Real World Application: Exposure

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Let’s say you’re shooting a portrait of a solitary hipster in front of a lake. You’re shooting at f11 because you want the hipster to be the main subject, but you nee the lake visible as well for context. You zoom in now and want a nice portrait of just the hipster so you go from f11 to f2.8, the widest open aperture setting your lens has. Now, if you’re shooting manual, the camera will do nothing for you; it’s your job to maintain the same exposure as the last shot. If you do nothing, you’ll end up with a blazingly bright exposure. Remembering that aperture follows the f-stop scale, you know that if you go from f11 to f2.8 you’ve increased the light hitting the sensor by 4 stops. That means 2^4 or 16 times the amount of light is now hitting your sensor. You need to reduce your exposure by those same 4 stops using ISO and/or shutter speed. Your ISO is set to 1600 so you twirl the dial from 1600 to 800, that’s one stop, from 800 to 400, there’s two stops, 400 to 200, three stops, and 200 to 100, four stops. You didn’t have to touch shutter speed; by compensating for the aperture change using ISO you’ve maintained the same exposure. But let’s say you were at ISO 800 and your camera can’t go lower than ISO 100. Now you’ll need to double your shutter speed to compensate for that extra stop of brightness. Let’s say you were at 1/160. In this case you’d need to make your shutter faster, setting it to 1/320.

The “Zone System”

You may hear about this so we’ll discuss it briefly. The Zone System is a photographic technique for determining optimal film exposure and development, formulated by Ansel Adams and Fred Archer.

A dark surface under a bright light can reflect the same amount of light as a light surface under dim light. The human eye would perceive the two as being very different but a light meter would measure only the amount of light reflected, and its recommended exposure would render either as Zone V. The Zone System provides a straightforward method for rendering these objects as the photographer desires. The key element in the scene is identified, and that element is placed on the desired zone; the other elements in the scene then fall where they may. With negative film, exposure often favors shadow detail; the procedure then is to

  1. Visualize the darkest area of the subject in which detail is required, and place it on Zone III. The exposure for Zone III is important, because if the exposure is insufficient, the image may not have satisfactory shadow detail. If the shadow detail is not recorded at the time of exposure, nothing can be done to add it later.
  2. Carefully meter the area visualized as Zone III and note the meter’s recommended exposure (the meter gives a Zone V exposure).
  3. Adjust the recommended exposure so that the area is placed on Zone III rather than Zone V. To do this, use an exposure two stops less than the meter’s recommendation.

Digital photography

The Zone System can be used in digital photography just as in film photography; Adams (1981, xiii) himself anticipated the digital image. As with color reversal film, the normal procedure is to expose for the highlights and process for the shadows.

Until recently, digital sensors had a much narrower dynamic range than color negative film, which, in turn, has less range than monochrome film. But an increasing number of digital cameras have achieved wider dynamic ranges. One of the first was Fujifilm’s FinePix S3 Pro digital SLR, which has their proprietary “Super CCD SR sensor” specifically developed to overcome the issue of limited dynamic range, using interstitial low-sensitivity photosites (pixels) to capture highlight details.[citation needed] The CCD is thus able to expose at both low and high sensitivities within one shot by assigning a honeycomb of pixels to different intensities of light.

Greater scene contrast can be accommodated by making one or more exposures of the same scene using different exposure settings and then combining those images. It often suffices to make two exposures, one for the shadows, and one for the highlights; the images are then overlapped and blended appropriately, so that the resulting composite represents a wider range of colors and tones. Combining images is often easier if the image-editing software includes features, such as the automatic layer alignment in Adobe Photoshop, that assist precise registration of multiple images. Even greater scene contrast can be handled by using more than two exposures and combining with a feature such as Merge to HDR in Photoshop CS2 and later. A simplified approach has been adopted by Apple Inc. as a selectible HDR option in later versions of the iPhone.

The tonal range of the final image depends on the characteristics of the display medium. Monitor contrast can vary significantly, depending on the type (CRTLCD, etc.), model, and calibration (or lack thereof). A computer printer’s tonal output depends on the number of inks used and the paper on which it is printed. Similarly, the density range of a traditional photographic print depends on the processes used as well as the paper characteristics.

Honestly, don’t worry too much about this. Just remember that your camera ‘meters’ gray at 50% so make sure to compensate for that 50% based on what your brain knows the lightness/darkness of the value should be.

Sunny 16

Want to know a general way of ballparking your exposure? On a sunny day, shooting at f16, your ISO will be the denominator of your shutter speed for any shot in broad daylight. So, shooting at f16 and ISO 200, a shutter speed of 1/200 will give you a decent exposure. Want to open your aperture? There’s where knowing that f-stop scale comes in handy again.


Alternative Approach?

Wouldn’t it be great if you didn’t have to expend such mental energy? You’re paying big money for a camera, can’t it do some of this heavy lifting. Yes it can, and in many cases you should let it. Learn more about shooting modes as we finally dive in to how to operate your camera.

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