Two different kinds of previews in Image Acquisition Tool

I used to think frames captured from camera should be scaled up or scaled down when I choose different resolutions to preview. But the preview results of Image Acquisition Tool in MATLAB don't prove my notion. In Image Acquisition Tool, I connect my usb camera and preview the video. Here are all the resolution options of my camera.

When resolution = 352x288 (default), the preview is normal: the len's contour is in the proper (middle) position.

But when I change the resolution to a more common size, like 320x240 or 640x480, the preview shows that the len seems to be biased. However, I don't want this kind of change and I hope the len is in the middle as well as the one is the default resolution.

I am not sure whether my problem is caused by the video capture tool in MATLAB (software) or my camera specification (hardware). Do anyone know whether my issue is a normal one?

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My work in on Windows 7 64-bit and Matlab R2012b. Tell me anything you want to know about my camera if it matters.

Answer 1

[Walter Roberson]

320 x 240 and 640 x 480 are both 4:3 aspect ratios. 352x288 is 11:9 which is close to 12:9 but not quite.

When aspect ratios are changed, the image can be be cropped to the right ratio, which loses part of the image but keeps proportions intact. Or the image can be stretched unequally, which does not lose part of the image, but loses proportions (so a circle would come out as an ellipse.)

Your camera provides only two resolutions with the 11:9 proportion: the 352 x 288, and the 176 x 144 (which is a 1:2 downsample). All higher resolutions provided by the camera are 4:3 and so will either be cropped or non-proportional.

The recommendation would be to back away slightly from the target and move the view slightly so that the higher resolution captures the entire target. You will not lose detail due to the longer distance if you increase the resolution sufficiently to match.

[Yudong]

I learn much from your answer. About your recommendation, sorry to tell you my camera must be touched the paper to read images. Thank you anyway!