镜头选择与立体角

一般而言相机设置例如相机之间的立体角和镜头选择并没有太多硬性规则因为校准过程会自动计算所有参数包括立体角和焦距。不过在搭建立体系统时仍需注意以下几点。1. 对于短焦距镜头应采用较大的立体角。这是因为如果使用短焦距镜头时立体角过小图像边缘会出现更多的噪声。关于镜头选择/立体角的经验法则如下对于焦距较短的镜头8mm、12mm应使用较大的立体角至少35度但越大越好对于中焦距镜头17mm应使用至少25度的立体角对于35mm及以上焦距的镜头立体角可降至15度对于超长焦镜头10度也可接受。通常情况下由于长焦镜头/高倍放大倍率固有的景深问题使用长焦镜头时实际上更倾向于采用较小的立体角。2. 如果因实验限制必须在短焦距镜头下使用较小的立体角请将样本置于图像中心。当立体角过小时图像边缘处的噪声会显著增加。在 Vic-Snap 中您可以点击“显示辅助线”来帮助将样本定位在图像中心。3. 使用过大的立体角可能会导致表面与传感器呈过大倾斜角度从而难以提取图案。这也会给景深带来挑战。当因镜头选择而不得不使用较大立体角时您需要权衡这些因素。如果图案与传感器呈很大倾斜角度您可能需要使用“初始猜测”功能来协助软件进行匹配。4. Correlated Solutions, Inc. 会对镜头和相机进行联合规格确认以确保它们兼容。我们还会在确定规格前对镜头进行测试以确保镜头质量适合 DIC 应用。劣质镜头可能会给系统引入噪声。 但是如果客户希望使用其他镜头建议先查阅该镜头可覆盖的传感器尺寸范围并与相机的传感器尺寸进行对比以确保镜头的成像圈能完全覆盖传感器且无暗角。5. 对于焦距较短的镜头您可能需要在“校准对话框”中选择更高的畸变阶数。更多信息请参阅校准问题故障排除6. 对于焦距较长的镜头或高倍放大应用仅当中心 x 和中心 y 值未能正确提取时才需要在“校准对话框”中选择“高倍放大”。更多信息请参阅高倍放大校准英文原文如下Lens Selection and Stereo AnglePosted by Elisha Byrne, Last modified by Micah SimonsenGenerally 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.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 angleFor lenses 35mm or longer, its 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. specs lenses and cameras together, making sure they are compatible. We also test the lenses before they are speced 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 see:Troubleshooting Calibration Problems6. 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 see:High Magnification Calibration