Sunlight, Contrast, and Exposure
By Carl Weese
There is a fairly common misperception that clear sunlight is "high contrast" light that challenges the capabilities of digital sensors and film. This isn’t so, but I think I know some of the reasons for the misperception.
Our perception of strong sunlight includes its harshness. In bright sun we squint, shade our eyes, reach for sunglasses. Sunlight feels harsh even though it can be flattering for some subjects. I like to photograph buildings in hard sunlight. People or flowers, not so much. But that's an aesthetic choice; it has nothing to do with the ability of photographic media to record a scene in strong sunlight in an accurate and convincing manner.
The dynamic range of a subject, and so the contrast we need to encompass on film or sensor, results from two distinct factors. First there is the reflectance of the subjects in the picture. Few natural objects reflect much less than 10% or much more than 90% of the light that falls on them (recall Ctein’s recent post dealing with the similar limit to the reflectance range of a photographic print). This means that under even lighting, sheer difference in subject reflectance is hard pressed to offer a 4.5 stop range: a white egg on a piece of black velvet is no problem even for a short range (high contrast) color transparency film. But if one part of a uniform subject—imagine a large piece of gray cloth, or a lawn seen from a distance—is in strong light and another in deep shadow, the difference in light reflected back to the camera can be enormous even though the subject is all one tone, a difference much greater than the potential reflectance range of different objects.
So the luminance range of a scene and the difficulty of recording it depends on the stuff in front of the camera, and even more on the nature of the light. Which brings us back to hard-edge, squint-inducing sunlight. When the sunlight comes from behind the camera ("near-axis") it is one of the lowest contrast situations you can encounter. All major surfaces of the subject receive precisely the same light. Axis light makes for shadows so narrow that they just don’t matter and we are left with only the reflectance range of the subject matter itself, not even five stops. Black and white film swallows this range so easily that a print from such a scene on normal contrast paper is apt to be dull and lifeless. Grade 3 or 4 is likely to be needed (or a Zone System style expansion development can be given to the negative). With a typical DSLR camera this range is so short that we’ll probably want to override the camera’s meter and add some exposure in order to "expose to the right" so the recorded data falls in the rich right-hand side of the histogram, then adjust the tones to a full range later, like changing paper grades in the traditional darkroom.
Waterbury, CT, 2008. The sun is behind and above the camera, near-axis light. The only significant shadow is that of the stop sign, and because it lands on light sidewalk, it's bright and open. A shadow on a light surface gives a middle tone, not a dark one.
What happens when the subject is lit from the side, say at an angle between 45° and 90°? Now the shadows widen and we probably want to see into them. That means the film or sensor has to record the reflectance range of the subject matter plus the difference of intensity in the light falling on highlights and shadows. But this is still no big deal.
Torrington, CT, 2009. Even with the sunlight fully from the side, the tonal range is no strain for the sensor of my Pentax K20D, in fact I raised the ACR slider for Black Point a bit to reinforce the deepest tones.
As the highly informative data sheets that used to come in film boxes would have told us, there is generally a three stop difference between full sunlight and “open shade” (open shade is a subject position where direct sunlight is blocked but the light of the open sky is available). In practice this means that the sunlight parts of the picture are receiving at most three stops more light than the broad (open) shadows.
Torrington, CT, 2009. The street and wall are in "open shade," lit only by the overhead blue sky with no direct or bounced light, but the sensor manages to hold detail all the way up to the white signs in full sun. I used a modest amount of both Recovery and Fill Light in ACR.
The exposure difference between light and shade is often less than three stops because of light bounce from objects in the environment. If there are clouds in the sky, shadow values can rise more than a stop because white clouds reflect sunlight down to the ground. So a scene with a typical distribution of subject values in a situation where there is a bit of bounce light and a few clouds in the sky may add only two stops of dynamic range to the reflectance range of the subject. So maybe seven stops. This pushes the range transparency film can record, but is well within range for modern sensors, and a piece of cake for negative film, as long as the exposure is set correctly.
All the dynamic range in the world won’t help unless the exposure makes use of that range intelligently.
Meters for reflected light, beginning with the earliest selenium cell units down to the computer-enhanced systems in modern digital cameras, all work on the basic principal that the average of values in a scene will be "in the Middle." Yes, modern equipment uses highly sophisticated algorithms to guess when a scene isn’t actually of middle value, but I've yet to encounter a system that won't render a snow scene as middle, or maybe light, gray, instead of a more accurate bright (but not burned out) white. A scene with mostly dark or shadowed objects and just a few smaller bright areas will also give the smartest camera systems fits.
More important for understanding how to expose, no photographic recording medium records values centered on what we perceive (and metering systems measure) as Middle Value.
