Move closer or shrink FOV?

There was this question posed by my professor in a computer graphics class. It was for bonus points (we love them, don’t we?) and sadly, I didn’t give a satisfactory answer. And to this date, I still don’t know what the answer should be.

To elaborate, first I need to explain what field of view or FOV is. Humans have an FOV of almost 180 degrees. For 3D graphics and computer games, it’s typically 90, 60 or 45 degrees (helps with cutting down processing calculations). What is it?

Simple illustration of field of view

Suppose you’re standing somewhere looking at a scene. You notice something in the distance, and you want to take a closer look or zoom in. For the purposes of this example, let’s just assume you have some bionic superpower that enables your eyes to function like a camera/binoculars thingy.

There are two ways to go about doing this. You can physically move closer. Or you can shrink your FOV. A smaller FOV means less is visible, but whatever is visible is enlarged, so to speak. Either way, the object of your attention becomes larger.

Move closer or shrink FOV?

The resulting rendered scene is of the same “width” in both cases. The object is enlarged in both cases. The question was, what’s the difference between moving closer and shrinking the FOV?

Let’s look at a normal rendered scene.

Scene with normal perspective

If we move closer and keep the FOV at 60 degrees, we get this:

Scene with camera closer

If we stay where we are and change the FOV to 45 degrees, we get this:

Scene with smaller FOV

Using the tree and cube to act as reference points, and the mountain as backdrop, can you spot the difference?

Now that I think about it, the answer probably has some similarities to the concept of ray tracing. Instead of having light reflecting off objects and enter your eye, think about shooting back rays from the eye towards the scene.

I guess I’ll have to talk more on this. Please share your answer and we can compare notes. Stay tuned.