Calibrating for a high magnification setup can present a few challenges. This article will discuss techniques for getting the best calibration result at small fields of view.
What is 'high magnification'?
A high magnification test is one where the lens magnification is roughly in the range of 1-4x; for example, with a standard 2/3" sensor, this would be fields of view between about 8mm and about 30mm.
Challenges & techniques
At small fields of view, the depth of field can become very limited. It may be difficult or impossible to achieve enough depth to allow for good tilting of the calibration grid. This will result in calibrations which have very poorly estimated, unrealistic values for "Center (X)" and "Center (Y)". In extreme cases you may see a "Sync Error" warning caused by these poor estimates.
- Use small apertures (high F-numbers). This will maximize depth of field; note that, at very high F-numbers, the resolution of a lens will suffer. Apertures above F/8 or F/11 may result in blurry-appearing images. It may be necessary to balance the need for depth of field with the increased measurement noise due to this blur.
- If you cannot get enough tilt to give a good center estimate, you can check the "High mag" option in the calibration dialog. This forces the center values to be at the geometrical center of the sensor. This is not optimal, but may allow the calibration to converge when it otherwise wouldn't, and is better than having a very poor value.
Additionally, it may be difficult to select a calibration target which fits well in a very small field of view.
- For fields down to approximately 10-15mm, a printed paper grid can work well, provided the paper is coated smooth, and a very high quality laser printer is used. Using standard copy paper will result in very visible paper texture at these magnifications; likewise, using a lower end printer will result in poorly formed target circles. This will result in images which fail to extract, or high calibration scores.
- Correlated Solutions provides glass calibration grids which have very accurate targets for fields of view down to 5mm. These grids must be backlit with a diffuse, even white field. Be sure that the transparent dots are evenly shaded, and that the correct side of the grid faces the camera (see grid documentation for details.) If the dots show shadows or uneven lighting, you may see consistently slightly high calibration scores in the 0.2-0.5 range.