Color transparency films, Polaroid direct-positive materials, and digital sensors all have a "High Middle." The correct exposure of middle values is not in the center of the available tonal range of the film/sensor, but closer to the high values. That is, if middle value subjects are exposed correctly, there is a relatively limited ability to record brighter values and a greater ability to read into the shadows. Also, the cutoff with overexposure, to clear film or clipped data, is abrupt and clear-cut. The slide into darkness is gradual and determined by our tolerance for muddier color, grain, or noise. Kodachrome could hold detail a good 2.3 stops above Middle, and as much as 3.5 stops below, depending on the specific picture and the way the transparency was to be used—viewed by projection, reproduced for the four color press through process separations, etc. When film separations were replaced by scanning beginning in the '80s, chrome films got effectively slower and more centered because scanners could retain more information from the thin highlights and less from the dense shadows, compared to good process camera seps. Digital sensors are rapidly improving, but even the best have less ability to read into the highlights than into the shadows.
Waterbury, CT, 2009. Full sunlight in the foreground and background, deep shadow (much darker than open shade) in the middle. I set the camera to "sunny 16" (1/320th @ ƒ/8 with effective E.I. of 80) to keep the highlights from burning out, then used a little Recovery in ACR to give them more substance along with a lot of Fill Light (stupid name but useful tool) to bring out some sense of texture in the deep shadows. Note the extra tone in the lower surface of the I-beams from light bounced back up from the pavement.
Negative films, either properly exposed and developed B&W or C-41 color negative, are also off-center, in an even more lopsided manner, but in the opposite direction. There is limited ability to read into the shadows, but an enormous ability to read up into areas much brighter than the Middle. In the picture above, using "sunny 16" with a negative film would be a disaster, leaving blank film in the shadows. To expose this scene on negative film, we need to measure the deep shadows and expose two stops less. The shadows will have full detail and while the fire hydrant technically will be overexposed, it will have plenty of tonal separations in the negative and so print or scan quite easily. On transparency film, all you could do is use sunny 16 and accept pure black in all the shadow areas. Or bring in an entire film crew to light the underpass.
Now for a digression: for as long as I’ve been using black and white films (since the early 1960s) experienced workers knew that to get the best tonality you needed to expose more (at least a stop more) and develop a bit less, than the manufacturer’s recommendation. In effect, the established standardized rating (known first as ASA and then ISO) was really about a one stop push. So Tri-X at its official speed of 400 wasn’t as bad tonally as Tri-X pushed to 800 or 1600, but it wasn't as good as when rated at 200 or 100. At the official rating, shadow values are weak and difficult to print even just two stops under Middle, and as film is pushed/underexposed this keeps getting worse while extended development makes negative highlights denser without necessarily improving their detail. (Thus comes the dictum of David Vestal about how to handle B&W film, "don’t underexpose, don’t overdevelop.") Careful densitometric testing of film speed to the Zone System standard of Zone I at 0.10 above film base plus fog (fb+f) also generally results in a tested speed one or one-and-a-half stops below the manufacturer’s rating. With proper handling, B&W film retains rich detail two stops or a little more below Middle and three, four, or even five stops above (with some darkroom work to retain those highlights.)
So, with negative film, you use an exposure that makes sure no important subject matter is exposed more than two stops below Middle, and know that the film will easily record subjects three or four stops above Middle, or even more with special development. When color negative films took a giant leap forward in quality starting in the late 70s, the big news was that they suddenly offered sharpness, fine grain, and saturation rivaling chrome films, but with at least the dynamic range of properly handled b&w negative films. Art photographers understood the significance and adopted color negative film immediately, while commercial photographers and, for a while, photojournalists, had to continue to use transparency films because clients refused to adapt away from the workflow of transparency film—but that’s a digression I won’t continue here.
With color transparency films, you make sure that no highlight is brighter than 2.0 or 2.3 (depends on the specific film) stops above Middle. Negatives or positives, you are trying to avoid winding up with clear film instead of recorded subject data. All of this means that with negatives you expose for the shadows and accept the generous highlights. With transparencies you expose for the highlights and accept what you get in the shadows, or fill them in with artificial light. With digital capture, it’s like transparency film, except there’s more range at both ends, while the short end with abrupt cutoff is still in the highlights so they are the critical exposure set point. The "paper towel test" is useful for getting this right with individual cameras.
Even full backlight is within range using a few tricks. In the shot above, there are bright buildings behind the camera, catching full sunlight over the roofs of the low buildings in the picture. A lot of light is bounced back to the building on the right. The full sunlight at center-left is glancing along the pavement at an acute angle which lowers its intensity. The exposure of 1/200th @ ƒ/5.6 is almost two stops more than "sunny 16" but everything is within range. What happened here is that a backlit subject has another backlit object behind it, so the sensor doesn't have to deal with the full power of direct sunlight.
So don't be afraid of the sun! The real problem of shooting in full sunlight is to find subjects that are flattered by that all-important point-source in the sky; the technical problems are really negligible. You can see more sunlight pictures among recent postings at my Working Pictures daily web log.
Next time, some lighting situations so much more demanding than sunlight that they really do present difficult, even impossible, problems not just for sensors but for negative films.