<p>There's no <strong>visible</strong> blur for the same reason you often don't see blur in very wide angle lenses: the circle of confusion is much smaller than a pixel. But it is still there. An extreme example of this is fisheye lenses.</p>

<p>If you had a camera with infinite resolution, such that you could zoom into the reflection on the ball, you would eventually see the blur. And it would be exactly the same blur as if you had aimed a flat mirror at the part of the scene you're looking at through the ball.</p>

<p>Now, we don't have such a camera, but we have something even better: software! So I built this simple setup with two perfectly sharp checkered planes, a sphere and a camera with some depth of field:</p>

<p><a href="https://i.sstatic.net/nKdZu.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/nKdZu.png" alt="scene setup"></a></p>

<p>Rendering it with some depth of field at 512x512, you'd think the reflection in the sphere is perfectly sharp:</p>

<p><a href="https://i.sstatic.net/K6EXH.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/K6EXH.jpg" alt="normal view"></a></p>

<p>But here is a central crop of the exact same scene rendered at 8192x8192:</p>

<p><a href="https://i.sstatic.net/UA3pm.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/UA3pm.jpg" alt="zoom into sphere"></a></p>

<p>And we can see the exact same blur on the checker pattern. It's as simple as that: you don't see the blur because it's too small.</p>

CC BY-SA 4.0
2019-11-30T06:53:31.047Z
Wow.....you could be right... is this **true**?  If you look at the [example photo](https://imgur.com/Cj0GBZD), the direct background seems a comparable size to the reflected background.. but it's hardly a definitive test! I'll try to persuade myself of  it by your  method before accepting.
Robin Betts
CC BY-SA 4.0
2019-11-30T13:01:20.560Z
@RobinBetts it isn't the same size. Top: one balcony is about 1/3 the frame width. In the ball: you see a whole building (~6 balconies) in about the same width. Clearly not the same effective field of view. I think I could make a convincing example with a dSLR too. I just need to find a nice shiny ball.
Olivier
CC BY-SA 4.0
2019-11-30T15:20:51.090Z
..and I've just confirmed your result
Robin Betts
CC BY-SA 4.0
2019-11-30T02:56:04.670Z
2019-11-30T02:56:04.670Z
Olivier
<p>Because the ball doesn't "see" the world through an imperfect lens system, whereas your camera in that case does. If you were to introduce an imperfect visual system around it, with a resulting blurring point spread function, you will get a blurry reflection.</p>

CC BY-SA 4.0
2019-11-24T08:08:59.683Z
2019-11-24T08:08:59.683Z
lightxbulb
<p>A <a href="https://blender.stackexchange.com/q/158942/35559">question</a> has come up recently in Blender SE, remarking on the <em>absence</em> of blur due to depth of field in reflections in a spherical surface. This is a real-life effect.. can anyone come up with a nice explanation of it?</p>

CC BY-SA 4.0
2019-11-25T19:22:40.737Z
@Rahul   That's the one, for me.. you should make that an answer?
Robin Betts
<reflection><depth-of-field>
Why are reflections in a spherical surface sharp, regardless of depth of field?
Robin Betts
# Why are reflections in a spherical surface sharp, regardless of depth of field?

(Asked by: username_0 on 2019-11-23)

A [question](https://blender.stackexchange.com/q/158942/35559) has come up recently in Blender SE, remarking on the *absence* of blur due to depth of field in reflections in a spherical surface. This is a real-life effect.. can anyone come up with a nice explanation of it?

**Tags:** reflection, depth-of-field

> **Comments:**
> **username_0**: @Rahul   That's the one, for me.. you should make that an answer?  

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## 2 answers

### Answer by username_1 on 2019-11-30. Score: 3 | Accepted answer

There's no **visible** blur for the same reason you often don't see blur in very wide angle lenses: the circle of confusion is much smaller than a pixel. But it is still there. An extreme example of this is fisheye lenses.

If you had a camera with infinite resolution, such that you could zoom into the reflection on the ball, you would eventually see the blur. And it would be exactly the same blur as if you had aimed a flat mirror at the part of the scene you're looking at through the ball.

Now, we don't have such a camera, but we have something even better: software! So I built this simple setup with two perfectly sharp checkered planes, a sphere and a camera with some depth of field:

[![scene setup](https://i.sstatic.net/nKdZu.png)](https://i.sstatic.net/nKdZu.png)

Rendering it with some depth of field at 512x512, you'd think the reflection in the sphere is perfectly sharp:

[![normal view](https://i.sstatic.net/K6EXH.jpg)](https://i.sstatic.net/K6EXH.jpg)

But here is a central crop of the exact same scene rendered at 8192x8192:

[![zoom into sphere](https://i.sstatic.net/UA3pm.jpg)](https://i.sstatic.net/UA3pm.jpg)

And we can see the exact same blur on the checker pattern. It's as simple as that: you don't see the blur because it's too small.

> **Comments:**
> **username_0**: Wow.....you could be right... is this **true**?  If you look at the [example photo](https://imgur.com/Cj0GBZD), the direct background seems a comparable size to the reflected background.. but it's hardly a definitive test! I'll try to persuade myself of  it by your  method before accepting.  
> **username_1**: @username_0 it isn't the same size. Top: one balcony is about 1/3 the frame width. In the ball: you see a whole building (~6 balconies) in about the same width. Clearly not the same effective field of view. I think I could make a convincing example with a dSLR too. I just need to find a nice shiny ball.  
> **username_0**: ..and I've just confirmed your result  

---

### Answer by username_2 on 2019-11-24. Score: 1

Because the ball doesn't "see" the world through an imperfect lens system, whereas your camera in that case does. If you were to introduce an imperfect visual system around it, with a resulting blurring point spread function, you will get a blurry reflection.


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