# 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:** 8\ **Page:** 2
**Author:** ![RickJay](https://avatars.discourse-cdn.com/v4/letter/r/bb73d2/32.png) [@RickJay](https://boards.straightdope.com/u/RickJay)\ **Post date:** [July 13, 2010, 4:55pm UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/21 "2010-07-13T16:55:52Z")
> [@Chronos](#): > > 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’. If you carefully line up common points of reference the shorter girl does seem to have gained an inch or two at some points. In Photo 1, Short Girl’s nose is level with the bottom of Tall Girl’s chin. In Photo 2, her nose is level with Tall Girl’s mouth, about a two inch gain. Their shoulders are also a few inches closer to each other in Photo 2. However, at other points they appear even. The relative position of their knees doesn’t change at all, for instance. The bottom of Tall Girl’s jacket is about even with the bottom of Short Girl’s white shirt in both photos. That suggests to me that the illusion in the photograph is completely different from the illusion experienced in real life. In the photo, there is very obviously no change in real height; the short girl’s coming up a bit taller solely because they are holding their heads differently, because Tall Girl is shrugging in Photo 1 (making her shoulders look higher) and because (this is very easy to see) Tall Girl is standing closer to the camera than Short Girl in Photo 1, whereas in Photo 2 they’re even. That’s it. The effect of the sloping rail doesn’t have much of an effect on your eyes in the 2-D photo. In PERSON, the illusion would be different (and probably much more striking) and would be created by the deceptive surrounding visual cues fooling your brain’s sense of depth and level. I assure you in person you’d be fooled; these places are phenomenally persistent illusions. ---
**Author:** ![Superhal](https://avatars.discourse-cdn.com/v4/letter/s/b4bc9f/32.png) [@Superhal](https://boards.straightdope.com/u/Superhal)\ **Post date:** [July 14, 2010, 3:05am UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/22 "2010-07-14T03:05:39Z")
I’m not sure what the question is. It’s an optical illusion, the girls are the same height in both pictures. Whenever I see this type of illusion, I test it by blanking out the background. In this case, I covered everything in the picture except their faces. The darker girl is taller in both pictures by about 3 inches, the top of the lighter girl’s head is at the eye level of the other girl. It works for [this picture](http://www.illusionking.com/moving-beans-optical-illusion/) too. If you make a hole in your partially closed fist and then look through it, the beans stop moving. ---
**Author:** ![Max\_Torque](https://avatars.discourse-cdn.com/v4/letter/m/e9bcb4/32.png) [@Max\_Torque](https://boards.straightdope.com/u/Max_Torque)\ **Post date:** [July 14, 2010, 2:22pm UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/23 "2010-07-14T14:22:10Z")
Adding another video to the mix: [the Ames Room scene from **Temple Grandin**](http://www.youtube.com/watch?v=chxCNEsu3YU). The trick is excluding outside visual cues that enable you to see how the space is constructed. ---
**Author:** ![BigT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bigt/32/12044_2.png) [@BigT](https://boards.straightdope.com/u/BigT)\ **Post date:** [July 15, 2010, 11:10am UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/24 "2010-07-15T11:10:36Z")
> [@Chronos](#): > > 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’. I was confused, myself. The board looks slanted, but they seem to be the same size. I thought the trick was that the board appeared slanted although it really wasn’t, and that the girls seeming to be the same height was the proof that it wasn’t. ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [July 15, 2010, 3:16pm UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/25 "2010-07-15T15:16:10Z")
> [@BigT](#): > > I was confused, myself. The board looks slanted, but they seem to be the same size. I thought the trick was that the board appeared slanted although it really wasn’t, and that the girls seeming to be the same height was the proof that it wasn’t. Well, of course, they _are_ the same size, and if you are going to carefully scrutinize the relative differences between the heights of the two girls’ heads in the photos, measuring with your eyes, as it were, you are going to find that the difference is much the same in each picture, because it _is_ the same. However, for most people at least, if they casually glance at the two pictures, in the left-hand one the darker girl appears to be distinctly taller than the fairer one, whereas, in the right hand picture, the darker girl looks only a little taller. That is what an illusion is. Something _appears_ to be different from how it really is, but that does not mean that you cannot discover how it really is if you examine it carefully enough. In some cases that may involve something like taking a measuring stick to it, but not in all cases. For some illusions, just looking more carefully can reveal the reality. Probably, when people are shown the effect “live” there is enough hurry and distraction that they are almost certain to only be able to take in a quick, impressionistic glance or two, under which conditions the fact that their relative heights do not really change would be much harder to spot. You will not be given the chance to carefully scrutinize. You (and **Chronos** ) may, by now, have trained yourselves into a way of looking at the photos such that you will not be able to see the illusion (in those particular photos) at all. Some (by no means all) illusions disappear or become much weaker when you know the truth. This may well be one. (I do not not mean when you know how the illusion works. That is not necessary. I mean when you know it is an illusion, and you know what the facts that you were being deceived about arein reality.) ---
**Author:** ![EVIL\_COW\_PIE](https://avatars.discourse-cdn.com/v4/letter/e/7ab992/32.png) [@EVIL\_COW\_PIE](https://boards.straightdope.com/u/EVIL_COW_PIE)\ **Post date:** [July 15, 2010, 6:23pm UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/26 "2010-07-15T18:23:57Z")
> [@RickJay](#): > > In PERSON, the illusion would be different (and probably much more striking) and would be created by the deceptive surrounding visual cues fooling your brain’s sense of depth and level. I assure you in person you’d be fooled; these places are phenomenally persistent illusions. Having been to the Mystery Spot I can attest that this is true. Interestingly the ground around these boards (outside of say a three foot radius) was on an incline. Our group of about 25 was standing in a semi-circle around the people on the boards. So different people at different heights viewing from different angles all elicited the same ooh-aaah response. Of all of the illusions at the Mystery Spot this one was the most perplexing. ---
**Author:** ![EVIL\_COW\_PIE](https://avatars.discourse-cdn.com/v4/letter/e/7ab992/32.png) [@EVIL\_COW\_PIE](https://boards.straightdope.com/u/EVIL_COW_PIE)\ **Post date:** [July 15, 2010, 6:32pm UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/27 "2010-07-15T18:32:36Z")
Missed the edit window. [Here is a photo](http://image12.webshots.com/13/3/18/70/150831870WexPMs_fs.jpg) shot from the other direction that will give you and idea of what I mean. There is no special vantage point that makes this illusion work. ---
**Author:** ![Normal\_Phase](https://avatars.discourse-cdn.com/v4/letter/n/5f9b8f/32.png) [@Normal\_Phase](https://boards.straightdope.com/u/Normal_Phase)\ **Post date:** [July 15, 2010, 8:43pm UTC](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478/28 "2010-07-15T20:43:11Z")
Can’t see the photo, but if the ground (other than right where they are standing) AND the fence line all suggest a slope, the illusion isn’t exactly difficult to explain. It just goes to show that the brain’s perception of height is not absolute, but strongly influenced by where “level” seems to be. (Or doesn’t.) [Previous page](https://boards.straightdope.com/t/help-me-understand-this-optical-illusion/546478.md?page=1)