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I help manage a small usability lab. One of the things we're trying to is to record paper prototyping sessions - with a user sitting at a table and a camera directly overhead (from a height of about 1.5m) recording what they're doing. However, we've had a lot of difficulty finding cameras and AV equipment that can do this with enough resolution to be satisfactory. We typically use CCTV cameras in the lab (around AUD$100-300 each) - these are perfectly fine for most recording, but when we try to record paper on a table it's impossible to read what the user is writing or see what they're doing in fine detail.

We've tried a number of high-resolution CCTV cameras (e.g. this 1080p camera) and different lenses (e.g. this 3.5mm-8.0mm lens and this 6.0mm-15mm lens), but the best we can achieve is something like this, where even reasonably large text written in black marker pen is barely legible. We've connected the camera directly to a screen and a projector and have confirmed it's the camera that's the issue, not any of the AV gear we use like mixers etc.

Even an iPhone camera from the same height seems to be able to do this job adequately, but we can't find a dedicated camera to match the quality. (And we can't use an iPhone camera for this - aside from the impracticality, we also need to be able to push the video output through the lab's AV system for mixing, recording, archiving etc).

Is there any good type of camera with a BNC connector that anyone can recommend for paper prototyping?

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Your linked image is 720x576px only. Is this one from the 1080p camera? I don't think so. –  FrankL Jan 22 '13 at 8:33
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3 Answers

up vote 4 down vote accepted

It's a little math to get your satisfying resolution, but it is not too hard.

So, first of all we have to look, what we want to capture or how good the resolution should be in or how the pixel/cm ratio is.


The bill is 8cm in width and one character has a size of 1mm width.

If you would scan the bill with 150 dpi, it's quite readable. 8cm x 59,06 (150dpi = 59,06dots per cm) = 472,44dots aka px

That means for reading the bill, it has to fill at least more than the half of your camera viewport, because your camera has 720px resolution in width.


If you say, bill doesn't matter but the notes, I would assume your paper is triple the width than the bill. And I assume one character is 1cm broad. 9px for it, like in your image, is too small. So, let us say 20 px is fine. That means we need a resolution of 20px per cm.

24cm x 20px = 480px

For reading the notes, it has to fill at least more than the half of your camera viewport, because your camera has 720px resolution in width. If you have a 1080 camera half of your viewport is fine. (Right now in your image, it is around a third of the viewport.)


Okay lets say you have a 1080 cam (it's easier to calculate and you will actually need it). Then we need half of the viewport fo the notes, which is 24cm width. I guess your prototypes are around this size or a little bigger. That means we have a total viewport of 48cm width.

Next we look at this graphic, which is from Unibrain and see - we know our distance D=1,5m and our desired width W=48cm. Then we take a look at the pdf and choose our needed focal lenght. Be aware, the 5th column is for 0,5m distance so just triple it. And you need to add 5mm to the focal length, because you need CS-mount and the table is for C-mount. An CS/C adapter adds 5mm extra.

enter image description here

Answer: The choice of camera is only half the story. Choosing the right focal length makes it complete.

You will need a focal lenght of 17mm or more using a 1080p. Or you reduce the distance to your target. Reading the bill won't work! Hope it helps.

Other notes: Your image has a lot of noise. Especially in white areas and with high contrasts - the black letters on white paper. Give it more light, it seems to run in low light conditions. And if possible switch to BW mode, this increases the quality a lot (because of a bayer filter in front of the CCD chip)

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Thanks so much for the information and explanation, Frank. That was very helpful. We ended up trying a number of different lenses (with varying focal lengths) and it did make a big difference - although as you noted, there are other factors (lighting etc) too that made it difficult. We've decided CCTV cameras probably aren't the right solution so are looking at alternatives. Thanks again! –  John Feb 20 '13 at 0:30
    
@John The SLR approach from JohnGB looks promising as well. Because you have a lot of pixel available, but no live preview. May be there are unofficial firmware hacks out there for your SLR, which give you sometimes this feature. –  FrankL Feb 20 '13 at 10:05
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I've had to do something similar in the past, and used an SLR and took a time lapse. I used a Nikon D7000 in this case, which has a nice time lapse setting. I found that taking a series of high resolution still images was a lot more useful than having a video most of the time. The resolution means that even tiny text is easily readable when you zoom in.

You can also end up with a video if you play the still images, but it is a nice sped up video which is nice to watch a rapid overview of the process. It's fun to watch a high speed video of the process.

If you need to have the second by second detail with audio, I would still use a good SLR but use the video setting, which on the D7000 at least is incredibly sharp. Much more so than on an equivalent resolution video camera.

The downside of a SLR is that it has a bigger sensor and therefore a lower DoF (Depth of Field), which may not be ideal. I have used a Canon S100 (which is much cheaper compact camera) for similar work and the video was almost as sharp as the D7000, but with a much higher depth of field. You just had to make sure that you had good lighting with the S100. The D7000 work well in even very poor lighting.


This is not a solution with a BNC connector, but I don't see any solid reason why a BNC connector is needed, and making it specifically for BNC connectors makes the answer less relevant for most other people.

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In video mode Nikon has Full-HD (1.920 x 1.080), but it's quite pricey around usd 900. –  FrankL Jan 22 '13 at 10:38
    
@FrankL it's not the cheapest option, but I use the camera for many more things. I've also done some work with a Canon S100 for something similar. I'll add that info to my answer. –  JohnGB Jan 22 '13 at 10:44
    
D7000 is great. If you have one, you are lucky ;) –  FrankL Jan 22 '13 at 10:53
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Khan Academy's Vi Hart uses a Canon VIXIA HF M40 to record all the writings that she does. There's an explanation behind how she creates her videos here: http://www.youtube.com/watch?v=z4x5FZyzLyQ. (At 4:00 in the video is where the camera model is mentioned).

As commented, she is using a tripod with a closer range, but perhaps this might lower the expenditure you need.

enter image description here

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In the beginning you can see, they use a tripod in front of the paper. But there camera is hanging on the ceiling in 1,5m height. –  FrankL Jan 22 '13 at 8:36
    
Perhaps they should switch to using the tripod instead of having a camera so far away. –  icc97 Jan 22 '13 at 8:44
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