Tech Update: Evolution And Revolution In Focus Control: Accuracy, Speed And Spontaneity

“Until the dawn of the digital era, the evolution of focusing systems in cameras was a running dialogue between two fundamental camera types—the view camera…and the viewfinder camera…”

Given that the physical and perceptual experience of making a photograph is shaped by technology, and that technology is also embedded in the resulting images, one of the chief and perhaps most profound changes in how we make an image has been the changes in focusing—and recently autofocusing—technology. There’s a reason that the documentary photojournalism of Lewis W. Hine (shot with a ponderous 5x7 view camera or a 4x5 Graflex SLR) has a qualitatively different feel from that of Alfred Eisenstaedt or Henri Cartier-Bresson (shot with pocket-sized 35mm rangefinder cameras). It’s not only framing—it’s responsiveness, spontaneity, and, perhaps, repose, that underlies what these image-makers showed us.

Two Tracks
Until the dawn of the digital era, the evolution of focusing systems in cameras was a running dialogue between two fundamental camera types—the view camera, which presents the image formed by the lens on a ground-glass screen used for viewing, composing, and focusing, and the viewfinder camera, which employs a separate, direct vision optical or wire frame finder for viewing the subject and composing the image, and uses a separate system, a rangefinder or manual focusing scale, for focusing the lens.

Over the course of time, the view camera evolved into the eye-level SLR that presents the laterally and vertically correct viewing image projected by the lens onto a textured viewing screen to the photographer’s eye. The viewfinder camera evolved along a parallel track, from the humble tiny reflex finders and wire frame aiming devices affixed to countless box and folding cameras to the glorious coupled range/viewfinder with projected, parallax-compensating viewfinder frame lines epitomized by the Leica M3 and its successors. Some cameras (e.g., the Speed Graphic and some Alpa 35mm SLRs) were hybrids that provided both these viewing and focusing options.

On-Sensor Phase-Detection AF
Pairs of phase-detection pixels under micro lenses above the sensor array are masked so they only detect light from one edge of the lens or the other, thus allowing the camera to compare their phase difference and quickly determine the proper focus point. This structure is equivalent to the two divided light paths in the conventional Phase-Detection AF (PDAF) systems in D-SLRs. While the masked pixels receive less light than normal pixels, they only represent a small percentage of the pixels on the sensor so overall sensor sensitivity is barely affected. Algorithms determine when data from masked pixels is used to record the image. The great advantage of on-sensor PDAF is that it enables Hybrid AF systems using both PDAF and Contrast AF to be built into a variety of compact cameras, including mirrorless Compact System Cameras (CSC) and point-and-shoots. Potentially it will also enhance the AF performance of D-SLRs with optical viewfinders. It’s an emerging technology with enormous upside potential.
Courtesy of Fujifilm

“Before 1995, most digital cameras did not have Live View, and although this feature was rapidly adopted in point-and-shoot cameras, it was more than a decade before Live View appeared in higher-end D-SLRs…”

Enter The AF Era
The first successful AF system in the world for cameras was the Leitz (Leica) Correfot shown at photokina in 1976. It was basically a “proof of concept” design that used a delicate “vibrating frame” to enhance focusing precision, had the camera connected by cable to a bulky computer to avoid the considerable expense of developing a dedicated IC chip to process the AF signals, and read out the correct focus point via LEDs rather than physically focusing the lens. Although it was never fully developed into a production camera, the Correfot pioneered many of the features built into the Phase-Detection Autofocus (PDAF) systems found in today’s D-SLRs. These include a sub-mirror that directs part of the incoming light to a PDAF line sensor (CCD) in the bottom of the camera, and a beam splitter that divides the incoming light into a pair of images and compares their light intensity patterns (peaks and valleys) electronically.

The PDAF systems built into the world’s first AF still camera, the Konica C35AF of 1977, and the first fully integrated AF SLR, the Minolta Maxxum 7000 of 1985, rely on the same basic concept. Essentially, it’s a simple electronic rangefinder with a base length determined by the diameter of the lens. By calculating the “separation error” between the two images on the line sensor, the system can immediately determine whether the object is in front of or beyond the point of optimum focus, and move the lens in the proper direction until the two images are “in phase,” at which point proper focus is achieved.

