校准最佳实践

do you need to take images at different distance for good range coverage as well ?

@Brian: It sounds like you are working with a camera that can do some light internal image processing (i.e. sharpening filters). These alter the raw image to make it appear sharper. Generally, we would recommend turning off all image processing during calibration. Depending on the exact implementation, these algorithms could bias the found locations of image saddle points or circles. However, the effects could very well be negligible.

First off I would like to thank your team for making such great calibration targets! My question deals with digital camera setting. I am using a fixed focal length camera and basically following the guidelines that you have set out in this document. Provided that you are getting a “good image”, do settings like sharpness, gamma, white balance, etc, impact the camera characteristics of camera calibration? If so, do you have any recommendation for such settings?
I keep going back and forth on whether the sharpness setting impacts the finding of the intersections of a chess board.

@Min-An Chao, yes you are correct that the pinhole camera model is ideal in the sense that it is focus free. Only with very small apertures do standard cameras begin to exhibit pinhole like characteristics. Unfortunately, in most cases we are required to open the aperture in order to collect more light and as a result we deviate from the pinhole model. However, in these cases we can assume pinhole like characteristics where the image is in focus and we perform a calibration at that focusing distance (fixed focal length) in order to estimate the pinhole model in that condition. By re-focusing we are effectively changing the distance between lense and sensor which affects the focal length directly. The change in focal length is quite small, but for precision applications it can put a measurement setup out of spec. For ideal optics the principal point (cx,cy) will theoretically not change. But in the real world, it might.

To visually see the change in focal length by focusing. Put your camera in manual focusing mode, close the aperture as much as you can and use a lot of light to illuminate a scene. Now, manually adjust the focus dial and observe how the image zooms in and out.

Hello,
Thanks for great guidelines and informative Q&A section here. I‘d like to follow up @Tim‘s question, since the possibility of calibration for autofocus camera has bothered me several times. Because pinhole camera model does not have focus problem, so when applying this model to describe a camera matrix, fine-tuning the distance between image sensor and the lens set should not change the pinhole focal length f, and should not change the origin point (cx, cy) projected on the image plane. Then what would autofocus affect or harm the calibration results? Because personally I did the calibration for one camera where I can set manual focus digitally and precisely and repeat the calibration with precisely controlled motorized stages and could not find signifacant differences on camera matrix and distortion parameters with opencv scripts. Could you shed some lights here? Thanks!

Min-An