<p>For some reason most sites choose to leave the projection plane out of the visualization of the view frustum. The focal length often labeled &quot;g&quot; is the distance from the camera to the projection plane. The projection plane (which is blue in the attached image) is almost always placed at a distance from the camera such that the distance from the center to the top and bottom is 1. This image from the fged website shows it well: <a href="https://i.sstatic.net/6wEgR.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/6wEgR.png" alt="enter image description here" /></a> This choice is a matter of convenience since it makes the math all work out nice and neat. As you can see the near plane can, and often is, behind the projection plane. (the green plane in the image is the near plane)</p>
<p>For your second confusion: One goal of the projection matrix is to force coordinates with a z value less then that of the near plane to fall outside of the near clipping region which is -1 in OpenGL. So any coordinate that has a z value less then near value will result in a z value below -1 after projection.</p>

CC BY-SA 4.0
2021-10-23T13:18:35.073Z
There is no ideal focal length, it is perfectly reasonable to choose a near plane and a projection plane that coincide. The near plane acts to clip geometry. The focal length acts like a lens that scales geometry.  Increasing the focal length zooms in on distant geometry because the projection plane is closer to it, or beyond it. This is exactly how most games implement a zoom effect, they simply move the projection plane farther from the camera by increasing the focal length.
pmw1234
CC BY-SA 4.0
2021-10-23T13:21:23.127Z
Thank you for those clarifications. I actually realized that my comment didn't make much sense because I got confused. : )
KeyC0de
CC BY-SA 4.0
2021-01-26T18:34:37.193Z
2021-01-26T18:39:37.597Z
2021-01-26T18:39:37.597Z
pmw1234
<p>The part confusing  about opengl during studying the projection was that the focal length, top, left, bottom, right, wasn't declared, and it magically did it with FOV and image aspect ration.   I found the wonderful maths in <a href="https://www.scratchapixel.com/lessons/3d-basic-rendering/perspective-and-orthographic-projection-matrix/opengl-perspective-projection-matrix" rel="nofollow noreferrer">this amazing site</a>, and found out how it is calculated based on FOV and focal length.<br />
Two points I noted for my confusion was:</p>
<ul>
<li>near clipping plane is the plane which limits the projected coordinates, and</li>
<li>focal length is the distance from the point of projection to the optical center( (0,0,0) in our case )</li>
</ul>
<p>Now, the question is, is the focal length always equal to the distance of the near clipping plane from the optical center, i.e. in other words, is the image <em>always</em> projected on the near clipping plane (or at least in openGL)? Yet in other words, is the near clipping plane and the plane of projection the same thing just with different terms.</p>

CC BY-SA 4.0
2021-01-25T17:30:04.307Z
but focal length is calculated for a simulated camera with some imaginary film size. So No.
joojaa
<opengl><projections>
is focal length equal to the distance from the optical center to the near clipping plane- openGL
juztcode
# is focal length equal to the distance from the optical center to the near clipping plane- openGL

(Asked by: username_0 on 2021-01-25)

The part confusing about opengl during studying the projection was that the focal length, top, left, bottom, right, wasn't declared, and it magically did it with FOV and image aspect ration. I found the wonderful maths in [this amazing site](https://www.scratchapixel.com/lessons/3d-basic-rendering/perspective-and-orthographic-projection-matrix/opengl-perspective-projection-matrix), and found out how it is calculated based on FOV and focal length.  
Two points I noted for my confusion was:

* near clipping plane is the plane which limits the projected coordinates, and
* focal length is the distance from the point of projection to the optical center( (0,0,0) in our case )

Now, the question is, is the focal length always equal to the distance of the near clipping plane from the optical center, i.e. in other words, is the image *always* projected on the near clipping plane (or at least in openGL)? Yet in other words, is the near clipping plane and the plane of projection the same thing just with different terms.

**Tags:** opengl, projections

> **Comments:**
> **username_1**: but focal length is calculated for a simulated camera with some imaginary film size. So No.  

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## 1 answers

### Answer by username_2 on 2021-01-26. Score: 2

For some reason most sites choose to leave the projection plane out of the visualization of the view frustum. The focal length often labeled "g" is the distance from the camera to the projection plane. The projection plane (which is blue in the attached image) is almost always placed at a distance from the camera such that the distance from the center to the top and bottom is 1. This image from the fged website shows it well: [![enter image description here](https://i.sstatic.net/6wEgR.png)](https://i.sstatic.net/6wEgR.png) This choice is a matter of convenience since it makes the math all work out nice and neat. As you can see the near plane can, and often is, behind the projection plane. (the green plane in the image is the near plane)

For your second confusion: One goal of the projection matrix is to force coordinates with a z value less then that of the near plane to fall outside of the near clipping region which is -1 in OpenGL. So any coordinate that has a z value less then near value will result in a z value below -1 after projection.

> **Comments:**
> **username_2**: There is no ideal focal length, it is perfectly reasonable to choose a near plane and a projection plane that coincide. The near plane acts to clip geometry. The focal length acts like a lens that scales geometry.  Increasing the focal length zooms in on distant geometry because the projection plane is closer to it, or beyond it. This is exactly how most games implement a zoom effect, they simply move the projection plane farther from the camera by increasing the focal length.  
> **username_3**: Thank you for those clarifications. I actually realized that my comment didn't make much sense because I got confused. : )  

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