HI Alex,
If you render out the Camera 1 and use it in the material (Color) for the Object that is in the view of Camera 2, then set the Texture Tag to Camera Mapping. Press the Calculate button, and remove the image (rendered and applied) from the Color channel: Now it should be very close.
What happens in your scene? The camera 1 is the source for the Camera Shader. This Shader is applied to object “Default.1”. The Camera is set but the texture tag was set to UV[W], whereby the set up doesn’t cover the whole image produced by the camera shader. (It is easy to see in the UV Editor). With the Camera Projection setting, this problem is eliminated.
The Camera is set to focal length 36mm and sensor size 36mm (which is the default). Which means, that any object width that is perpendicular to the camera will have the same need in distance to be horizontally covered, i.e., fit in the horizontal field of view. This is always true if the two numbers are the same. So a sensor with 40mm width needs a 40mm lens to achieve that.
In your scene the object is 3cm wide, which would require a 3cm distance to the camera. The camera is at P.Z. -3.134cm, and the points that creates the surface are P.Z. -0.3cm. So, the object needs to be set to P.Z. +0.166cm (it was zero before.
Then we need to have the camera shader cover the front of that object, which can be done in the UV editor for example. While that front part UV is selected, UV Commands> Fit UV to Canvas. But the object hight is not fitting. But that is now an easy match mathematically, based on the render settings. But scale and move the object not a few points, otherwise the UV will distort the result.
Anyway (you can ignore the explanation set in a gray font): Set up the camera mapping and delete the UV tag (copy the object before and set it to invisibility, back up). While the object and the texture tag is selected, go to Object Manager>Tags>Generate UVW Coordinates. This sets the UV mesh to the camera projection.
Another difference shows up, while the Anti Alias settings are on Geometry, that works for camera.1 but camera 2. has to work with an image.
Scene file
https://www.amazon.com/clouddrive/share/k9b4AfQSsoACLbD9en11VAGbIMGpwf9Ovhn7JXRnbYE
All the best
