# Photography and vision correction **URL:** \ **Category:** Factual Questions\ **Created:** [September 28, 2010, 2:21am UTC](https://boards.straightdope.com/t/photography-and-vision-correction/555308 "2010-09-28T02:21:03Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![Moonlitherial](https://avatars.discourse-cdn.com/v4/letter/m/4bbf92/32.png) [@Moonlitherial](https://boards.straightdope.com/u/Moonlitherial)\ **Post date:** [September 28, 2010, 2:21am UTC](https://boards.straightdope.com/t/photography-and-vision-correction/555308/1 "2010-09-28T02:21:03Z")
I am near sighted, although not very and my mild prescription is making real world tests difficult. When taking a photograph of something far in the distance, looking through the viewfinder would my eyes focus as though it were close (distance to the mirror) or far (distance to the subject). ---
**Author:** ![Shark\_Sandwich](https://avatars.discourse-cdn.com/v4/letter/s/f04885/32.png) [@Shark\_Sandwich](https://boards.straightdope.com/u/Shark_Sandwich)\ **Post date:** [September 28, 2010, 2:56am UTC](https://boards.straightdope.com/t/photography-and-vision-correction/555308/2 "2010-09-28T02:56:50Z")
The Master [speaks.](http://www.straightdope.com/columns/read/810/why-does-a-mirror-image-look-fuzzy-to-a-nearsighted-person-even-if-the-mirror-is-close) ---
**Author:** ![Moonlitherial](https://avatars.discourse-cdn.com/v4/letter/m/4bbf92/32.png) [@Moonlitherial](https://boards.straightdope.com/u/Moonlitherial)\ **Post date:** [September 28, 2010, 2:59am UTC](https://boards.straightdope.com/t/photography-and-vision-correction/555308/3 "2010-09-28T02:59:52Z")
Awesome, exactly what I was looking for! ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [September 28, 2010, 3:05am UTC](https://boards.straightdope.com/t/photography-and-vision-correction/555308/4 "2010-09-28T03:05:23Z")
> [@Shark\_Sandwich](#): > > The Master [speaks.](http://www.straightdope.com/columns/read/810/why-does-a-mirror-image-look-fuzzy-to-a-nearsighted-person-even-if-the-mirror-is-close) If we’re talking about an SLR camera, it’s nothing like a mirror. The viewfinder of an SLR is basically a piece of frosted glass (focusing screen) with a magnifying glass behind it. The camera’s lens forms an image on the focusing screen. You’re looking **at** an image formed on the screen, not **through** the screen at something beyond. However, the “magnifying glass” part of the finder makes it look like the screen is much further away. So if you are severely nearsighted, your eyes may not be able to focus on the screen. But many SLR cameras have an adjustment dial next to the finder to adjust focus, so if you’re not _too_ nearsighted, you can probably adjust it to your eyes. ---
**Author:** ![GaryM](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/garym/32/241_2.png) [@GaryM](https://boards.straightdope.com/u/GaryM)\ **Post date:** [September 28, 2010, 1:44pm UTC](https://boards.straightdope.com/t/photography-and-vision-correction/555308/5 "2010-09-28T13:44:17Z")
> [@scr4](#): > > If we’re talking about an SLR camera, it’s nothing like a mirror. The viewfinder of an SLR is basically a piece of frosted glass (focusing screen) with a magnifying glass behind it. The camera’s lens forms an image on the focusing screen. You’re looking **at** an image formed on the screen, not **through** the screen at something beyond. > > However, the “magnifying glass” part of the finder makes it look like the screen is much further away. So if you are severely nearsighted, your eyes may not be able to focus on the screen. But many SLR cameras have an adjustment dial next to the finder to adjust focus, so if you’re not _too_ nearsighted, you can probably adjust it to your eyes. And even some cameras with viewfinders, not SLRs, have an adjustment to accommodate differences in vision. I have a canon G3 here at work that has an optical viewfinder and it can be adjusted to bring the view of the subject into sharp focus. I believe that “the Master’s” explanation does not hold true for convex and concave mirrors. But I’m not smart enough to explain it. :smack: ---
