I've been working on a "introductory" post. I'm not done with it, but I figure it can still help you get an idea of why we pick the cameras that we pick and why we tell people to stay away from most of the popular consumer grade cameras sold today......
Let me talk about the difference between a good performing camera and a poor performing camera. A good performing camera is going to provide footage with enough detail and clarity to clearly identify an unknown subject. The litmus test that should be used is this:
Is the footage clear and detailed enough for the police to identify and ultimately arrest someone based on your footage alone? For most of us, a poor performing camera is any camera that cannot provide this level of footage at all times of the day and night. It's not that the poor performing camera will be completely burry and unrecognizable, but even a tiny amount of blurring will cause all the important distinguishing features of a subject to be lost and therefore useless to identify an unknow subject. For example, with a poor performing camera you will probably still see a subject’s face with eyes, a nose, and a mouth, but without all the small details needed to positively identify the subject. If all you can tell is that a subject is "a white male, 15-35 years old, wearing a hoodie" then that camera has failed at its job which it to provide clear enough footage for the police to arrest someone. That is the goal we are trying to achieve here.
There are three camera hardware specifications that play a critical role in determining if a camera is going to be a “good performer” or a “bad performer”. The first two go hand and hand together - the camera’s overall resolution (ie 2mp, 4mp, 5mp, 8mp) and it's sensor size (ie 1/2.8”, 1/1.8”, 1/1.2” ,etc). The third hardware specification is the focal length of the camera. This is usually listed in mm – such as 2.8mm, 3.4mm, 6mm for fixed cameras, or a range like 2.8-12mm or 6-18mm for a variable focal length cameras. The focal length is important because it has a direct relationship to the DORI concept we will discuss later.
First let’s talk about a camera’s resolution and sensor size and why it is important to choose these wisely. During the day, almost any camera can provide detailed enough images to be a "good performing camera." However, it's at night when you will start to see real differences between cameras....... If the camera is set for auto exposure, at night the camera will slow the shutter speed down and crank the gain up until it produces a very nice bright image. These images look great and people get fooled into thinking there camara is a “good performer” based on this bright, clear, static image. The problem begins however as soon as they is something moving in the frame. Then the slow shutter speeds and high gain will result in blurry and ghosting subject (ghosting is where parts, or even all of the moving subject will disappear in some frames of the recorded footage). To prevent this from happening, we want to turn a camera to full manual exposure and set the shutter speed fast enough to prevent blur and the gain low enough to prevent ghosting. However this requires that a camera has great low light performance to ensure that the overall image after these settings are made is still bright enough to be useful.
The camera’s overall resolution plays a role in the low/no light performance of a camera. It comes down to physics. If you have a 8mp camera (that’s generally 8,294,400 pixels) or even a 4mp camera (that’s 4,085,760 pixels) with an image sensor that is 1/2.8" (roughly 1/3") , experience has told us that the pixel size that these two specs create is going to be so small that the camera cannot give great low/no light images. Of course if you have your camera mounted in a bright area (perhaps there are some bright street lights nearby), then it may be able to produce great results. But the vast majority of people are going to have a tough time getting the gain low enough to prevent ghosting, the shutter speed high enough to prevent blurring, and still be able to get an image that is bright enough to be useful.
On the other hand, a camera with only 2mp that uses the same size sensor (1/2.8") will have pixels that are two to four times as large as the pixels in a 4mp or 8mp camera because there are only 2,073,600 pixels on the same size sensor vs 4,085,760 on a 4mp camera or 8,294,400 pixels on a 8mp camera. Because each pixel is larger on the lower resolution camera, it can absorb two or four times as much light in the same amount of time as the higher resolution cameras can. This results in much brighter images. A 2mp resolution is generally the highest resolution that we suggest on a 1/2.8" sensor size to ensure that we can set the gain low enough to prevent ghosting, set the shutter speed fast enough to prevent blurring, and still have a bright enough image to be useful. Any higher pixel count and the image will be too dark after setting appropriate shutter and gain settings.
With this in mind, we have a set of maximum resolutions per common sensor size that we like to recommend:
- For a 1/2.8” sensor size (roughly 1/3”) we recommend a resolution of no greater than 2mp
- For a 1/1.8” sensor size (roughly 1/2”) we recommend a resolution of no greater than 4mp
- For a 1/1.2” sensor size (roughly 1”) we recommend a resolution of no greater than 8mp
Keep in mind that the vast majority of popular consumer grade cameras use a sensor that is 1/2.8” yet have 8mp resolution. That’s the smallest sensor trying to pack on the highest number of pixels which results in an extremely small pixel size and therefore terrible low/no light performance. By the time you turn your shutter speed up enough to prevent blurring and your gain down enough to prevent ghosting, you are left with a completely black image. Those same settings on a 2mp camera would still provide an image that is bright enough to be useful because the individual pixel size is larger.
Second, let’s talk about DORI and how a cameras focal length can make or break a camera's performance.
DORI is an industry standard measured that helps users know how far away a person can be from the camera and still provide clear enough footage to “Detect”, “Observe”, “Recognize”, or “Identify” that person. Keep in mind that this calculation is a mathematical equation that measures the number of “pixels per foot”. It does not reflect the quality of the camera or the camera’s ability to provide good low/no light performance. It is a good starting place when trying to determine what camera to select for a particular situation.
Here is what Dori stands for:
- “Detect” – You can tell “something is out there” but that’s it.
- “Observe” – You can see a person and tell what they are doing, but you can’t tell who it is
- “Recognize” – There is enough clarity/detail that you can pick out people that you know from the footage, but it’s not clear enough to identify strangers
- “Identify” – The footage is clear/detailed enough to positively identify a stranger. The police could use your footage to identify and arrest a person based on your footage alone.
To keep the discussion as simple as possible, we are going to say that a camera’s DORI numbers are primarily affected by the camera’s resolution, and focal length. For example, the higher the resolution, the farther away an object can be from the camera and still be identified, and the narrower a camera’s focal length is, the father an object can be from the camera and still be identified as well.
Therefore it is important to choose the correct focal length camera for situation. Far too often people buy wide angle (2.8mm) cameras and stick them on their four corners of their house because they can “see everything.” The problem with that is with such a wide angle of view, the “Identify” range of the camera is likely only 0-15’. This means that even though the camera can “see your entire yard”, unless a subject comes to within 15’ of the camera, the footage will not have enough detail to properly identify a person. Having a wide angle camera like this isn’t wrong, but don’t expect it to provide useable “identifiable” coverage of a car parked 20’ from the camera. You might need to get a 6mm camera or even one with a narrower field of view to cover your driveway and mailbox at the street.
A good tool to use to help decide which cameras would work well in your situation is the
IPVM Calculator. With this tool, you can enter your actual address and see a satellite view of your property. You can then select actual models of cameras and overlay them on the view. The
tools will show you the camera field of view coverage and give you a visual representation of the DORI numbers via the shade of blue in the coverage area. Dark blue is "Identify" and then it gets lighter blue for each level. Please keep in mind that this calculator is not perfect, but it is a useful tool to decide which focal length to purchase.
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