past this critical skip. You need to first be clear about what you want to 'see' - is it the outline of a car or a license plate, the presence of a person or the details of their face, etc. The difference here is massive. With the first of these options you might be able to see hundreds of meters or more, with the later, you might be lucky to get 50 meters.
To be clear, this is the same for determining a 'regular' fixed camera but the perception is that zooming in overcomes this. However, once the PTZ is zoomed in all the way, it becomes a 'regular' fixed camera with a very narrow FoV. Just like any other camera, at that point, the size of the object significantly impacts how clearly one can 'see' it.
Example: The Axis 'Text' Spin Job
Manufacturers routinely confuse users with misleading market campaigns. For instance, an Axis promotional video with over 50,000 views brags that an "Axis HDTV 1080p PTZ dome with 20x zoom lets you read texts 1.6 km (1 mile) away." This is amazing, right? Reading text a mile away. What kind of magic cameras are Axis now selling? However, the video shows text on the side of a nearly 300 foot long tanker [link no longer available], shown below:
Each letter is likely as tall as an adult human being - maybe 5 or 6 feet high or ~25,000% greater than the size of license plate characters. If you are part of the 0.1% of Axis customers who are using PTZs to read the names off of tankers, this is great. For everyone else, it's pointless at best.
Bottom line, until you are clear about what exactly you need to see, speculating how far you can see is a futile endeavor.
Why Optical Zoom Ratios Are Misleading
We have met many people who think that a 36x zoom camera can see 4 times as long as a 32x zoom camera because the former camera is 4x 'more' than the latter. This is completely untrue. In most cases, the 36x camera may only see very slightly more than the 32x zoom. Indeed, in some cases the lower zoom camera can see farther.
Instead the two key elements are:
- The minimum (most telephoto) lens angle
- The pixel resolution of the camera
The zoom expresses the range between widest and most telephoto FoV. If you want to see far, the range and the widest FoV are irrelevant. What you care for is the most telephoto field of view, best expressed in degrees. Below is an example from a manufacturer spec sheet:
In this example, the key number is the degrees (in the case, 1.7), the most telephoto angle the camera supports. Focus on degrees rather than millimeters as this controls for differences in camera's image sensor (note that this is a 35x camera (119 / 3.4 = 35)). The smaller the degree of the most telephoto horizontal FoV, the farther a PTZ will likely see (assuming the same resolution).
However, resolution is also key. In the past, when all PTZs were SD, the angle of view was key. Now, HD PTZs have far lower optical zoom ranges but increasingly equal or superior ability to see across far distances. This is because the increased resolution compensates for a somewhat wider maximum FoV. See our HD vs SD PTZs results for details.
Why PPF Mislead Especially So
Regular IPVM readers realize that pixel density or pixel per foot (ppf) as a metric of image quality suffers from many issues, such as lighting variance and low lighting. In perfect lighting, you might need 50 PPF to identify facial details but with shadows or direct sun, this may jump to 70 or 80ppf and at night may need 100 or 150ppf (or may not be able to see anything at all).
It gets worse with PTZs. Typically, PPF measurements assume a 'normal' fixed camera with a regular wide angle lens. However, when PTZs zoom far away, they need to use relatively long lenses (100mm+ are common) and view across long distances (hundreds of feet is close range). This creates many more issues - F stop is much higher meaning far less light is received; focusing becomes much harder, atmospheric issues are magnified, the image may shake, etc.
Here's an example from our test at ~220 feet distance with ~100ppf in the middle of the day. The image quality should be exceptional - like a passport photo (if ppf was valid) but it is fairly medoicre:
How Far You Should See?
Here are some very rough rules of thumbs:
- To make out facial details or license plates, expect no further than ~100 meters / ~300 feet from the camera
- To identify a person's outline, expect no further than ~500 meters / ~1500 feet from the camera
Of course, keep in mind that this presumes a high end PTZ (SD at ~36x zoom, HD at ~20x zoom) and even bright lighting. Any expectation of seeing things miles away must presume you are interested in only seeing huge things like ships, buildings, airplanes, etc. or are using a hugely expensive specialist camera with a meter or longer lens, costing $100,000+ USD.
Why An On Site Test is So Important
All of this said, if a prospect wants to know how far their PTZ can see, it is crucial to do a test on site. There are too many factors that can come into play and too much possibility for misunderstanding to just throw out a number (unless you seriously underestimate it). Get a portable test kit, go out and spend a few hours (at night too, if that's important to the prospect) and document what realistically can be done on that specific site.
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