Pixels per foot / Pixels per meter is the most fundamental and valuable, though imperfect, metric for specifying video surveillance image quality.
In a single number, this metric (e.g., 10ppf, 40ppf, 100ppf) conveys important information about what the projected quality that a camera can provide.
The image below, taken from our Design Calculator, demonstrates examples of common pixel per foot (ppf) levels:
PPF / PPM Established Metric
PPF has become a critical established metric for several reasons:
Broad camera manufacturer support: Most major manufacturers use this metric.
Common A&E specifications: Architects and engineers who plan large projects regularly use PPF / PPM as the basis for their designs and surveillance plans.
Need for Something: With so many resolution options today (from 1MP to 12MP and beyond), the old metrics which used percentage of screen covered make no sense. PPF has filled this void.
The Goal of PPF
PPF is a single metric (e.g., 10, 50, 90, etc.) that when specified should deliver a specific level of quality. For example: "the parking lot camera must deliver 75 PPF." Instead of guessing or just specifying more resolution, using this metric should enable the selection of the ‘right’ resolution for the scene. The final image, following the PPF metric, will then deliver a more predictable level of quality.
PPF Problems
However, PPF suffers from many problems that must be factored in:
Assumes even lighting and ignores the impact of bright sunlight
Assumes day time lighting and ignores the impact of night time / low light viewing
Disregards differences in lenses and compression
Disregards that image quality needs are subjective and debatable
Fails to specify related and critical metrics to complement PPF
Despite this, PPF does have value for estimation and planning. It just cannot be used blindly or simplistically. Inside the PRO Member’s section, we explain:
How to calculate PPF
How to recognize PPF limitations and make adjustments
However, in our tests, lens quality is rarely a major concern, despite fine quality differences which are perceptible in some lenses...
Has there been a specific test of lenses and their effect on apparent PPF? Some tests have indicated that in certain cases there might be substantial differences, A Major Flaw in Long Lenses and PTZs Found.
Plus with imager pixel density increasing, lenses are needed to have higher MTF.
Note, we use PPF throughout this report, but sometimes the metric equivalent, Pixels Per Meter (PPM), is used. However, PPF is commonly used, even in many regions standardized on the metric system.
This bias for anything non-metric forever bemuses me.
Setting aside your 'regional' comment for just a moment, there are only 3 countries in the world not using the metric system. And with no offense to Burma or Liberia, it's the USA that you are acquiescing to here. So I ask you (IPVM) to reflect on PPF versus PPM. As 'the world's leading video surveillance information source, IPVM provides the best reporting, testing and training for 10,000+ members globally' [my emphasis on the areas in bold], who are you serving? What is the more exact measurement tool?
You say, 'PPF is commonly used, even in many regions standardized on the metric system. I wonder what you base that comment on? More importantly, if that is true, could the reason be because PPF is what we are force fed?
The US National Institute of Standards and Technology (NIST) has a very interesting paper on the subject of metric systems in the USA at https://www.nist.gov/sites/default/files/documents/pml/wmd/metric/1136a.pdf
I work in metric but was schooled in imperial (UK), I have no bias, but, I feel in this realm that ppf is more granular than ppm.
Of course, we all constantly work in many different units depending on which tasks we are doing and what information we are accessing from where; Farenheit/centigrade/kelvin, lux/candles mph, mpg, litres/litres of milk, fl oz of cream, in the end it doesn't really matter, does it?
The reason that 5MP has a lower horizontal pixel count than a 4MP camera is the difference in aspect ratios, i.e., 5MP cameras are 'taller' than 4MP ones:
I just read the PPF or PPM notes. To my understanding, the PPF value changes based on FoV. Thus, the PPF value does not represent a fixed specs of a camera.
Example: Using the same camera, by zooming to a FoV without shifting the camera, we infact change the FoV value with the resolution remain as a constant. The PPF will be different with a different zoom point.
Yes, your understanding is correct. The example from the report shows this:
PPF also decreases with distance. The angle of view may remain the same, but the further a subject is from the camera the greater the field of view width will be and the lower the PPF. See the example below:
That is odd, I just ran the same calculation in the Calculator and got the expected 128PPF. Your calculation is correct, I am not sure why the Calculator is saying 72PPF:
Try opening a new project in the Calculator and running the calculation again. If you still have the issue, email me at sean@ipvm.com
Thanks for the question! This is an interesting case, but the Calculator is actually correct here. I can match the values you have displayed when the camera in question is in corridor mode, e.g., when the camera's field of view has been rotated by 90 degrees. You can see the matching values (corridor mode selection highlighted in red):
Since the FoV is rotated in corridor mode, the horizontal resolution is now 720 pixels, instead of 1280. Hence the PPF calculation is 720 pixels / 10 feet = 72.
I've learned something new..."corridor mode". I tried the other option and set the same width to 10 ft, the PPF change to 128 for the 720p resolution which is correct. Thank you for the clarification.
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Comments (18)
Undisclosed #1
Has there been a specific test of lenses and their effect on apparent PPF? Some tests have indicated that in certain cases there might be substantial differences, A Major Flaw in Long Lenses and PTZs Found.
Plus with imager pixel density increasing, lenses are needed to have higher MTF.
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Gavin Botha
Great article.
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James Drymiller
This bias for anything non-metric forever bemuses me.
Setting aside your 'regional' comment for just a moment, there are only 3 countries in the world not using the metric system. And with no offense to Burma or Liberia, it's the USA that you are acquiescing to here. So I ask you (IPVM) to reflect on PPF versus PPM. As 'the world's leading video surveillance information source, IPVM provides the best reporting, testing and training for 10,000+ members globally' [my emphasis on the areas in bold], who are you serving? What is the more exact measurement tool?
You say, 'PPF is commonly used, even in many regions standardized on the metric system. I wonder what you base that comment on? More importantly, if that is true, could the reason be because PPF is what we are force fed?
The US National Institute of Standards and Technology (NIST) has a very interesting paper on the subject of metric systems in the USA at https://www.nist.gov/sites/default/files/documents/pml/wmd/metric/1136a.pdf
Thanks for your consideration and Happy New Year.
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Eddie Moss
Can anyone explain why the Horizontal Pixel count of 2592, for 5MP resolution, is less than the figure of 2688, for 4MP resolution?
This question is in reference to the table on page for of this tutorial.
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Undisclosed End User #2
Give a better idea of ppm.
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Undisclosed #3
Clear explanation of changes in HFoV and change of PPF according to difference parameter. Great information.
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Muon Cheun Cheng
I just read the PPF or PPM notes. To my understanding, the PPF value changes based on FoV. Thus, the PPF value does not represent a fixed specs of a camera.
Example: Using the same camera, by zooming to a FoV without shifting the camera, we infact change the FoV value with the resolution remain as a constant. The PPF will be different with a different zoom point.
Is my theory correct?
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Muon Cheun Cheng
I’m disturbed by the following comparison between my calculation and the camera calculator.
For a camera resolution of 720P:
Horizontal pixel: 1280
Vertical pixel : 720
PPF = Horizontal pixel / FoV width
With a FoV width = 10 ft
The PPF = 1280 / 10
PPF = 128 pixels
The camera calculator shows
My calculation shows 128 PPF for a camera resolution of 720p.
The camera calculator show 72 PPF for the same resolution.
What went wrong with my calculation as compare with the camera calculator?
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