# Help me understand this optical illusion **URL:** \ **Category:** Factual Questions\ **Created:** [July 13, 2010, 1:07am UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478 "2010-07-13T01:07:59Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![PlainJain](https://avatars.discourse-cdn.com/v4/letter/p/5daacb/32.png) [@PlainJain](https://boards.straightdope.com/u/PlainJain)\ **Post date:** [July 13, 2010, 1:07am UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/1 "2010-07-13T01:07:59Z")
This past weekend I had relatives in town. My mom was interested in visiting [The Mystery Spot](http://www.mysteryspot.com/) ([Wiki article](http://en.wikipedia.org/wiki/Mystery_Spot)). I decided to bring a small bubble level with me just for the heck of it. Most of the illusions there are frame of reference illusions. They were fun to experience but pretty easy to figure out. However the one I could not figure out was the very first one which took place outside the “house”. The set up is that there are two parallel, level boards (2 X 4’s). Two volunteers stand on each end, then trade places and their respective sizes appear the change. The photos I took were a bit blurred but [I found these on the internet](http://farm5.static.flickr.com/4135/4788632336_8373c265c5_b.jpg) which I feel are a good representation. You can tell from the boards that are visible at the bottom that both photos are taken at a near level angle, from the same angle and about the same distance away. After the guide made a big flourish of demonstrating that the boards were level, I asked if he minded if I tried my level. He joyfully invited me up. Both boards were level lengthwise and level to each other. When done, the effect was quite dramatic and drew oohs and aahs from the crowd. I looked at the [Shimaura and Prinzmetal research paper](http://socrates.berkeley.edu/~shimlab/1999_Shim-Prinz_MystSpot.pdf) (PDF) and it didn’t seem to address this particular illusion. How could two people just 2 or 3 feet apart from each other seem the change heights of about 5" (in relationship to each other) standing on level boards? They did this with 4 or 5 pairs of visitors of varying heights with no change of results. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [July 13, 2010, 2:28am UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/2 "2010-07-13T02:28:30Z")
The wikipedia article explaines it. ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [July 13, 2010, 2:32am UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/3 "2010-07-13T02:32:46Z")
The board they are standing on is level, but the railings behind make it look as though the ground is sloping down to the right. In both those photos, the darker skinned woman is taller - the top of her head is above the top of the other woman’s head - but in the left hand photo her tallness is exaggerated because she appears (thanks to the railings behind) to be standing on lower ground, even though she is not. ---
**Author:** ![GuanoLad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/guanolad/32/42_2.png) [@GuanoLad](https://boards.straightdope.com/u/GuanoLad)\ **Post date:** [July 13, 2010, 2:57am UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/4 "2010-07-13T02:57:24Z")
I think the ground is level, but the platform is not. The fenceline is giving the illusion of uneven ground, allowing you to believe the platform has been adjusted to be level, but in fact it’s adjusted to be uneven. Maybe. ---
**Author:** ![PlainJain](https://avatars.discourse-cdn.com/v4/letter/p/5daacb/32.png) [@PlainJain](https://boards.straightdope.com/u/PlainJain)\ **Post date:** [July 13, 2010, 3:13am UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/5 "2010-07-13T03:13:24Z")
> [@njtt](#): > > The board they are standing on is level, but the railings behind make it look as though the ground is sloping down to the right. In both those photos, the darker skinned woman is taller - the top of her head is above the top of the other woman’s head - but in the left hand photo her tallness is exaggerated because she appears (thanks to the railings behind) to be standing on lower ground, even though she is not. I’m not convinced. I printed out the photo and taped it to my drafting table. There seems to be a measurable difference in height in the two photos. ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [July 13, 2010, 3:18am UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/6 "2010-07-13T03:18:46Z")
Also note that the darker skinned woman is standing closer to the camera in the left hand photo – look at where her feet are, compared to her friend’s. ---
**Author:** ![needscoffee](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/needscoffee/32/1076_2.png) [@needscoffee](https://boards.straightdope.com/u/needscoffee)\ **Post date:** [July 13, 2010, 3:28am UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/7 "2010-07-13T03:28:01Z")
> [@Nametag](#): > > Also note that the darker skinned woman is standing closer to the camera in the left hand photo – look at where her feet are, compared to her friend’s. Look at the second photo at where the woman on the right’s feet are. You can see how much higher that spot is than where the other woman is. ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [July 13, 2010, 3:29am UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/8 "2010-07-13T03:29:59Z")
