3 Point Poser

You're familiar with how, in a strong upshot or downshot, the verticals of buildings "fan out" or converge, rather than being parallel. We see this in 3-point and 2-point-vertical views, as well as fisheye. Those verticals converge upon the VP that we're most nearly looking at, and widen on the opposite end of the picture. You can see this effect dynamically by panning around within Street View in Google Maps or Google Earth: One end of the picture squishes and one broadens as we mouse to and fro, the squishing happening where a VP comes near our frame of view.

So why is the opposite happening in this picture, made long before Photoshop made such distortions easy? The horizon isn't high in the frame (the view seems almost as much upshot as downshot) and the buildings and cable car which we are necessarily looking up at--given their appearing above the horizon--broaden at the top like thunderclouds. Weird, right?

Archives San Francisco Chronicle

Shouldn't those verticals be, if anything, leaning in toward the top?

I have two hypotheses, one simple, one conjectural:

One

I think what happened here was that the photog shot with a short (wide-angle) lens aimed down, putting his center of vision on the ground, not among the Santas' bellies but their boots. Then the bottom of the picture was cropped off. Just that simple. The horizon was in fact high in the frame, but the crop conceals that.

BTW, we don't see things like this this thunderhead effect happening with our eyes, because our peripheral vision is weak and lacks detail.

But in cameras the image receiving surface (where film once lay) is flat. So whatever perspective logic is dictated by the lens setup -- in relation to the physical scene -- prevails over the whole flat receiving surface. Point a wide-angle camera at the ground a few feet away, and the verticals converge toward the bottom and widen at the top, consistently, mathematically. There is in this case no muting of the effect: no fisheye to curve the straight lines, or imperfect spherical retina to lose details. The resulting perspective prevails both above and below the horizon, as all perspective does. That basic fact is one of the first things beginning students either grasp, or get caught up short on.

Two

2A

The second hypothesis is worthy mostly just because it leads to a movie that feels like a tiny tropical vacation (You'll see.), but I advance it seriously:

It is that some cameras allow the user to tip the receiving surface. That could create this thunderhead effect. If the surface was tipped to make the image of the building tops have to travel a tiny bit farther in the camera's inner space, they would spread bigger.

2B

The same could be accomplished with the now-quaint optical print-making device known as the enlarger. The challenge in either method would be to not have major parts of the image go out of focus. This could probably be managed by using smaller aperture and longer exposure.

The thunderhead effect may have been in vogue in news photography, c. late 60s -70s - ? Yes, to judge by the collection of vintage SF Chronicle cable-cars photos I got this from, which contained at least one other example of similarly counter-logical perspective. Shooting this way adds jazzy, energizing diagonals, something I'm forever telling comics students and myself to do, to avoid dull drawings.

So how do I know that tipping that receiving surface risks out-of-focus results? Because that very adaptation, called tilt shift, can be used to create an artificially abbreviated depth of field. This is one of the several tricks used to accomplish this bizarre and utterly singular effect (see below), which you may also have seen in insurance commercials on TV. The other tricks are heightened color saturation and sped-up motion. Uncanny how they combine to create a world that looks real and toy at the same time, isn't it?

JH

Mod 2 -- I guess you had to be there-- and I mean right there.

I was walking through my unglamorous corner of San Francisco and saw this sight:

Unremarkable, right? Maybe at first and second glance. But it reminded me of a mistake that beginning students often make with cast shadows: Having the shadow take on a simple shape, like this triangle, that falls onto the ground intact without respect to any incidental objects it may hit, such as the weeds. As in this D-winning student work, where the (totally unrealistic) shape of the cast shadow just powers through the second flower pot as if it wasn't really there, or...

...was flat artwork like this trompe l'oeil chalk art.

So how could this be happening in real life? How could the shadow just act as if the weeds weren't there? The short answer is that it was totally dependent on my having a particular position, and thus a particular line of sight. Similar to the way that this sidewalk art...

...only looks right from one precise position in space. (The very same position where last night the cunning chalk artist set up a projector on a tripod to show him exactly what to draw.)

The shorter lesson of this is: distrust a shadow that's a simpler shape than the object what done made it.

The more involved answer is that I happened to be standing where two shadow-edges with two totally different directions appeared continuous. I'm betting it could only be seen thus from that one position, among the infinite others possible.

The weeds, by being very robustly 3D and--with their fine leaves--effectively semi-transparent, were showing me something that you don't normally see except in fog or airborne dust: the passage of a cast shadow through 3D space. I just happened to be standing where the sloping plane of the tent-like volume of the cast shadow falling through the weeds was (1) raked by my eyeline, (2) was foreshortened from my end-on vantage point as to appear to be a line and not a plane, and (3) just happened to appear continuous with the edge of the cast shadow of the old house's gutter on the sidewalk, despite being at a radically different angle in space.

Just a step and a half to the left, and the view was quite different, and befit the real complexity of the situation:

JH


P.S.: If you are really into this kind of thing, you will understand why the sloping sidewalk was necessary for the creation of that first scene above: The slope disrupted the normal parallel relationship that the gutter and its shadow would have on level ground.

Welcome to Spring!

Dear ILL625 Spring '11 Students,
Please check out this video, which may help mentally prepare you for this class. Here's wishing everybody a great semester!
(BTW If you chance to look at older blog entries, you will find different and more involved terminology than we're using now, which may or may not be helpful to you.)

JH