**Author:** ![Giles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giles/32/60_2.png) [@Giles](https://boards.straightdope.com/u/Giles)\ **Post date:** [September 28, 2010, 2:11pm UTC](https://boards.straightdope.com/t/photography-and-vision-correction/555308/6 "2010-09-28T14:11:43Z")
> [@scr4](#): > > > [@Shark\_Sandwich](#): > > > > The Master [speaks.](http://www.straightdope.com/columns/read/810/why-does-a-mirror-image-look-fuzzy-to-a-nearsighted-person-even-if-the-mirror-is-close) > > If we’re talking about an SLR camera, it’s nothing like a mirror. … Agreed. What you are looking at is the image as it would appear on the film (in a film SLR), or on the sensor (in a digital SLR). So, if the image would be out-of-focus after you push the button, it’s going to be out-of-focus on the image inside the camera that you looking at. To see it in focus, two things must be right: (1) the lens at the front of the camera must be in the right position, and (2) the lens at the back of the viewfinder must be in the right position. So, if you are making adjustments to the lens at the back of the viewfinder, so so with a scene that you know is in focus – which it ought to be if you are using the camera’s automatic focus, or you are looking at a landscape and the lens is set at infinity. It’s quite different with a non-SLR camera, where you are not viewing the scene through the camera’s lens when you use the viewfinder. ---
**Author:** ![DHMO](https://avatars.discourse-cdn.com/v4/letter/d/ecc23a/32.png) [@DHMO](https://boards.straightdope.com/u/DHMO)\ **Post date:** [September 28, 2010, 5:11pm UTC](https://boards.straightdope.com/t/photography-and-vision-correction/555308/7 "2010-09-28T17:11:27Z")
> [@GaryM](#): > > I believe that “the Master’s” explanation does not hold true for convex and concave mirrors. But I’m not smart enough to explain it. :smack: Actually, it does “hold true,” but it requires a minor detour into the field of optics and virtual images. All reflecting surfaces (mirrors) form an “image,” so to speak. For a flat (plane) surface, the reflected rays of light leave the mirror at the same angle that the incoming rays made to the normal (perpendicular) to the surface at that point. Parallel incident rays remain parallel after reflection, and the “focus” is at infinity, so there is no true image; the rays of light have merely been redirected. Your eye then perceives objects in the mirror as if they were located the same distance _behind_ the mirror as they are in reality in _front_ of it. For a _convex_ mirror (bulging out toward you,) the reflected angle = incoming angle still applies, but, since the surface is curving continuously, the normal, hence, the direction of the reflected ray, changes for every point on the mirror. Parallel incoming rays will _diverge_ upon reflection. But if you project them back into the mirror, it will appear that they originated at a “virtual focus” (since it does not really exist.) An object viewed in such a mirror will appear _closer_ than it is in reality (such as the rear-view mirrors on the passenger side of many cars.) An object at infinity (or very far away) will appear to be at a distance twice the radius of curvature of the convex surface, _behind_ the mirror. A _concave_ mirror, on the other hand, forms a “true image” in _front_ of the mirror. The angle of reflection = the angle of incidence, as before, but now the surface is curving the opposite way, and parallel rays striking the surface will _converge_ upon reflection. Projecting the reflected rays through the mirror, they _diverge_ from the real focus in front of the mirror. If your eye is located between the focus and the mirror, objects will appear closer, and magnified, compared to how they are in reality (like many concave make-up mirrors.) If your eye is farther from the mirror than the focus, reflected objects appear inverted (upside-down/umop-[SIZE=2][SIZE=3]əp!sdn[/SIZE][/SIZE].) If your eye is in line with the axis of the reflecting surface, you can see an inverted actual image floating in the air at the point of focus. Concave mirrors are used for virtually all large telescopes, and the real image is formed at the Prime Focus. Sometimes, instruments are placed at the prime focus (spectrometers, cameras, etc.) Or, you can place an eyepiece beyond the focus and examine the image, as you would a real object with a magnifying glass. In all cases, the reflected image is perceived as coming from behind the mirror, not from its surface. So, The Master’s explanation holds for all cases, he merely confined it to the context of the case at hand. ---