The important thing to bear in mind is that traditional PDAF in film SLRs and D-SLRs uses a system that is separate from the viewing image or the image captured by the sensor to determine the distance to the subject. The fact that PDAF identifies the subject as part of the AF process is why it is inherently faster than Contrast AF (CAF) systems that focus the camera by maximizing the contrast of the on-sensor image. It’s also why all focus-tracking systems in use rely on PDAF rather than CAF.

With steady advances in design, such as multiple cross-pattern AF sensors providing multiple AF points that are sensitive to subjects with horizontal, vertical, or oblique line patterns, vastly improved performance (especially in low light and with low-contrast subjects) due to wider area coverage, impressive increases in the sensitivity of today’s AF sensors, and amazing upgrades in the speed, torque, responsiveness, and quietness of AF motors, the PDAF systems in D-SLRs have elevated the picture-taking experience for millions of photographers worldwide.

By automating one of the key picture-taking parameters, it has had the effect of prioritizing timing and composition, and enhancing overall speed, responsiveness, and precision, giving shooters a better shot at capturing decisive moments. Even at its present advanced state, however, AF is not a panacea, which is why all D-SLRs and Compact System Cameras offer Manual Focus (MF) mode. It’s still not perfect, but autofocus, particularly PDAF, has certainly transformed the experience of making pictures.


Live View And Contrast-Detection AF (CAF)

One of the most important turns in the autofocusing road occurred in 1995 with the advent of Live View (a.k.a. Live Preview) in the Casio QV-10 and Ricoh RDC-1. The concept, derived from TV video cameras, is essentially to take a continuous live feed directly off the camera’s image sensor and display the image directly, either on the camera’s Liquid Crystal Display (LCD) and/or in its Electronic Viewfinder (EVF). Since the Live View image is formed by the camera lens, defined (“framed”) by the sensor’s dimensions, and completely free of parallax error, it is the functional equivalent of the image you see when you look through the viewfinder of an SLR.

However, it took camera designers quite a while to realize the full potential of this similarity. Before 1995, most digital cameras did not have Live View, and although this feature was rapidly adopted in point-and-shoot cameras, it was more than a decade before Live View appeared in higher-end D-SLRs (as an alternative mirror-up option to the OVF), and even longer before it became the technological centerpiece of a new generation of mirrorless Compact System Cameras (CSC) with interchangeable lenses, vastly upgraded EVFs, and OLED monitors.

Indeed, most early Live View systems had serious limitations, and the first general-use D-SLRs to provide Live View for both exposure simulated preview and framing preview were the Canon EOS-1Ds Mark III and the EOS 40D of 2007. The most sophisticated type of Live View displays the exact look of the exposure electronically and allows the photographer to alter the exposure in real time by adjusting the shutter speed, aperture, and ISO before the photograph is taken.

Live View And CAF
Live View is practically synonymous with Contrast Detection (CAF). It’s the AF system used in all true D-SLRs when shooting video, and the one used in most CSCs. CAF is achieved by measuring the contrast within the Live View image projected by the lens onto the image sensor in real time. The intensity difference between adjacent pixels in the area being focused on increases as the lens approaches the correct focus point, and when maximum contrast is detected, the optimum focus is achieved. Note that this AF system, while commendably precise, does not have any independent method of calculating the subject distance or identifying the subject, because it does not use a separate sensor like PDAF. However, CAF, which is based on software implementation, can be more flexible and potentially more accurate than PDAF. However, it is inherently slower, and it cannot identify a subject and predict where it will be when the shutter fires, so Predictive AF and Focus Tracking options are off the table.

“Live View is practically synonymous with Contrast Detection (CAF). It’s the AF system used in all true D-SLRs when shooting video, and the one used in most CSCs.”

Emerging AF Tech
The hottest new trend in AF is hybrid CAF/PDAF systems that incorporate dual-purpose phase-detection/image capture pixels in the CMOS image sensor itself. This emerging technology promises not only faster, more decisive autofocusing in mirrorless Compact System Cameras, but also the ability to provide Continuous AF and Focus Tracking before and during the exposure with D-SLRs, which is crucial when shooting at high burst rates or capturing HD video. While today’s best Contrast AF systems have been tweaked to provide much improved speed and performance, on-sensor PDAF looks to be the next really big thing in the inexorable advance of autofocus technology.

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