A Major Flaw in Long Lenses and PTZs FoundBy Ethan Ace, Published Aug 26, 2014, 12:00am EDT
Theoretically, long lenses should let you capture faces and license plates very far away.
For example, over 900 feet away from a 1080p camera one should get over 40ppf from a 100mm lens.
But how well does this work in practice?
In this test, we took 3 long lens cameras, including:
- An Axis 1080p PTZ
- A Dahua 720p PTZ
- A Fujinon 8-80mm varifocal lens [link no longer available] with a 720p Samsung SNB-5004 camera
We ran tests from 100 to 500 feet away from the cameras, comparing the theoretical PPF to the actual achieved.
All 3 cameras tested showed significant image quality problems at longer lens lengths, with degradation compared to theoretical max performance averaging 50%
This is a major issue when using camera calculation / estimation, one that we suspected but wanted to confirm with actual testing.
A few camera manufacturers privately confirmed that they were not surprised as the MTF, or resolving power, of long varifocal lenses have known, though not publicly acknowledged, problems. Lens manufacturers are reluctant, if not paranoid, about disclosing technical details. The poor MTF at longer lens lengths is most likely source of the image quality issues.
Handling Depth of Field Issues
Advanced surveillance professionals will recall that depth of field is a problem for longer lens, like the 80mm+ used in these tests.
To control for this, we manually focused each camera at each spot tested. For example, a focus was done at the 100' distance test, again at the 300' distance test, etc.
Though this is unrealistic for lenses added to fixed cameras, we did this to eliminate depth of field as being the cause of image quality issues.
Using the 8-80mm Fujinon lens at its longest focal length, visible quality was equivalent to images of nearly 50% its actual PPF, seen in the following examples.
Using the 8-80mm lens, despite actual PPF being 63, image quality is roughly equivalent to ~35 PPF.
At 500', the same is true, with effective image quality of only about 20 PPF, about half of actual (38 PPF).
At close range, ~100' from the camera, the same holds true, with effective PPF around 80, less than half of actual. However, effects are much less pronounced at pixel densities this high, making them less noticeable.
These findings held true for PTZ cameras, as well. Looking at examples from our integrated IR PTZ test.
In the example below, using a 720p PTZ with a 4.7-94mm lens, effective image quality at ~420' was similar to ~30 PPF, instead of actual (~55 PPF).
Image quality was also greatly reduced using the 1080p PTZ, effectively about ~36 PPF vs. the 87 which should be delivered.
Depth of Field
In addition to image quality reductions, users should be aware of depth of field issues when using long lenses. Focused at ~500', the field of view began to soften around 300', with severe blurring at 200' and below due to subjects being out of focus.
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