Webcam Focal Length

1. Jan 20, 2008

joeyar

Is there a formula for calculating the required focal length for a webcam given the width of the field of view and the distance away from the subject the camera is?

Also is there a relationship between the focal length of a webcam and the 'fish-eyedness' or distortion of the picture?

2. Jan 20, 2008

binzing

Most webcams say the specs right around the lens, so you shouldn't need to calculate anything if you just wanna know what is focal length is.

3. Jan 20, 2008

joeyar

No, I wanna know what focal length webcam I will need to fit an object of a given width from a given distance away into shot

4. Jul 31, 2009

jackqpublic

I think you're confused as to the technical meaning of 'focal length' in photography

5. Jul 31, 2009

mgb_phys

That's not quite enough, you would also need to know the size of the imaging chip.

6. Jul 31, 2009

junglebeast

Yes of course, because the triangle formed between the imaging plane (having a distance f) is similar to the triangle formed by the view frustum.

f = focal length
d = distance to object
W = desired width of viewable area at distance to object
w = width of imaging plane (CCD size)

(W/2) / d = (w/2) / f

So,

f = (w/2) / ((W/2) / d)
= w*d / W

Of course, you need to know w but this should be something you can look up in the camera specifications. Focal length is adjustable.

Yes, radial distortion is more pronounced with wider field of view. However radial distortion is an effect of low quality lenses so it doesn't follow a particular geometric relationship. Typically removing for radial distortion requires calibration against known images of straight lines and then minimizing some energy function to find the radial coefficients that cause the lines to appear straight, which can then be used to apply an inverse transformation to undo the effects in future images.

7. Jul 31, 2009