Question: I run simple test for OpenCV camera pose estimation. Having a photo and the same photo scaled up (zoomed in) I use them to detect features, calculate essential matrix and recover camera poses.

```
Mat inliers;
Mat E = findEssentialMat(queryPoints, trainPoints, cameraMatrix1, cameraMatrix2,
                         FM_RANSAC, 0.9, MAX_PIXEL_OFFSET, inliers);

size_t inliersCount =
    recoverPose(E, queryGoodPoints, trainGoodPoints, cameraMatrix1, cameraMatrix2, R, T, inliers);

```

So when I specify the original image as the first one, and the zoomed image as the second one, I get translation T close to [0; 0; -1]. However the second camera (zoomed) is virtually closer to the object than the first one. So if Z-axis goes from image plane into the scene, the second camera should have positive offset along Z-axis. For the result I get, Z-axis goes from the image plane towards camera, which among with other axes (X goes right, Y goes down) forms left-handed coordinate system. Is that true? Why this result differs from the coordinate system illustrated [here](https://stackoverflow.com/a/18022846/3635669)?

Answer: 
According to the OpenCV [document](http://docs.opencv.org/3.0-beta/modules/calib3d/doc/camera_calibration_and_3d_reconstruction.html#recoverpose), the algorithm in the function recoverPose is based on the paper "Nistér, D. An efficient solution to the five-point relative pose problem, CVPR 2003." From equations in Section 2 in this paper, we know it uses the basic triangle relationship (see figure [here](https://i.stack.imgur.com/7ELRD.png)):

x2 = R\*x1 + t 

Therefore, translation t is the vector from cam2 to cam1 in cam2 frame. This explains why you get the answer t close to [0; 0; -1].
In this diagram 

[![diagram ](https://i.stack.imgur.com/8tS3L.png)](https://i.stack.imgur.com/8tS3L.png)

origin is located at top-right corner. Hence it satisfies right hand coordinate system. Positive Z-axis of image and camera coordinate systems are in SAME direction.

However, in openCV, image coordinate system is located at top-left corner. So, it satisfies left hand coordinate system. Positive Z-axis of image and camera coordinate systems are in OPPOSITE direction.