Of course there “seems” to be a difference, “seeming” is what illusions are all about. You do not really thing the women’s heights changed, do you? In addition, the darker skinned woman is cocking her head to one side in the right-hand photograph, which lowers it slightly. Also, a very slight, otherwise imperceptible, change in tilt of the camera could exaggerate the effect in photographs like these (or diminish it, depending on the direction of the tilt), as could slight differences in how close each woman is to the camera. Thus the effect might be exaggerated in some pairs of photographs, perhaps including these. However, the effect as seen “live” (and most of it as seen in photographs) arises as I said. (If I understand your OP correctly, you ruled out **GuanoLad** ’s idea when you examined the boards.) ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [July 13, 2010, 3:36am UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/9 "2010-07-13T03:36:21Z")
> [@needscoffee](#): > > Look at the second photo at where the woman on the right’s feet are. You can see how much higher that spot is than where the other woman is. No. That is the illusion (the biasing effect of the background) deceiving you. It is precisely because you _think_ her feet are higher when she stands on the left that she does not look as tall as she does when she stands on the right. Anyway, if the boards they were standing on were actually at significantly different heights, that would be fairly obvious to anyone actually present, observing the illusion “live.” > [@Nametag](#): > > Also note that the darker skinned woman is standing closer to the camera in the left hand photo – look at where her feet are, compared to her friend’s. Yes indeed. This exaggerates the effect in these particular photographs, although it would make little difference “live,” where our visual systems can judge and compensate for distance effects much better. ---
**Author:** ![panamajack](https://avatars.discourse-cdn.com/v4/letter/p/47e85d/32.png) [@panamajack](https://boards.straightdope.com/u/panamajack)\ **Post date:** [July 13, 2010, 3:56am UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/10 "2010-07-13T03:56:13Z")
I’m having trouble even seeing the ‘illusion’ in the photos (I can see it if I concentrate). In both pictures, the taller girl’s nose is nearly level with the other one’s eyebrows, and the only difference is that the tilted head makes the comparison a bit off. But the point where the front of her neck enters the jacket is about in line with the mouth. The shorter girl might even be subconsciously straightening up. Her pants leg looks smoother, and her back looks straighter, although that could be the angle. Doesn’t help that the taller girl seems to be a touch closer to the camera in the left shot. I’ve visited the Mystery Spot once, and I seem to recall that the fence adds to the illusion by slanting slightly away from the observer, thereby increasing the perception of the slope. I wouldn’t be surprised if it’s even exaggerated by varying the thickness of the fence boards. Most of the site’s other illusions work that way. ---
**Author:** ![Spoke](https://avatars.discourse-cdn.com/v4/letter/s/a88e57/32.png) [@Spoke](https://boards.straightdope.com/u/Spoke)\ **Post date:** [July 13, 2010, 3:59am UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/11 "2010-07-13T03:59:52Z")
The one woman cocking her head to the side in the second photo really enhances the illusion. Even if you ignore the backdrop entirely the cocked head makes the women seem like they are closer to being eye-to-eye in the second photo. ---
**Author:** ![PlainJain](https://avatars.discourse-cdn.com/v4/letter/p/5daacb/32.png) [@PlainJain](https://boards.straightdope.com/u/PlainJain)\ **Post date:** [July 13, 2010, 4:47am UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/12 "2010-07-13T04:47:02Z")
> [@njtt](#): > > (If I understand your OP correctly, you ruled out **GuanoLad** ’s idea when you examined the boards.) Not sure if I understand you, but I placed my level on the place on the boards these girls are standing on. > [@njtt](#): > > > [@Nametag](#): > > > > Also note that the darker skinned woman is standing closer to the camera in the left hand photo – look at where her feet are, compared to her friend’s. > > Yes indeed. This exaggerates the effect in these particular photographs, although it would make little difference “live,” where our visual systems can judge and compensate for distance effects much better. If all I had was one photo to judge by I would be looking for all of the little staged things that could have been done to enhance the illusion. As **njtt** points out it was different live, these were just a few pairs of giggling tourist who did nothing to enhance the effect. In person the effect was startling. I’m sure it’s enhanced by the railing, but I believe the difference is measurable in the photos. ---
**Author:** ![GuanoLad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/guanolad/32/42_2.png) [@GuanoLad](https://boards.straightdope.com/u/GuanoLad)\ **Post date:** [July 13, 2010, 5:07am UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/13 "2010-07-13T05:07:57Z")
> [@PlainJain](#): > > Not sure if I understand you, but I placed my level on the place on the boards these girls are standing on. All directions? If so, then my guess is clearly bollocks. ---
**Author:** ![In\_Winnipeg](https://avatars.discourse-cdn.com/v4/letter/i/a8b319/32.png) [@In\_Winnipeg](https://boards.straightdope.com/u/In_Winnipeg)\ **Post date:** [July 13, 2010, 5:10am UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/14 "2010-07-13T05:10:40Z")
\ It’s in California now? I have a bunch of postcards from some trip my grandparents took back in the 50s, and this used to be in the UP of Michigan. The postcards are very similar to the pictures at the website in the OP. ---
