Friday, July 20, 2012

Are you sure you need all those megapixels? (Maybe you can get by with a little help from your lens)



When it comes to selecting an IP camera the first question out of some people’s mouths is how many megapixels does it have?  
But before asking this question, you should ask yourself how many megapixels do I need? 
And before asking that the question make sure you know what degree of resolution you need?  

Do you want to see the flow of foot traffic in your department store?


 or do you want to see the facial features of the woman pocketing that watch?


Resolution
Resolution is defined as the number of picture elements comprising an image. When dealing with IP cameras, the pixel is the unit of measure. Resolution may be expressed either horizontally x vertically (640x840), or as a total number (1.4 megapixels). The finer the detail you want to see, the greater the resolution (more pixels) you should seek. However it’s not quite that simple. Megapixels do equal greater resolution but there are a few more considerations, some plus, some minus.
(+)     Megapixels add greater resolution to the equation and you get a higher quality image.
(-)      But now you need more bandwidth and have higher storage requirements.
(+)     Compression and frame rate adjustments can solve this.
(-)      But this could lower the quality of your image.
Bearing all this in mind, the most practical thing to do is decide how many megapixels you need without going overboard?

PPF
 Camera developers have come up with a minimum of 40 pixels per foot (ppf) as the standard for facial recognition.PPF refers to the resolution of the final video frame and is based on the size of the area being recorded. However Recognition is a broad term, implying that this is a person is already known to the viewer. In most cases recognition is based not only on facial characteristics but also on familiarity with the subject’s body build and perhaps choice of clothing. If the person is looking away from the camera or wearing sunglasses, or a hat is obstructing their face, you may sort of, kind of think you recognize then but you can’t be sure. The ideal resolution would allow you to identify them. Notice I made a distinction between recognizing and identifying. For identification the standard has been raised to 80 ppf (across the person’s face.  So if you are watching a section of the store that is 20 foot wide, say the jewelry counter,  you need a camera that gives a horizontal resolution of at least 1600 pixels wide (20 feet x 80 pixels). 

                          40ppf                                                                                               80ppf


                  
                                                       
           







                                                                                                                                                  
                
                                                        On Second Thought
                                                                                                        
Suppose you want to cover the 20 foot jewelry counter plus 10 feet on either side, your total area of surveillance is now doubled to 40 feet. Picture a triangle here, by moving back you have now widened the base of the triangle but the angle remains the same. You have widened the field of view. And with a wider field of view comes the need for higher resolution to maintain that optimum facial identification.  Since you have doubled your area to 40 feet, using that original camera with a megapixel resolution would now cut your final resolution in half to 40ppf adequate for facial recognition but not identification.  So if you want to keep that requisite 80 ppf, you could consider purchasing a camera with a higher resolution (more pixels per foot, in this case, 40 feet x 80 pixels) of at least 3200. But now you have those storage and bandwidth excesses we spoke of. But there is another way. 

Consider the Lens
If you remember from previous blog entries, the lens plays a major part in the quality of the image and the field of view. It determines how far away you can see something and it also defines how wide an area you can cover.
  If we are monitoring an area closer to the camera we need a wider angle lens with low magnification thus a smaller focal length (less mm). But if we are monitoring something farther away, our angle of view becomes narrower and we need a higher vocal length (more MM) which will give us greater magnification. With a fixed angle lens this is an either/or proposition but in the expanded area situation described above we need both. There is a way, however, to widen the angle of view, maintain focus and still use the original camera.
You could use a variable lens. Variable lenses give you a choice of fields of view. If the field of view is changed, focus can be maintained within a range of focal lengths. Variable lenses are labeled according to zoom capability. For example a lens that states having an 8x zoom refers to the ratio between its longest and shortest focal length, and might give you a range of 6mm to 48 mm.


This would be an efficient move. You still have facial recognition but since you have not increased the resolution, you haven’t increased the bandwidth. Furthermore if in the future, a decision is made to once again change the viewing area, this lens can adapt, giving you added flexibility.


 Carpenters used to have an old saying when it came to making sure they weren’t wasting wood. Measure twice, cut once. The same could be applied to choosing the right camera. Consider your measurements.  Granted, choosing a camera without adequate resolution won’t give you the surveillance you need, but at the other end of the scale, purchasing a camera on the sole basis of megapixels can very well give you more resolution than you need.  This means you’re not only wasting budget dollars but eating up bandwidth without reaping any additional benefits.
If you need advice in choosing a camera or putting together a system that will give you the most for your investment, call Kintronics at 800-431-1658 or go to our website and fill out a request form.

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