:: Manual exposure
|Camera:||Canon EOS-1Ds Mark II|
Metering and exposure is more about results than technique: If you’re getting the exposures you want, it doesn’t really matter how you meter. But if you’re one of those photographers who shies away from manual exposure because you’re overwhelmed by all the settings options (and maybe even flashback to learning to drive a manual transmission), let me assure you that manual exposure is really quite straightforward.
Some exposure facts
- A meter thinks the entire world is a middle tone and does everything it can to make it so: Sunlit snow? Without any input to the contrary, your camera will turn it a murky gray. Moonlight coal? Without any input your camera will turn it…, wait for it…, a murky gray.
- We measure the amount of light reaching the sensor in “stops”—think of a stop of light as you would a cup of sugar: Too sweet/bright? You need less sugar/light. Not sweet/bright enough? More sugar/light.
- Your camera has three exposure variables that control the amount of light reaching the sensor (or film):
- Aperture (the size of the opening that allows the light in; it’s measured by f-stops) controls depth: Large f-stops (it’s a ratio, so the smaller the f-stop number, the larger the aperture) allow more light, but they also shrink the range of sharp focus.
- Shutter speed (the duration that the sensor is exposed to light) controls motion: Long shutter speeds let in more light but they also blur anything that’s moving
- ISO determines how sensitive the sensor is to light: The higher the ISO, the greater the sensitivity, but the worse the image quality
- For every one-stop change in one exposure variable, you can make a corresponding one-stop change in another setting that will keep the amount of light reaching your sensor unchanged.
Understanding these basic facts is the cornerstone to controlling your camera and images.
Getting in the mode
Manual exposure is all about control: you choose an exposure target and dial in its tone–the camera guides you, but you make the ultimate exposure decision. A spot meter is the most precise way to pinpoint your target and therefore should be your metering mode (the way the camera “sees” the scene) when you expose manually.
Most of today’s DSLRs offer a spot meter; if your camera doesn’t, select the metering mode that uses smallest percentage of the frame (such as Canon’s partial mode). Whether you’re metering in spot or partial mode, zooming your lens as tight as possible will further increase your metering precision.
Controlling scene variables
Photographers have three scene variables to manage: motion, depth, and light. Not coincidentally, our cameras provide three exposure settings for managing the scene variables: ISO, f-stop (aperture), and shutter speed.
Landscape shooters on a tripod can take advantage of a one-to-one relationship between the three scene variables and the three camera exposure settings. In other words, for each scene variable, there’s a corresponding exposure setting. Understanding this relationship is the foundation for mastering manual exposure—once you get it (it doesn’t take long), you’re well on your way to full control of any metering situation nature throws at you.
Motion: ISO in a static scene
If you’re on a tripod photographing a static scene, motion isn’t part of the equation and you can automatically go with your camera’s native (best) ISO, usually 100 or 200. Pretty simple.
Depth: F-stop for depth and sharpness
With ISO handled, it’s time to determine the f-stop. Your f-stop choice starts with the understanding that there’s an ideal f-stop for each shot. Really. The two f-stop factors to consider are, in this order: your desired depth of field (DOF); and the f-stop at which your lens is sharpest.
Start with the f-stop that returns your desired depth of field, without going any smaller than necessary. In scenes where there’s no noticeable difference in DOF between f-stops (such as when everything in your frame is at infinity), select your lens’s sharpest f-stop. So what’s a lens’s sharpest f-stop? You can take the time to test each lens at a variety of f-stop/focal-length combinations; or you can go with the usually safe assumption that a lens’s sharpest f-stop is in the middle of its f-stop range, usually f8 to f11. Either way, don’t deviate from your lens’s sharpest f-stop unless the DOF calls for it.
Why not shoot everything f22 (or whatever the lens’s smallest aperture is) to maximize DOF? Well, sometimes you want less DOF, not more (see the example below). And even when you want lots of DOF, in addition to the fact that lenses tend to be less sharp at their extreme apertures (and without getting into too much technical detail), the smaller the aperture, the greater the diffraction that robs your lens of resolving power. You can usually assume you’re safe from significant diffraction at f-stops of f11 and larger (smaller f-stop numbers) and should go with a smaller aperture unless you really need the extra DOF.
My general approach is to always default to an f-stop in the f8-f11 range, then adjust smaller or larger if I think I need more or less DOF. Mastering DOF and focus point are an art in themselves, so if you’re not comfortable with determining and controlling DOF, take a little time to learn hyperfocal focusing.
Light: Shutter speed for exposure
Now that we have motion and depth handled, let’s get down to the business of managing light. Notice that we haven’t even started thinking about exposure, and we already have two of our three exposure variables set.
The entire foundation of exposure is understanding that your meter wants to make everything it sees a middle tone. In manual exposure mode, if you point a spot meter at any object and dial the exposure so the meter indicator registers zero, you’ve made that object a middle tone, regardless of its brightness; dial the exposure to +1 and you’ve made the object one stop brighter than a middle tone, -1 and it’s one stop darker than a middle tone, and so on.
