cdifoto wrote in post #6607803
I'd like to see a source on that. Something legitimate, like straight from Canon. Maybe I am wrong but I'd like it to be proven. I always thought the camera looked for contrast to focus, which would (at least to my mind) explain why the red beam of a Speedlite can allow focus in pitch black. And why you can focus on a small white light in an otherwise dark room.
Also, Canon touts its cross-type sensors as being able to detect contrast on both the vertical and horizontal plane, as opposed to the line type only detecting contrast on one plane.
I think you may just be reading into the word too much. The camera needs contrast to differentiate it from the background and get a distance, but it doesnt judge contrast to achieve focus.
Passive autofocus
Passive AF systems determine correct focus by performing passive analysis of the image that is entering the optical system. They generally do not direct any energy, such as ultrasonic sound or infrared light waves, toward the subject. (However, an autofocus assist beam of usually infrared light is required when there is not enough light to take passive measurements.) Passive autofocusing can be achieved by phase detection or contrast measurement.
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Phase detection system
Phase detection is achieved by dividing the incoming light into pairs of images and comparing them.
SIR TTL passive phase detection (secondary image registration,
through the lens) is often used in film and digital
SLR cameras. The system uses a
beam splitter (implemented as a small semi-transparent area of the main reflex mirror, coupled with a small secondary mirror) to direct light to an AF sensor at the bottom of the camera. Two
optical prisms capture the light rays coming from the opposite sides of the lens and divert it to the AF sensor, creating a simple
rangefinder with a base identical to the lens' diameter. The two images are then analysed for similar light intensity patterns (peaks and valleys) and the phase difference is calculated in order to find if the object is in
front focus or
back focus position. This instantly gives the exact direction of focusing and amount of focus ring's movement. Although AF sensors are typically one-dimensional photosensitive strips (only a few pixels high and a few dozen wide), some modern cameras (
Canon EOS-1V,
Canon EOS-1D,
Nikon D2X) feature
Area SIR sensors that are rectangular so as to provide two-dimensional intensity patterns.
Cross-type (CT) focus points have a pair of sensors oriented at 90° to one another, although one sensor typically requires a larger aperture to operate than the other. Some cameras (Canon EOS-1V, Canon EOS-1D,
Canon EOS 30D/
40D) ) also have a few 'high precision' focus points with an additional set of prisms and sensors; they are only active with '
fast lenses' of certain
focal ratio. Extended precision comes from the increased diameter of such lenses, so the base of the 'range finder' can be wider.
Contrast measurement is achieved by measuring contrast within a sensor field,
through the lens. The intensity difference between adjacent pixels of the sensor naturally increases with correct image focus. The optical system can thereby be adjusted until the maximum contrast is detected. In this method, AF does not involve actual distance measurement at all and is generally slower than phase detection systems, especially when operating under dim light. This is a common method in
video cameras and consumer-level
digital cameras that lack
shutters and reflex mirrors. Some
DSLRs (
Olympus E-420,
Panasonic L10, Nikon D300 in Tripod Mode) use this method when focusing in their
live-view modes.
I was using my 17-85 and aiming it a contrasty coffee cup on my desk. Probably took ~5 seconds of hunting back and forth. Switched off LiveView and went the old fashioned Phase-detect, and ZIP.....Immediate focus lock.
