A good calibration will have marking residuals all under about 1.0 pixel. If you have a few points over 1.0 residual but less than 2.0 pixels then one would generally not worry, especially when images are high resolution. If a large number of points (more than 10) have residuals over 1.5 then there is a more serious problem. Imprecision by the marker may be the result of undue sharpening by a digital camera (turn it to softest setting if possible), or color artifacts from the digital camera (try the camera’s monochrome/black-white mode if it has one).
Some digital cameras and some office flat bed scanners perform an automatic operation called "unsharp masking". This makes photographs seem sharper to us visually. Unfortunately this operation can greatly increase the noise and false edges in a scene like the calibration grid. It will be worse if the lighting is not strong and the contrast is low. The added image noise and false edges throw off the Camera Calibrator's automatic marking algorithm because it does not see smooth black to white transitions in the image. This can cause the auto- marked points to be poorly marked, which in turn can cause a poor calibration or a calibration that fails altogether.
The solutions are:
• Try turning down the sharpness setting on your digital camera or scanner.
• If your digital camera has a monochrome (black and white) mode try that as it may reduce artifacts.
• Pull the images into an imaging program like PhotoShop and do noise filtering and blurring. The images can look very blurry to a human and be quite acceptable to the Calibrator.