<p>Since a projection plane and a center of projection is used for achieving perspective projection, the shape of the view frustum doesn't change changing the focal length(which is the distance from the center of projection to the plane of projection i.e. the screen or buffer)  </p>

<p>Then how is zooming done in computer graphics? It is mentioned in my textbook that this is done by changing the size of the view frustum and that the relationship that <em>field of view is inversely proportional to the zoom</em> holds(somehow I don't know) but looks to me more like a bit conterintuitive, e.g.:  </p>

<p><a href="https://i.sstatic.net/wCGAf.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/wCGAf.png" alt="enter image description here"></a></p>

CC BY-SA 4.0
2020-02-17T11:26:33.243Z
Your virtual camera film becomes smaller, but your screen remains the same size, meaning that a smaller region of the world is projected on the same area of the screen - that is a zoomed in region.
lightxbulb
CC BY-SA 4.0
2020-02-17T11:41:19.367Z
@lightxbulb , does focal distance and focal length refer to the same thing in CG? It seems to be different in photography , because in my textbook it is mentioned that when you change the focal distance you also change the frustum, but that doesn't change if you change the plane of projection. Do you instead change the center of projection to change the focal distance(or is it focal length), could you please explain a bit about these?
juztcode
CC BY-SA 4.0
2020-02-18T02:53:24.487Z
@DanielMGessel , I think the mistake I was making in understand the entire textbook was that I was assuming the plane of projection is the one that is changed, but to increase or decrease d, also the center of projection can be changed, which one is usually changed?
juztcode
CC BY-SA 4.0
2020-02-18T05:45:09.243Z
@DanielMGessel , I also did that, but the problem is that the frustum and FOV would remain the same if you change the plane of projection
juztcode
CC BY-SA 4.0
2020-02-18T05:48:23.080Z
@DanielMGessel , and once we have -ze/n as the value for `w` the homogeneous coordinate, why and how can it be set to -ze? Even the math is making no sense to me
juztcode
<projections><perspective>
how is zooming done in computer graphics
juztcode
# how is zooming done in computer graphics

(Asked by: username_0 on 2020-02-17)

Since a projection plane and a center of projection is used for achieving perspective projection, the shape of the view frustum doesn't change changing the focal length(which is the distance from the center of projection to the plane of projection i.e. the screen or buffer)

Then how is zooming done in computer graphics? It is mentioned in my textbook that this is done by changing the size of the view frustum and that the relationship that *field of view is inversely proportional to the zoom* holds(somehow I don't know) but looks to me more like a bit conterintuitive, e.g.:

[![enter image description here](https://i.sstatic.net/wCGAf.png)](https://i.sstatic.net/wCGAf.png)

**Tags:** projections, perspective

> **Comments:**
> **username_1**: Your virtual camera film becomes smaller, but your screen remains the same size, meaning that a smaller region of the world is projected on the same area of the screen - that is a zoomed in region.  
> **username_0**: @username_1 , does focal distance and focal length refer to the same thing in CG? It seems to be different in photography , because in my textbook it is mentioned that when you change the focal distance you also change the frustum, but that doesn't change if you change the plane of projection. Do you instead change the center of projection to change the focal distance(or is it focal length), could you please explain a bit about these?  
> **username_0**: @DanielMGessel , I think the mistake I was making in understand the entire textbook was that I was assuming the plane of projection is the one that is changed, but to increase or decrease d, also the center of projection can be changed, which one is usually changed?  
> **username_0**: @DanielMGessel , I also did that, but the problem is that the frustum and FOV would remain the same if you change the plane of projection  
> **username_0**: @DanielMGessel , and once we have -ze/n as the value for `w` the homogeneous coordinate, why and how can it be set to -ze? Even the math is making no sense to me  

---

## 0 answers