**Author:** ![PlainJain](https://avatars.discourse-cdn.com/v4/letter/p/5daacb/32.png) [@PlainJain](https://boards.straightdope.com/u/PlainJain)\ **Post date:** [July 13, 2010, 5:29am UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/15 "2010-07-13T05:29:21Z")
Here’s a[video I found on YouTube](http://www.youtube.com/watch?v=Dovu3H4zfvg&feature=related). Note the section between 00:28 and 00:35. It goes by too fast to really get the feel of it but I put my own level on this spot. I guess the part that was confusing to me was how close these people stand to each other. The explanation at the end does “expose” all of the effects except this one. ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [July 13, 2010, 5:49am UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/16 "2010-07-13T05:49:00Z")
> [@PlainJain](#): > > Not sure if I understand you, but I placed my level on the place on the boards these girls are standing on. > > If all I had was one photo to judge by I would be looking for all of the little staged things that could have been done to enhance the illusion. As **njtt** points out it was different live, these were just a few pairs of giggling tourist who did nothing to enhance the effect. In person the effect was startling. I’m sure it’s enhanced by the railing, but I believe the difference is measurable in the photos. I don’t know what you mean. Nothing of any significance is “measurable” from casual photos like these, and if you were to actually measure the women (or the people you saw) there is no question that you would find that their heights did not change. The illusion you saw when you were there was caused by the railing in the background (and perhaps other similar environmental cues) which fooled your visual system into thinking that the two people were standing on a slope, and so had their feet planted at different heights, when in fact the feet were all at the same height. This (in the example of the pictures you linked to) leads to the impression that the woman on the right, in each of the photos, is taller than she is, relative to the other, because she appears (falsely) to be standing further down the slope. (When you actually look at the feet it may be obvious enough that they are at at the same level, but when you are trying to estimate the people’s relative tallness, you are looking at their heads, and the visual system “forgets” what it may previously have known about the feet, and concludes that the people are standing with one further down the slope than the other from the cure of the railing behind.) The other factors mentioned simply exaggerate the effect slightly, probably purely by chance, in this particular pair of photographs. The main cause of the illusion, and probably the entire cause when you are seeing it “live,” is the deceptive line of the railing, that makes it appear that they are standing on a slope. ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [July 13, 2010, 6:02am UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/17 "2010-07-13T06:02:29Z")
A lot of the other “Mystery Spot” effects appear to arise from things that are actually sloping being disguised to look as though they are not sloping (or sloping in the opposite direction). Obviously this illusion could be accomplished in that way too. The boards the people stand on could be at different heights. I am assuming that is not the case for this particular illusion because, first of all, the OP says she checked the boards and found them to be level and, secondly, because it seems to me that any difference in height of the standing places would be very obvious to the people who actually stand there, and then change positions. Thus, in this case, it appears that the illusion is induced by making it look as though there is a slope when there actually is not. ---
**Author:** ![ZipperJJ](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zipperjj/32/211_2.png) [@ZipperJJ](https://boards.straightdope.com/u/ZipperJJ)\ **Post date:** [July 13, 2010, 1:25pm UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/18 "2010-07-13T13:25:37Z")
> [@In\_Winnipeg](#): > > \ > > It’s in California now? I have a bunch of postcards from some trip my grandparents took back in the 50s, and this used to be in the UP of Michigan. The postcards are very similar to the pictures at the website in the OP. In the linked Wikipedia article, it says there are other mystery spots around America, including in the UP. ---
**Author:** ![Wheelz](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wheelz/32/5718_2.png) [@Wheelz](https://boards.straightdope.com/u/Wheelz)\ **Post date:** [July 13, 2010, 3:57pm UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/19 "2010-07-13T15:57:08Z")
It seems to me that the fence is mostly what creates the illusion. The shorter girl is also definitely standing up straighter in the second photo. In fact, both girls appear to be standing straight up in the spot on the right and leaning backwards a little when on the left. Something must be encouraging them to stand that way. ---
**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:** [July 13, 2010, 4:17pm UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/20 "2010-07-13T16:17:13Z")
I’m not seeing any effect at all in those linked photos. In both of them, the dark-skinned girl appears to be taller, by about the same amount. Maybe my brain (and **panamajack** ’s) just uses a different set of perceptual cues than most folks’. [Next page](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478.md?page=2)