# Vision question

**URL:** <https://boards.straightdope.com/t/vision-question/970157>\
**Category:** Factual Questions\
**Created:** [August 23, 2022, 10:13am UTC](https://boards.straightdope.com/t/vision-question/970157 "2022-08-23T10:13:34Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Danger\_Man](https://avatars.discourse-cdn.com/v4/letter/d/6de8d8/32.png) [@Danger\_Man](https://boards.straightdope.com/u/Danger_Man)\
**Post date:** [August 23, 2022, 10:13am UTC](https://boards.straightdope.com/t/vision-question/970157/1 "2022-08-23T10:13:34Z")

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How come when I close one eye and put my finger really close to the other eye I can see a translucent outline of my finger?

I can understand with two eyes how there are some parts that are only seen by one eye, but I don’t understand what is happening with a single eye open.

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**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\
**Post date:** [August 23, 2022, 10:15am UTC](https://boards.straightdope.com/t/vision-question/970157/2 "2022-08-23T10:15:52Z")

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Lack of focus causes blur.

> **[Defocus aberration](https://en.wikipedia.org/wiki/Defocus_aberration)**
>
> Tilt
> Spherical aberration
> Astigmatism
> Coma 
> Distortion
> Petzval field curvature
> Chromatic aberration
> In optics, defocus is the aberration in which an image is simply out of focus. This aberration is familiar to anyone who has used a camera, videocamera, microscope, telescope, or binoculars. Optically, defocus refers to a translation of the focus along the optical axis away from the detection surface. In general, defocus reduces the sharpness and contrast of the image. What should be sha...

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**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\
**Post date:** [August 23, 2022, 9:18pm UTC](https://boards.straightdope.com/t/vision-question/970157/3 "2022-08-23T21:18:01Z")

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Let’s say that you’re looking at a small, distant object. Light from that object can hit any spot on the lens of your eye, and it’ll still be bent to hit the same spot on your retina. That’s what a lens does.

But now put your finger right in front of your eye. Some paths from the distant object to parts of your lens miss your finger, but some other paths are blocked by your finger. So that spot on your retina is now getting light both from the distant object and your finger.