Viewfinders, metering and exposure technology

How a camera shows the scene and measures light shapes how a photographer works. Optical and electronic viewfinders offer different experiences, and modern electronic viewfinders and rear screens can show exposure, histograms, highlight warnings and focus aids directly in the image. Metering systems have evolved from single averaging cells to multi-zone and recognition-based systems that analyze the whole scene. PHOT-101 covered how to use metering modes and histograms; this lesson looks at the technology behind them: optical and electronic viewfinders, rear screens, how metering systems work, exposure previews and their limits, histograms and zebras, auto ISO, bracketing, and in-camera multi-shot modes such as HDR and live composite.

Optical viewfinders

An optical viewfinder (OVF) shows the scene directly through glass. In an SLR, the image comes through the lens via the mirror and pentaprism, so it shows exactly the framing and focus of the lens, but at the brightness of the lens's maximum aperture rather than the final exposure, and with no preview of white balance or exposure. Rangefinders and some compacts use a separate optical window beside the lens, with frame lines showing the field of view. Optical viewfinders have no lag, consume no power, show the full dynamic range the eye can see, and are favored by some photographers for the direct connection they provide with the scene. Their key specifications are magnification, coverage (the percentage of the final frame visible) and eyepoint (how far the eye can be from the eyepiece, important for spectacle wearers).

Electronic viewfinders

An electronic viewfinder (EVF) shows the image from the sensor on a small high-resolution display, usually OLED, viewed through magnifying optics. It previews the final image, including exposure, white balance, picture style and depth of field, and can overlay histograms, level indicators, focus peaking and highlight warnings. It shows a bright image in dim light and lets photographers review images without taking the eye away. Early EVFs suffered from lag, low resolution, smearing and blackout during bursts; modern high-end EVFs offer resolutions of around 9 million dots or more, refresh rates of 120 Hz or higher, and blackout-free shooting with stacked sensors. Remaining drawbacks include power consumption, a slight delay, limited dynamic range of the display, and the difference between the display's rendering and the final processed raw image.

Rear screens

Rear LCD or OLED screens show live view and playback. Tilting and fully articulating screens help with low and high angles, video and self-portraits; touch screens allow focus point selection and menu navigation. Screen brightness is adjustable and affects how bright images appear in playback, a common cause of misjudged exposure, so the histogram is a more reliable guide than appearance. Screens are hard to see in bright sun, when an EVF or a hood helps.

How metering works

Camera meters measure reflected light and calculate the exposure that would render the metered area as a mid-tone. In SLRs, a separate metering sensor in the viewfinder housing measures the image on the focusing screen; early meters used a few zones, while modern SLR meters use RGB sensors with hundreds of thousands of pixels that can recognize faces and colors. Mirrorless cameras meter directly from the image sensor, using every pixel. Evaluative, matrix or multi-zone metering divides the scene into zones, compares the pattern of brightness, color, focus position and distance with a database of scenes or a trained model, and chooses an exposure intended to look right: for example, recognizing a bright sky at the top of the frame and avoiding underexposing the landscape below. Center-weighted and spot metering measure a defined area, giving predictable results that skilled photographers can interpret. Highlight-weighted metering, offered by some cameras, protects the brightest areas from clipping, useful on stage and in high-contrast scenes.

Exposure preview and its limits

An EVF's exposure preview shows how the camera's JPEG would look, which is valuable but can mislead. The preview reflects the picture style, contrast and white balance settings, not the raw data, so highlights may look clipped when the raw file still holds detail, or shadows may look blocked when they can be recovered. Some cameras offer a "flat" or log-like view, or let photographers set a neutral picture style to make the preview and histogram closer to the raw data. When using flash, the preview cannot show the flash's contribution, so cameras disable exposure preview in flash modes or show a setting effect that represents ambient light only.

Histograms, highlight warnings and zebras

The histogram shows the distribution of tones from black on the left to white on the right. A histogram piled against the right edge indicates clipped highlights; against the left edge, crushed shadows. RGB histograms show each color channel separately, revealing when one channel, often red in sunsets or flowers, has clipped while the overall histogram looks fine. Highlight warnings, or "blinkies", flash the clipped areas in playback. Zebras, borrowed from video, overlay stripes on areas above a chosen brightness in live view, before the picture is taken. Because these tools are based on the JPEG rendering, testing your camera tells you how much extra headroom the raw file has, often about half a stop to a stop.

