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:

faster shutter

30 second shutter speed:

30 second shutter

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.)

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