cgardner
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In real life we get many of the clues about shape from our stereo vision, but photography is based on optical illusion; tricking the brain into thinking a 2D pattern of contrast represents a 3D object. There are many forms of contrast: tone, color, relative sharpness, relative size, etc. and the brain relies on them all to match the patterns it sees with the memories of stored objects.
Human perception is an interest of mine and something I've studied outside of the narrow confines of photography. It starts with an understanding of the physiology of the eye. The color sensing cones are concentrated in the center 2 degrees of our FOV - about twice the width of a thumb held at arms length - and their sensitivity is more analogous to the "a" and "b" channels of an Lab file than an RGB file. The balance of the retina is covered with rods which are only sensitive to a relatively narrow range of blue/green light. They are also about 3000x more sensitive than the cones. That physiology explains why red lights are used on ships at night and why at night you notice movement out the corners of your vision before you notice it in the center.
Because the color sensing cells are in the center of the FOV the brain tends to tune out everything in the periphery, a form of tunnel vision. We don't consciously think about the shallow DOF our eyes have because the eye darts around and our short-term subconscious memory stitches together the small focused snippets into the perception of a panorama which seems to be entirely in focus and ideally exposed.
When shallow DOF is used in a photograph it mimics the way the brain tunes out all the less important stuff. It sends a subliminal clue to the brain of the viewer that the sharper stuff is more important and will pull attention to it. The fact the rods are much more sensitive means that bright contrasting (with the background) areas on the edges of a photo can be very distracting and pull attention off what is in the center.
One of the reasons the rule of thirds works very effectively when a dynamic feeling is desired in a photo is because it will tend to pull the eye of the viewer across the 2/3rds of the photo containing the less important details, but then show relatively little space beyond the main center of interest. It tends to produce an ideal buffer of "negative" space which stops the movement of the eye across the frame on what is intended to be the center of interest. Still photos which cause the eye to move across the frame that way evoke a sensation of movement in the mind of the viewer: if you trace a path in photo you feel as if you've walked down it. The flip-side of the ROT is centered static image, which keeps the viewer focused in the center because there's no clue where to go next, left or right. Off center horizontal crops of headshots don't work very effectively in the compositional sense because the "negative" space is so large it tempts the viewer off the face to go look at it, causing the last conscious thought to be "Wow! Great Bokeh..." instead of "What compelling eyes!"
Its really not about following rules or breaking them, simply understanding the cause and effect of image placement in the frame on perception, both physiologically and emotionally. The process of vision is mostly one of pattern matching. There are specialized part of the temporal lobe, the Fusiform Gyrus which a dedicated to remembering faces and familiar objects and connecting the images with emotional reactions. Its part of way strangers will react to to a photo put up for C&C differently and often more objectively than the photographer who took in and has first hand knowledge of the subject and also emotional connections to the act of taking it.
The strongest clues about 3D shape come from tonal contrast. One of the more interesting jobs I had was making maps at National Geographic. I'd take the hand-drawn relief plates created in pencil by the cartographic artist and create halftones and color separations. In the case of simple B&W relief plates the artist would simply draw the shadows the mountains would cast when the sun was at a 45 degree angle. That's enough to fool the brain into thinking there are 3D mountains on the map. Short lighting a face works in a similar way for same reason. Placing the key light 45 degrees from the nose and above the eye line causes the shadow of the nose to fall along the base of the nose and over the top of the nostril, covering about 1/2 of it. Our brains, seeing the shadow, can infer the shape of the nose more accurately that with other patterns. That's why a short lit face looks more "real" and 3D than a face lit with most other patterns.
But the strongest illusion of 3D shape in a photo occurs when the strongest light hits the object from behind and the camera shoots into the shadow side. Just consider how the moon looks in its phases: flat when flat lit, but like a sphere when the light hits from behind. A good exercise for someone seeking to understand the relationship of light angle and shape is to take an object like an egg, illuminate it with a single light, then walk 360 degrees around it taking a photo every 15 degrees.
http://super.nova.org/TP/Egg.jpg
The conventional four-light portrait set-up addresses the need to create spacial separation and define shape with contrast. The foundation is fill, necessary because the camera can't record the full range of contrast in a 3D scene lit with a single-source:
http://super.nova.org/TP/LE01.jpg
In nature the fill comes from the sky and wraps around everything 360 degrees. The closest practical equivalent indoor is to place the fill over the camera where it will illuminate every nook and cranny the camera sees. Fill placed to the side will create shadows and result in unfilled voids.
On a dark background a separate light is used to create visual separation...
http://super.nova.org/TP/LE02.jpg
The need for a background light can be eliminated by simply selecting a lighter background and letting the fall-out from the fill illuminate it.