**Author:** ![DHMO](https://avatars.discourse-cdn.com/v4/letter/d/ecc23a/32.png) [@DHMO](https://boards.straightdope.com/u/DHMO)\ **Post date:** [September 28, 2010, 8:43pm UTC](https://boards.straightdope.com/t/photography-and-vision-correction/555308/8 "2010-09-28T20:43:37Z")
> [@DHMO](#): > > But if you project them back into the mirror, it will appear that they originated at a “virtual focus” (since it does not really exist.) **An object viewed in such a mirror will appear _closer_ than it is in reality** (such as the rear-view mirrors on the passenger side of many cars.) Bloody hell.:o The other way 'round. I knew that. :smack: ---
**Author:** ![FatBaldGuy](https://avatars.discourse-cdn.com/v4/letter/f/ecd19e/32.png) [@FatBaldGuy](https://boards.straightdope.com/u/FatBaldGuy)\ **Post date:** [September 28, 2010, 8:53pm UTC](https://boards.straightdope.com/t/photography-and-vision-correction/555308/9 "2010-09-28T20:53:05Z")
If you’ve had an eye exam recently, you’ve probably been in a small room where you sit in a chair and read letters off of the chart. Most of the time, the chart is projected from the back of the room onto a mirror at the front of the room, which then bounces back to another mirror at the back of the room, and finally to a screen at the front, which you look at. The reason for this is to simulate a distance much longer than the size of the room. ---
**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:** [September 28, 2010, 9:18pm UTC](https://boards.straightdope.com/t/photography-and-vision-correction/555308/10 "2010-09-28T21:18:57Z")
> [@](#): > > Bloody hell. > > The other way 'round. No, you had it right the first time. The label on the car mirror is wrong. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [September 28, 2010, 9:26pm UTC](https://boards.straightdope.com/t/photography-and-vision-correction/555308/11 "2010-09-28T21:26:44Z")
> [@Chronos](#): > > No, you had it right the first time. The label on the car mirror is wrong. Never thought about it like that, but you’re right, in terms of distance to the image. The label on the mirror should really say “Objects in mirror are **larger** than they appear.” ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [September 28, 2010, 10:20pm UTC](https://boards.straightdope.com/t/photography-and-vision-correction/555308/12 "2010-09-28T22:20:05Z")
[quote=“scr4, post:4, topic:555308”] > [@Shark\_Sandwich](#): > > If we’re talking about an SLR camera, it’s nothing like a mirror. The viewfinder of an SLR is basically a piece of frosted glass (focusing screen) with a magnifying glass behind it. You are correct for some SLRs. SWMBO has an SLR that forms a virtual image and then has an eyepiece that looks at it. There is no frosted glass. Pretty much like a telescope. I didn’t know this and she had me take pictures of her and her friend when her friend was moving away. I didn’t have my glasses on, and all those pictures are out of focus due to my mild myopia. I was in a bit of grief over that. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [September 30, 2010, 8:20pm UTC](https://boards.straightdope.com/t/photography-and-vision-correction/555308/13 "2010-09-30T20:20:24Z")
> [@Kevbo](#): > > You are correct for some SLRs. > > SWMBO has an SLR that forms a virtual image and then has an eyepiece that looks at it. There is no frosted glass. Pretty much like a telescope. > > I didn’t know this and she had me take pictures of her and her friend when her friend was moving away. I didn’t have my glasses on, and all those pictures are out of focus due to my mild myopia. I was in a bit of grief over that. I’m curious to know who makes cameras like that. How can you focus using such a camera, even if you have 20/20 vision? The eye’s focus mechanism would correct for the focus error, and you’d see a sharp image even if the camera is in focus.