Connecting focal length to scale

[ragtag]

I have an object 1 cm in front of the camera, that I want to cover exactly the same amount of the image, no matter what I do with focal length (the object is a child of the camera, so it follows it wherever I move it).

What would be the formula for connecting focal length to the scale of this object so it would remain the same apperent size in the image?

What would be the formula for adding the option to translate it to and from the camera (in Z), and have it remain the same size?

Thanx,

Ragnar

[ragtag]

To answer my own question, in case someone else needs the same. The expression for this goes something like this.

plane.scaleX = ( ((10/camera.focalLength) * 3.4) * plane.translateZ );

Repeat for scaleY and Z. 3.4 was the scale I wanted the object in when the focal length was 10. I also added in translate of the plane. So the plane can be moved nearer or further from the camera, and still remain the same size as seen through the camera.

There are probably too many parenthesis in it

Ragnar