The "key" light overlapping the fill creates the illusion of shape. The perception of the light and object being "hard" or "soft" is a clue the brain gets mainly from the tone of the shadows. The darker the shadows (a function of the fill) the harder the lighting will become. I learned this back in the 1970s shooting weddings with two direct flashes. Precise placement of the key light relative to the face at 45 degrees created flattering short lighting pattern, and making that pattern seem "soft" was simply a matter of independently adding fill to reveal the detail in the shadows.
http://super.nova.org/TP/LE03.jpg
The placement of the key light at 45 degrees from the nose is cause and effect photographers learned by trial and error. There's no "rule" it simply a common sense solution based on the need to get light into the recessed eyes and model the nose in a natural but non distracting way. Like the map relief, lighting a still life subject at a 45 degree angle will mimic the angle of objects seen outdoors in the morning and afternoon (10 and 2 o'clock) and will make the shape of the objects be perceived more "naturally". But as mentioned the strongest illusion of 3D comes when an object is back-lit, so the addition of a "hair" or rim-light component its the forth ingredient in a coventional four-light scenario.
http://super.nova.org/TP/LE04.jpg
The "magic" thing about the photographic process is that when both the shadows and highlights are both exposed optimally, all the tones in-between fall perfectly into place because the process is engineered to create a linear perceptual response. "Zone 5" middle gray always falls between white and black perceptually. What changes with various media -- B&W neg, color neg, color transparency, digital - is the overall dynamic range measured in f/stops between the point where highlight detail is retained naturally and the camera can no longer resolve the shadow detail.
Its trivial to match the range of a scene to the sensor indoors with artificial light by starting with enough fill to record the darkest shadows, then adding other overlapping lights from different angles to create "key" and "accent highlights as illustrated above. Outdoors the problem is different because in most situations the ambient lighting exceeds the range of the sensor. But the lessons learned indoors about controlling contrast to create a realistic full tonal range can be applied outdoors.
Recording a full tonal range outdoors starts with keeping the ambient highlights under clipping. That takes a leap of faith like bungie jumping because in a cross-lit scene exposed for highlight detail will look like this:
http://super.nova.org/TP/DR_Backlight.jpg
The midtones and shadows are rendered darker than normal in that 20D shot exposed to retain detail in the white highlights because the range of the scene exceeded the range of the sensor. But shooting into the shadow side of the ambient light, with minimal overlap, allows flash to be used to lift the shadow side INDEPENDENTLY.
http://super.nova.org/TP/DR_FlashFill.jpg
That independent alteration of the shadow side of the ambient lighting is what allows the foreground to be lifted to where it is in perceptual balance with the background. The flash actually doesn't "fill" in the technical sense because it creates highlights over the top of the fill from the sky and the parts of the face and body not hit by the flash will have about the same tone as it did before the flash was added as illustrated in this single flash outdoor shot.
http://super.nova.org/TP/BelenBacklitEyes.jpg
The illusion of 3D in the face in that single flash outdoor shot is created by the contrast between the flash created highlights and the sky-lit shadows. Its easier to see the pattern when the image is blurred..
http://super.nova.org/TP/BelenBacklitBlur.jpg
I selected the river as the background so the white sweatshirt wouldn't distract from the face and used the natural rim light to define the shape. I relied on the flash to create a flattering highlight pattern of the face - pretty much a no-brainer with a flash bracket and a full-face pose. Combined the overall effect of the contrasting dark hair and pattern of highlights within the frame of the hair would hopefully guide the eye of the viewer to the face and hold the attention there because there aren't any other distractions.
http://super.nova.org/TP/TwoStages.jpg
When shooting outdoors I always have a flash on a bracket on the camera and look for opportunities to put the sun behind the subjects because I know: 1) its the only way technically to match scene to sensor and get a full range of tone, and 2) it creates the strongest illusion of 3D in a 2D photo. The bracket is a critical element because it is the downward angle of the flash which makes the light it contribute look more natural by mimicking the angle of natural light at 10 or 2 o-clock..
http://super.nova.org/TP/FillFlashOutdoors1.jpg
http://super.nova.org/TP/FillFlashOutdoors2.jpg
http://super.nova.org/TP/FillFlashOutdoors3.jpg
http://super.nova.org/TP/LilleyDock.jpg
http://super.nova.org/TP/LilleyMotor.jpg
http://super.nova.org/TP/InsectFly.jpg
http://super.nova.org/TP/InsectMantis.jpg
http://super.nova.org/TP/InsectSpider.jpg
Pro body? All the shots above where taken with a 5MP Minolta D7Hi. The ones below where taken with my 20D:
http://super.nova.org/TP/Longwood1.jpg
http://super.nova.org/TP/Longwood2.jpg
http://super.nova.org/TP/MC06.jpg
http://super.nova.org/TP/10mm_ReflectingPool.jpg
http://super.nova.org/TP/10mm_NatHist.jpg
http://super.nova.org/TP/154.jpg
http://super.nova.org/TP/WTcake1.jpg
http://super.nova.org/TP/BRLogoGear.jpg
http://super.nova.org/TP/BR_Golf_0350.jpg
http://super.nova.org/TP/BR_Aceman1.jpg
http://super.nova.org/TP/BBB_Apples4.jpg
It's not the gear, its knowing how to fool the brain of the viewer with contrasting tone, color, sharpness, etc...
Chuck
Edited on Nov 25, 2009 at 05:30 PM · View previous versions
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30D and that's what ticked me off. 
