Your Eyes Will Fool You

This is not my lunch’s view after I swallow:

It is, rather, a picture I took inside a steel tube post – an HSS section, to be pedantic – used as part of some newish dunnage on an old industrial building. I positioned my phone’s camera lens and flash over the end of the tube and here’s the photo.

The thing that looks like a bar crossing the interior space of the tube is actually a plate, which extends out on both sides to provide bolted connections for two beams. The fabricator cut a slot in the tube wall at two locations 180 degrees apart, slid the plate through, and then welded it to the outside face of the tube, leaving the rather rough and obviously unreachable edges you can see here on the inside.

It sure looks like the tube is oval in section, longer in the direction that the plate spans, and shorter the other way. It’s not: your perception is being fooled. The outside of the tube is clearly round (within the usual tolerances) and I’ve never encountered a steel tube where the wall thickness varied much at all. At the plate, you’re seeing only as far as the plate – it’s visible length is the diameter of the tube. On either side of the plate, you’re seeing further back into the tube, which, being further away, looks smaller. Your brain, without asking you, comes to the logical conclusion that the tube must be oval since it’s clearly longer on one axis than the other. Even knowing what’s going on doesn’t change the optical illusion.