Auto ISO

Auto ISO lets the camera vary ISO to achieve correct exposure while the photographer chooses aperture and shutter speed, a combination sometimes called manual mode with auto ISO. It is useful when light changes quickly and both depth of field and motion control matter, such as at events, sports and wildlife. Settings include the maximum ISO the camera may use and, in aperture priority, the minimum shutter speed, which many cameras can link to the focal length to follow the reciprocal rule automatically. Exposure compensation still works in manual mode with auto ISO on most modern cameras, adjusting the ISO the camera chooses.

Bracketing

Exposure bracketing takes a series of frames at different exposures, typically three, five or more, spaced one or two stops apart. It insures against misjudged exposure and provides the raw material for HDR merging, in which software combines the frames into one image with the highlight detail of the darkest frame and the shadow detail of the brightest. Most cameras can bracket automatically, ideally from a tripod with a self-timer or remote release to avoid movement; handheld bracketing works with fast frame rates and alignment in software. Bracketing also applies to focus (Lesson 7), white balance and flash power.

In-camera multi-shot modes

Cameras increasingly perform multi-frame processing themselves, borrowing techniques from smartphones. In-camera HDR merges brackets into a single JPEG. Live composite modes, used for star trails, light painting and fireworks, take repeated short exposures and add only new bright areas to the accumulated image, letting the photographer watch the image build without overexposing the background. Live time and live bulb modes show the developing exposure during long exposures. Live ND modes simulate a neutral density filter by averaging many short frames, smoothing water and clouds without a physical filter. Multiple-exposure modes combine frames creatively. These features save time in the field, but the results are usually JPEGs or composite raw files with fewer processing options than separate frames merged later.

Viewfinder specifications explained

Viewfinder specifications can be confusing. EVF resolution is usually quoted in dots, where each dot is one red, green or blue subpixel, so a 3.69-million-dot finder has about 1.23 million full-color pixels, roughly 1280 × 960. Magnification describes how large the view appears compared with the naked eye, quoted with a 50 mm lens on full frame; 0.8× or more feels large and immersive, while values below 0.6× feel like looking down a tunnel. Some manufacturers quote magnification for an equivalent lens or sensor size, so comparisons should be careful. Refresh rate affects how smooth moving subjects appear and how easy it is to follow action; 60 Hz is acceptable for slow subjects, while 120 Hz or 240 Hz modes help with sports and wildlife at the cost of battery life and sometimes resolution. Diopter adjustment lets photographers with mild short or long sight see the viewfinder sharply without glasses.

Spot metering and the zone system idea

Spot metering measures a small area, usually 1 to 5 % of the frame, and is the most precise way to meter a specific tone. Because any meter assumes that what it measures should be a mid-tone, the photographer must decide how bright the metered area should be and adjust accordingly. This idea comes from the zone system, developed by Ansel Adams and Fred Archer around 1940 for black-and-white film, which divided tones into zones one stop apart, from pure black to pure white, with middle gray as zone V. A photographer spot-metering a sunlit snowfield, for example, knows that the meter will render it gray, and adds about 1⅔ to 2 stops so that it appears bright white with texture. Spot-metering a dark coat and subtracting a stop or more keeps it dark. With practice, this approach gives consistent, predictable results, and spot meters on cameras that link the metering spot to the autofocus point make it easy to meter a subject's face.

Handheld meters

Handheld light meters remain useful despite sophisticated in-camera metering. A handheld meter can measure incident light, the light falling on the subject, by pointing a white dome from the subject's position toward the camera; incident readings are unaffected by the subject's brightness, so a black cat and a white cat in the same light receive the same exposure. Incident metering is especially valuable in studio work with flash, since flash meters measure the burst of light from strobes, allowing photographers to set lighting ratios between key and fill lights precisely before taking a picture. Spot meters with a 1° angle let landscape photographers measure distant tones and calculate the scene's brightness range. PHOT-210 covers metering for studio lighting in detail.

Exposure compensation and metering locks

Exposure compensation shifts the camera's chosen exposure in semi-automatic modes, usually in one-third-stop steps over a range of ±3 to ±5 stops. Auto exposure lock (AE-L) holds a meter reading so that the photographer can meter one area and recompose, useful with center-weighted or spot metering. Some cameras let photographers fine-tune each metering mode permanently if their camera consistently overexposes or underexposes compared with their preference. Understanding when the camera's metering will be fooled, as in snow, backlit scenes, dark venues or subjects against bright skies, and what correction to apply comes with experience, but the histogram is the final check.

Dual-display and waveform tools

Some cameras now offer waveform monitors, a video tool that shows brightness across the width of the frame, letting photographers see exactly which parts of the scene approach clipping. False color displays, also borrowed from video, map brightness levels to colors, so skin at the right level appears in one color and clipped highlights in another. These tools take practice but give much more precise exposure information than a histogram, particularly for hybrid photographers who also shoot video.