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Scruit - 26 Nov 2007, 12:50 pm
I have a BW camera (520TVL Sony) that has a 60mm lens and 850nm IR Pass filter attached to it.
When I try to focus on an object in the middle of the picture I can get a good clear image, but there is a 20% wide strip across the top and sides (not the bottom) that is blurry.
The camera is mounted abut 10' above the ground and reads license plates from passing cars (no OCR or anything). When a car first comes into shot the license plate appears in the top right and travels to the bottom left. It's not until the plate is in the middle of the screen that it's clear enough to read. I believe the focus issue is because objects at the top and sides of the screen are 10-20' behind the focal point.
Is this a product of the zoom lens? ie Is it normal for an object that is in focus at 80' to be out of focus at 100'?
I have removed and cleaned the lens and IRPass filter and they are perfectly clean. The ccd sensor appears to be clean but I haven't tried to clean it for fear of causing damage.
The biggest issue that I have is that the during a motion event the bulk of the framerate of the DVR is allocated to channels with motion and and the rest drop to 1fps. This means that a car leaving my house will trigger the motion sensing on the front cameras and the license plate camera will drop to 1fps. When the car passes under the license plate camera it's a 50/50 chance that the 1fps just happens to catch the plate when it's in focus. Once the car reaches 100' from the house the plate is no longer in focus (even though it's still large enough to read if it was). If the whoel image was in focus then the lower framerate wouldn't be so bad becuase I'd still capture at least one good image.
There now follows lots of nonsensical rambling. Read at your own risk:
As a temporary measure I'm going to reinstall my old 4ch DVR that does 30 frames per second and use the loop output from the primary DVR to feed the license plate images to the secondary DVR. The secondary DVR will always record at 30fps so it will always get a shot of the plate while it's in the range of focus. heck, I can feed the 3 door cameras to the secondary DVR too - they'll all record at 7.5fps but that's still good enough and will provide a backup DVR for the most important cameras (the ones that provide evidence that can lead to a suspect, rather than just fuzzy pictures on an untraceable bad guy ripping me off) in cae of failure of the primary DVR (has happened once this year due to failed HD - system was down 3 days before I realized it)
The licence plate camera is useful for about 10'. At 1 fps a car has to be travelling under 10 feet per second to be assured of a good image (assuming the plate is otherwise readable). That's about 6.5mph. A car leaving at 13mph will be missed 50% of the time. However with the DVR running at 7.5 frames per second the bad guy would have to travel at 75 feet per second to clear that 10 feet between frames. That's over 50mph, and he only had about 60' of acceleration space, on gravel.
When I try to focus on an object in the middle of the picture I can get a good clear image, but there is a 20% wide strip across the top and sides (not the bottom) that is blurry.
The camera is mounted abut 10' above the ground and reads license plates from passing cars (no OCR or anything). When a car first comes into shot the license plate appears in the top right and travels to the bottom left. It's not until the plate is in the middle of the screen that it's clear enough to read. I believe the focus issue is because objects at the top and sides of the screen are 10-20' behind the focal point.
Is this a product of the zoom lens? ie Is it normal for an object that is in focus at 80' to be out of focus at 100'?
I have removed and cleaned the lens and IRPass filter and they are perfectly clean. The ccd sensor appears to be clean but I haven't tried to clean it for fear of causing damage.
The biggest issue that I have is that the during a motion event the bulk of the framerate of the DVR is allocated to channels with motion and and the rest drop to 1fps. This means that a car leaving my house will trigger the motion sensing on the front cameras and the license plate camera will drop to 1fps. When the car passes under the license plate camera it's a 50/50 chance that the 1fps just happens to catch the plate when it's in focus. Once the car reaches 100' from the house the plate is no longer in focus (even though it's still large enough to read if it was). If the whoel image was in focus then the lower framerate wouldn't be so bad becuase I'd still capture at least one good image.
There now follows lots of nonsensical rambling. Read at your own risk:
As a temporary measure I'm going to reinstall my old 4ch DVR that does 30 frames per second and use the loop output from the primary DVR to feed the license plate images to the secondary DVR. The secondary DVR will always record at 30fps so it will always get a shot of the plate while it's in the range of focus. heck, I can feed the 3 door cameras to the secondary DVR too - they'll all record at 7.5fps but that's still good enough and will provide a backup DVR for the most important cameras (the ones that provide evidence that can lead to a suspect, rather than just fuzzy pictures on an untraceable bad guy ripping me off) in cae of failure of the primary DVR (has happened once this year due to failed HD - system was down 3 days before I realized it)
The licence plate camera is useful for about 10'. At 1 fps a car has to be travelling under 10 feet per second to be assured of a good image (assuming the plate is otherwise readable). That's about 6.5mph. A car leaving at 13mph will be missed 50% of the time. However with the DVR running at 7.5 frames per second the bad guy would have to travel at 75 feet per second to clear that 10 feet between frames. That's over 50mph, and he only had about 60' of acceleration space, on gravel.