Here is something some of you might want to try.
You'll need
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An old 35mm SLR camera (the type that has the distance between the camera and the subject etched on the lens barrel.
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A Post it Note
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A tape measure
Stand in front of a mirror. Measure the distance between yourself and the mirror with the tape measure. Let's say fro example that it's 3 feet.
Put the Post It note on the mirror.
First, focus your camera on the Post It note. Look at the lens barrel, and write down the distance.
Second, focus your camera on your reflection. Look at the lens barrel, and write down the distance.
My results in this were:
Distance the lens barrel gave for the Post It note: 3 feet.
Distance the lens barrel gave me to my reflection: 6 feet.
Why the discrepancy? The camera is focusing on the surface of the mirror in both cases, after all.
I'm sure someone versed in optics can explain this, but it's got me flummoxed.
1
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9 comments
Feel free to reply to any comment by clicking the "Reply" button.using an old Zenit 35mm SLR, bought new 1976, photo quality equal to any modern digital camera, subject plus distance of focus = 3 x 2 = 6,. try using a macro bellow lens for even weirder resuls
magicwatch
Level 7
Mar 3, 2018
0
That just proves that the mirror is actually a gateway to a parallel universe.
JimG
Level 8
Mar 2, 2018
0
I was wondering when someone would suggest that. 
"The camera is focusing on the surface of the mirror in both cases, after all."
Here is your mistake. If you look into ray-tracing, the process used to render 3d objects in computers, you'll quickly figure this one out.
ScottAHurst
Level 6
Mar 2, 2018
1
I'd like tom thank everyone for their comments. I still can't quite wrap my head around it. Maybe if I had prayed more in public school. 
Robotbuilder
Level 7
Mar 2, 2018
0
Ah this is like my Optician's office where he has a mirror on the wall to increase the distance to reading the letters by reflecting it.
He was so impressed when I knew what that was all about.
ie Small room - needed more distance for testing.
Oh and I come from an Optical Town. My Dad was a Tool & Die maker in an eyeglass/lens factory.
RavenCT
Level 9
Mar 2, 2018
0
I tried it with my new Nikon and it's exactly what @jnei said. If your 7 feet from the mirror and you focus on the post it the camera says 7 ft and if you foucus on yourself the camera says 14 ft because that's how far the light travels from you to the mirror and back again.
gearl
Level 8
Mar 2, 2018
3
Jnei:
Ooh, which Nikon have you got?
Jnei:
@gearl Nice bit of kit, the D7200. Shutters are always a bit of an unknown - sometimes they'll go well before the manufacturer's rating (in which case you can get them repaired at a reduced rate, which could be worth looking into), sometimes they'll last much longer. I've exclusively used Nikons for many years and still regularly use my two D300s alongside my new D500; both of them are well over half a million shutter actuations but still going strong, which is pretty good going (especially as one of them rolled about 20m down a hill a couple of years back).
are you using manual or automatic focusing? I guess the clue hear is in the word "focus" as your image would not be in focus at 3 feet as it would be double the reflected distance seen in the mirror, I guess
magicwatch
Level 7
Mar 2, 2018
1
To focus, the camera finds the image point, where light rays originating or reflecting from the object you wish to focus on converge. So in the first case, the camera is finding the point where light rays reflecting from the Post-It note three feet away converge. In the second case, it's finding the point where the light rays reflecting from you converge - and those light rays have travelled from you to the mirror and back to the camera. Stick the Post-It note on your forehead and try focusing on it again and it should be about six feet.
Here's a really amazingly professional-looking diagram to illustrate what I mean. I really ought to have been a graphic designer.
Jnei
Level 8
Mar 2, 2018
2
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