Motion Blur - When Is It Acceptable?
The below discussion
We've been discussing low light performance in class lately and different ways to obtain a usable image in low light conditions. The cameras gain can be increased but the resulting noise may result in a low quality image. Decreasing shutter speed can be extremely effective in obtaining a low noise image but it comes at the cost of motion blur. Below are two images taken just a few minutes apart. The first was taken with a relatively quick shutter speed, the second with an extremely long 30 second shutter speed. You'll notice that the second image is very sharp considering how long the shutter is open. This is because there is no motion in the scene (except for cars and fireflys). [comments continues after pictures]
(click images for full size versions)
'Quick' / 'normal' shutter speed:
30 second shutter speed:
I wanted to share the second picture to emphasize that with a long shutter speed you will still have a sharp image everywhere in the picture EXCEPT where there is motion.
Is there an application for low noise/sharp image but blurred motion in surveillance?
I say yes in some instances. In the above example, if a person were trespassing I would not be able to see that person in the first image. In the second image I would only be able to see a fuzzy object (maybe a person, maybe an animal) but I would know something is there and i need to investigate. If i were using motion detection video analytics using camera shutter speed in the first picture, nothing would trigger an alarm unless the moving object had its own light source. Using the shutter speed of the second picture, the analytics would alarm on the motion even though the object moving would be just a blur. The analytics don't care how 'sharp' the moving object is, it just care if the pixels change or not from frame to frame.
Alternatively, using a high gain level in lieu of long shutter speed may create so much noise the the analytics software would not be able to analyze the image or would give continuous false alarms. (John, let me know if i'm wrong about this)
Does anyone else sees an application for a "sharp everywhere except the motion" image in surveillance? I image it would be limited to intrusion detection and appear/disappear and i'm curious if anyone has successfully implemented using this approach. (Surely there are better options if you have the budget, IR or thermal imaging would do the trick, but perhaps an owner has a limited budget or it's an existing system they are trying to get the most out of.)
Doug, great question. Nice images. I've moved it from the class to the main discussion so everyone can participate.
There may be a slow shutter / motion blur that makes sense but it's not 30 seconds :)
A 30 second shutter, means a frame rate of .033 fps which is issue 1.
Within 30 seconds, an average adult can run 400 feet no problem. As such you are going to wind up with a long streak that no analytics will detect on because the phenomenon does not look like a human.
We've talked to analytic manufacturers about this, and they strongly recommend against slow shutters because it's hard to reliably detect blurs.
That said, if you just want to document activity or see if something is still there (e.g., is the building still standing), a 1 or 2 second slow shutter might work.
I am curious who is using slow shutter (by design, not accidently :) and what examples they might offer?
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