Generally speaking, there are not many rules to setting up cameras, such as the stereo angle between cameras and lens selection because the calibration calculates all the parameters, such as stereo angle and focal length. However there are a few things to keep in mind when setting up your stereo system.
1. For short lenses, use a large stereo angle. The reason for this is that we get more noise around the edges of the image if a short lens is used with too small of a stereo angle. This is explained in detail here (starting on slide 57):
Minimizing Noise and Bias in DIC
Rules of thumb for lens selection/stereo angle are here:
- For shorter focal length lenses (8mm, 12mm), you should use a large stereo angle (at least 35 degrees, but the higher the better)
- For mid-range focal length lenses (17mm), use at least a 25 degree stereo angle
- For lenses 35mm or longer, it's acceptable to go down to a 15 degree stereo angle (for very long lenses, 10 degrees is OK). Typically, a smaller stereo angle with longer lenses is actually preferred due to depth-of-field issues inherent to longer lenses/higher magnifications.
2. If you must use a small stereo angle with a short lens due to experimental constraints, keep the specimen in the center of the image. The noise is much higher along the edges of the image when the stereo angle is too small. In Vic-Snap, you can click the Toggle Lines on to help position the specimen in the center of the image.
3. Using a very large stereo angle can cause the surface to be so oblique to the sensor that the pattern is hard to extract. It might present challenges with the depth of field too. This is something you need to balance when having to use a large stereo angle due to lens selection. The the pattern is very oblique to the sensor, you might have to use the Initial Guess feature to help the software with the match.
4. Correlated Solutions, Inc. spec's lenses and cameras together, making sure they are compatible. We also test the lenses before they are spec'ed to make sure the lens quality is suitable for DIC. Poor quality lenses can introduce noise into the system. However, if the customer has other lenses they would like to use, it is suggested that the look up what sensor sizes it can cover and compare that to the sensor size of the camera in order to make sure the image circle of the lens properly covers the sensor with no vignetting.
5. For shorter lenses, you might need to select a higher Distortion Order in the Calibration Dialog. More here:
Troubleshooting Calibration Problems
6. For longer lenses/high magnification applications, you might need to select "High Magnification" in the Calibration Dialog if, and only if, the center x and center y values did not extract correctly. More here: http://correlatedsolutions.com/support/index.php?/Knowledgebase/Article/View/6/0/high-magnification-calibration
Nick Lovaas
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