Twilight

September 02, 2020  •  Leave a Comment

Birdhouse fencepost, northern CO Birdhouse fencepost, rural road, northern Colorado; HDR merge

Birdhouse fencepost, rural road, northern Colorado; HDR merge.

Twilight. It's not about cute vampires. It's about blood-sucking overexposure. Bone-chilling underexposure. Palpably inadequate tonal range. A challenge that occasionally comes up in landscape photography is shooting at twilight into the setting sun. The sky tends to be blown out in that situation — the highlights are "clipped" or contain no picture data — and the foreground, in low light and shadow, is usually too dark. One answer? Extreme exposure bracketing. Exposure bracketing is the process of shooting multiple photos of the same subject and adjusting the camera's exposure settings to brighten or darken the image. We're not talking about three frames at 1/3 intervals here, but five frames separated by a full stop (one EV), or three frames at two stops.

Using these intervals, you'll be capturing the scene with exposures ranging from a quarter of the brightness to four times the brightness of your starting exposure at a given ISO. And then you can merge the files later to adjust for the great differences in light. Because it can get confusing talking about exposure compensation in relation to exposure values (EV), let's use the abbreviation eC to refer to compensation settings and back up a little to explain what EV actually is.

Technically, EV is the result of a formula that takes into account both aperture and shutter speed and expresses it in a single number, using the binary logarithm (log2): EV = log2(N²/t), where N is the aperture and t is the shutter speed. You can figure it out easily enough with a log base 2 calculator (there's one at miniwebtool.com/log-base-2-calculator/). For f/2.8 (2.8² = 2.8×2.8 = 7.84), for instance, at a shutter speed of half a second (7.84/.5= 15.68), the EV is log2(15.68) = 3.97. Let's call it 4. And frankly, that's not a great deal of help to you unless you're planning on creating an exposure chart to consult for particular lighting situations — something that the technology in your camera nowadays makes unnecessary (though slightly more important if you use a hand-held light meter).

But some folks like the math side of things, and if you want to spend an exhilarating evening (if you're mathematically excitable) or an excruciating, soul-deadening one (if you're not), the Internet has literal pages and pages of this stuff. Photography is optics. Optics is mathematics. Knock yourself out. But photography is also an art.

The reason I took the detour into the exposure value formula is to make the point that all you really need to know about exposure bracketing is that most cameras today allow you to compensate for challenging light conditions by changing the photo's exposure in parts or multiples of 1EV. In other words, you only need to attend to the relative light values of the frames rather than what they represent, bearing in mind that because aperture, shutter speed and EV are all directly related to one another, full intervals have a straightforward 1-1 relationship.

The difference between an aperture setting of f/4 and f/5.6 at ISO 100, with shutter speed constant at 1/30, is 1EV. Between a shutter speed of 1/125 and 1/250 at f/8? 1EV. And so forth. So we're not interested in the formula as much as the relative effect of changing your settings by +/- 1EV (one stop), or a fraction thereof.

The exposure compensation dials of most current DSLRs allow adjustments in one-third (.3) or two-thirds (.7) stops, in addition to single stops, two stops etc. And with auto exposure bracketing, you set the intervals, tell the camera which side of your starting exposure setting to make the changes, and then fire off a burst of three or five frames, or more.

Piece of cake. The art comes in deciding what intervals are going to cover the scene's lightest and darkest features. You want the lightest photo to illuminate all the detail in the darkest areas; you want the darkest photo to provide some picture information for the lightest areas. In fact, software like Photomatix Pro will give you a report: of your three images, say, the underexposed photo covers the brightest parts of the scene well, but the overexposed photo may not cover the darkest parts of the scene well enough, in which case you may want to include a longer exposure in the group. We'll show an example of just that with our twilight shot shortly.

