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.

  1. David

    My thoughts:

    When FOV decreases, so does the perspective effect. Distances between near and distant objects will appear to decrease (i.e. the mountain and box appear closer when FOV is lower).

    If FOV was zero, then I believe everything would appear isometric (i.e. no perspective effect at all)

    Cheers,
    David 🙂

  2. Vincent Tan

    David, I believe when the FOV decreases, the perspective doesn’t change at all. If you look closely, you can shrink the smaller-fov screenshot and it should fit nicely into the original screenshot.

    I believe a smaller FOV simply achieves the normal camera zoom effect. It makes things larger, but keeps the perspective.

    Moving closer changes the final view more. If you look closely, the mountain “shrunk” a little. Maybe that’s misleading… The cube and tree was enlarged “more” (or faster) than the mountain, hence the mountain appears to have shrunk a little.

    This is why I wasn’t able to give a good enough explanation for the bonus points… I couldn’t describe the phenomenon adequately…

    I’m not sure what would happen if FOV is zero though. That’s something to think about…

  3. xero

    David is right here. Decreasing the FOV will decrease the effects of perspective aka the depth-of-field, it will also decrease the effective resolution of the scene and cause aliasing (not always noticeable on computer graphics). If you wrote an app that included a distance-based LOD algorithm, you’d notice it acted very different if “zooming-in” was toggled between a FOV change and a position change.

    The same principle applies to photography, except the FOV effects are noticed more in focus than DOF. The smaller the FOV, the more narrow the focus. In a camera, the idea is that less light enters at the same angles but is stretched over the same dimensions of film/pixels.

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  4. Vincent Tan

    Yes xero, you’re right! I’m sorry David, I was thinking of the perspective, as in the angle. Both of you are referring to the effects of the perspective being diminished.

    In a sense, decreasing the FOV approaches the orthogonal view, where perspective doesn’t matter. That’s what you’re referring to, right?

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