“Blown” (hopelessly overexposed) highlights are death to most images, so it’s usually best to ensure that you haven’t overexposed anything in your frame. This is easily achieved by pointing your camera’s spot meter at the brightest thing that you want to have detail (unless the sun is in your frame, you’ll usually be pointing at the brightest thing in the scene) and adjusting the shutter speed until that bright target is the tone you think it should be (and no brighter than your camera can handle).
You don’t need meter on the brightest thing in the scene, but that just makes the most sense to me. Viewers’ eyes are most drawn to the brightest element in the frame, one reason white highlights are usually a worse problem than black shadows. If you’ve metered on the brightest thing in the frame and set it to a value that won’t be overexposed, you’ve made the entire scene as bright as possible without overexposing anything (if the brightest thing isn’t overexposed, nothing is). And the beauty of manual exposure is that you can remove your camera from your tripod and point at whatever you want—a sunlit cloud, a patch of bright blue sky, granite reflecting direct sunlight, or whatever you think is the brightest thing in the scene.
Shooting in raw output mode, I know I can retain detail all the way up to (about) +3 stops (over a middle tone), though going this bright often washes out color (overexposes or “clips” one or more of my RGB channels). I usually have pretty good success setting the brightest thing in my frame to +2 or +2 1/3 over a middle tone.
After shooting, check your LCD. If you see blinking highlights or any clipped RGB channels, increase your shutter speed (shorter duration) and re-shoot. If your shadows are too dark and you have room to increase exposure (move the histogram to the right) without clipping any of the three color channels, reduce your shutter speed and re-shoot. And if you can’t find an exposure setting that won’t clip (cut off the histogram) on both the shadow (left) and highlight (right) sides, you’ve probably exceeded your camera’s dynamic range and need to resort to multi-exposure blending, a graduated neutral density filter, or simply accept black shadows or white highlights. And if you’re in raw mode, you have a little margin for error beyond what the histogram shows, so if you’re close, you may still be okay.
Motion: ISO when something’s moving
The steps above will handle most exposure situations. But sometimes we have to contend with movement in the scene: flowing water, wind-blown leaves or flowers, the night sky, and so on. In these situations I adjust my ISO until I get the shutter speed necessary to achieve the desired motion at the ideal aperture I’ve already chosen.
When you want a little more motion, for example to blur flowing water, you don’t have as much wiggle room because your native ISO is already pretty at or close to the camera’s lowest. Check to see if your camera offers an “expanded” ISO option that provides an ISO a stop lower than the camera’s native (ideal) ISO. (I only use my camera’s expanded ISO when I have to because I’ve notice a 1/3 stop loss of dynamic range when I do.)
Exposure steps summarized (and simplified)
That’s a lot of explanation for a very simple process. What it all boils down to is three (and occasionally four) steps:
- Start with your camera’s best ISO.
- Choose the f-stop that gives you the depth you want; if depth isn’t a concern, just go with your lens’s sharpest aperture, usually in the f8 to f11 range.
- Spot meter and dial in the exposure you want by adjusting the shutter speed.
- If you can’t achieve the shutter speed that freezes or allows the motion you want, adjust the ISO accordingly.
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Some manual exposure examples
The extreme dynamic range in this spectacular Eastern Sierra sunrise provided very little margin for error, so I really needed to nail the exposure. I probably could have gotten by with ISO 100, but the wind was howling on this frigid January morning, so to be safe I opted for ISO 200. Depth of field wasn’t a concern, so I went with f8, the largest aperture in my lens’s f8-f11 prime sharpness zone. To hold the color in the brightly lit clouds, while still allowing enough light to capture some shadow detail, I spot-metered the brightest part of the sunlit cloud and dialed my shutter speed until the viewfinder meter indicator pointed to +2 1/3, as bright as I could make the clouds without blowing them out. After capture, a quick check of the histogram confirmed that I’d maxed my highlights without overexposing them, while retaining just enough shadow detail. Mission accomplished.
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My primary objective when composing these poppies was to blur the background poppies into a smear of color with minimal DOF. A thin layer of clouds subdued the sun slightly, but it was still bright enough to comfortably allow ISO 100 at my wide open f2.8 aperture. I spot-metered on the brightest part of the closest poppy and dialed my shutter speed until my viewfinder indicator pointed at +1 2/3, then composed, very carefully focused on the leading edge of the foreground poppy, and gently squeezed my remote release trigger. While my luminosity histogram indicated I’d captured all the highlight detail, a check of the RGB histogram revealed a clipped red channel, so I dialed my shutter speed up 2/3 of a stop (faster) and re-shot.
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The sky, just starting to brighten ahead of the rising sun, was still quite dark when the moon peeked above Half Dome. My goal was to underexpose the sky enough to create a silhouette that emphasized Half Dome’s outline, the sky’s rich twilight blue, and the moon’s delicate shape. Too bright and I’d loose color; too dark and I’d lose contrast. To avoid motion blur in the moon, I dialed up to ISO 800; because Half Dome was eight miles away, DOF wasn’t a concern (even at 400mm) and I opted for f8. Pointing my spot meter at the sky just behind Half Dome, I dialed my shutter speed until the viewfinder indicator pointed at + 1/3 (in this case a 5 second shutter speed).