Now, exposure bracketing may vary depending on your camera's make or model. I'm a Nikon guy, and my D850 has a dedicated button for bracketing (either or both of exposure and flash). The exposure bracketing options are considerable: up to nine frames at 1EV intervals, five frames at three full stops (3EV intervals), and various choices for three frames from 1/3 intervals to 3EV. In short, more than you'll likely need in any lighting situation. But even my old CoolPix B700 (an ultra-zoom "bridge" camera, not a DSLR) has exposure bracketing, though a more humble version, allowing 1/3, 2/3, or 1 full stop either side of the selected exposure compensation setting. I'd say either side of 0.0. eC, but it's possible to set the starting exposure to -1/3, say, and bracket either side of that. The curious thing about the Nikon line is that the D3500, a true DSLR, albeit an entry-level crop sensor camera, doesn't have auto exposure bracketing. You'd have to shoot three frames of the same subject, manually changing the settings between shots. 

Why do we need exposure compensation, then? Aren't the technological marvels we use as photographers today capable of getting it right the first time, every time? Well, no. The cameras are incredibly sophisticated, true, but some situations can still trick the sensors into giving us inadequate readings, and some conditions are simply too extreme to be captured satisfactorily in a single shot. Snow and white sand, for instance, throw off the camera's judgment. It overcompensates and underexposes the image. With snow, counter-intuitively, we need to increase the exposure by 1/3 or 2/3 to adjust for the faulty reading. To compensate for the over-compensation, if you like. Sometimes, we just want to hedge our bets. Maybe the lighter image will be best, maybe the darker. With extreme conditions, where the range of light is too great — well, we get back to our twilight evening.

It's a rural road in northern Colorado, horse properties and houses on one side, with their roosters and vegetable gardens, cornfields on the other. Traffic is light: a few commuters are coming home. There's an old dairy down the road, family owned, well-established, and given new life by partnering with the Australian yogurt company Noosa, which shares the site now. A black pick-up heads north to the feedlot. Black Angus cattle dot the hillside, a skunk follows the shoulder, along the field's edge; mule deer watch from under shade trees. It's a warm mid-summer evening, dark earlier now than at the solstice.

I usually like to shoot the little birdhouse on the fencepost in winter, under steel-blue skies. The corn stubble is patched with snow then, and the incongruous wooden facade of the nest box rises above the austere geometry of the landscape. But the corn is high now, close to harvest. The picture elements are still spare — fence, corn, sky — but looking west, with the sun near setting, the corn stalks lie in deep shadow, the horizon luminous. What are the choices? I'm on aperture priority, ISO set at 1250 and auto sensitivity off. I don't want the ISO to change between shots, just the shutter speed. I can take five frames at 1EV intervals, but I elect instead to shoot three frames at two-stop intervals. The reason? There's enough of a breeze that any movement of the leaves or the tassels behind the fencepost will need to be aligned when the frames are merged later, and the more frames there are, the longer the total shooting time will be, and the greater the chance of movement. Even with a tripod, then, the final image may show some haloing or ghosting where alignment isn't exact. So, faster is better.

When it comes to merging the exposures later, I also have choices. We could import the three images into Adobe's Lightroom, select all, and then choose Photo > Photo Merge > HDR Merge. In Photoshop, on the other hand, we go to the File menu, then Automate > Merge to HDR Pro. But I favor Photomatix Pro for its controls and the smoothness of the result. Result in hand, you'll still want to make adjustments, reduce noise, dodge or burn here or there, convert to grayscale perhaps — but you should have an image with plenty of picture data and tonal range to work with, particularly in bringing out the sky and clouds. As it turned out, Photomatix Pro wasn't entirely happy with me: it didn't think the lightest photo did a good enough job of revealing the darker details, so I added as fourth image, more overexposed, and adjusted the intervals so I had two frames with 2EV between them on the light side of my starting exposure (0.0 eC here), and only one on the darker side. It did the job just fine.

8398-8401_progression 14008398-8401_progression 1400

Taking the actual photos on the dairy road, with a little preparation and experience, was the work of a few minutes. It was still warm when I left, but the light on the cornfields had dimmed. A skunk sniffed at my shoes. Bats took to the air. Sia popped out from between head-high stalks and started singing "Dusk Till Dawn". I could have imagined that last one. Rural night plays tricks with the senses. Time to get home and get merging.


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