Find out more about what the terms "line sensor" and "cross-type sensor" mean as I break them down and explain how they differ. (Reported by: Ryosuke Takahashi)
AF sensors are made up of line sensors—single sensor points (CMOS or otherwise) arrayed in a line—that calculate the distance of a subject from the camera based on the information obtained. Here, technology called “phase detection” is used, which is the same basic mechanism as that of phase-difference detection AF. Horizontal line sensors detect in a vertical direction, whereas vertical line sensors detect in a horizontal direction.
A cross-type sensor is an AF sensor with intersecting line sensors. This allows for greater measurement accuracy as line sensors that detect along one direction are overlapped on the same point with line sensors that detect along the “opposite” direction. In other words, cross-type sensors are able to detect in both a horizontal and a vertical direction. This not only reduces any detection errors, but also prevents issues such as having the centre out of focus (when the focus is unintentionally placed on the background) as well as AF errors from occurring even if the subject lacks three-dimensionality, such as in the cases of a horizontal line or a single grove of trees.
A cross-type sensor
A: Vertical line sensor—Detects lines of the subject in the horizontal direction
B: Horizontal line sensor—Detects lines of the subject in the vertical direction
Line sensors are used to detect lines either in the horizontal or vertical direction for that AF position. In contrast, cross-type sensors can detect both the horizontal and vertical lines for the same AF position, achieving high accuracy AF.
In principle, it can be said that the higher the number of cross-type sensors used, the higher the AF detection performance, with stable focusing accuracy at each AF point. Furthermore, as the size of the camera body is closely connected with the arrangement of the AF sensors, it is generally easier to equip large cameras with large AF sensors. Thus, large cameras tend to have a greater number of cross-type sensors.
AF pattern illustration for the EOS 7D Mark II
A: Cross-type AF: f/5.6 (vertical) + f/5.6 (horizontal)
B: Dual cross-type AF: f/2.8 (right diagonal) + f/2.8 (left diagonal) + f/5.6 (vertical) + f/5.6 (horizontal)
For example, the EOS 7D Mark II, Canon’s flagship APS-C camera, uses cross-type sensors for all 65 of its AF points. Extremely high AF accuracy is achieved because the sensors detect lines in both the horizontal and vertical directions.
EOS 70D/ EF70-200mm f/4L IS USM/ FL: 200mm (320mm equivalent)/Aperture-Priority AE (f/4, 1/320 sec, EV-0.3)/ ISO 100/WB: Auto
Cross-type sensors make it easy to focus on animals that are constantly moving around. Here I was able to establish focus on the subject’s eyes, just as I intended. The 19 AF points on the EOS 70D are all made up of cross-type sensors.
EOS 70D/ EF70-200mm f/4L IS USM/ FL: 200mm (320mm equivalent)/ Aperture-Priority AE (f/4, 1/1,600 sec, EV+0.3)/ ISO 100/ WB: Auto
It’s hard to go wrong with focusing if your camera uses cross-type sensors, as they detect lines in both directions even for subjects that tend to blend into the background. Cross-type sensors are particularly effective for thinly-shaped subjects such as the poles in the above image.
Born in Aichi in 1960, Takahashi started his freelance career in 1987 after working with an advertising photo studio and a publishing house. Photographing for major magazines, he has travelled to many parts of the world from his bases in Japan and China. Takahashi is a member of the Japan Professional Photographers Society (JPS).
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