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I have an object of size 37.5um x 37.5um, and captured a 640 x 480 resolution image of the object with a CMOS sensor. I see that the object is around 200 x 200 pixels in the image. How do I calculate the magnification factor from the original object to the camera image?

The original object:

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The image from the camera:

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    I have a Zen question for you: How big is a pixel? – mattdm Jul 3 '13 at 11:44
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You'll need to know the physical size of the camera's sensor. An object of length 37.5 µm appearing on the physical sensor as an image of length x µm implies a magnification of (x / 37.5).

In order to determine the length of the image on the sensor, look at the number of pixels it takes up, and compare that to the total pixel count along that edge of the sensor. 200 pixels length out of 480 pixels is 0.42 of the height of the sensor; if the sensor is y high, then the image has a height of 0.42y. This will give you x to be used in the calculation above.

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