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Traditional phase detect autofocus system runs mainly in an open loop configuration: take measurement, send focus distance to lens. Contrast detect autofocus is closed loop, it has to take a contrast measurement, move the lens, take a contrast measurement, move lens etc. which is clearly much slower.

The reason phase detect on sensor isn't nearly as fast as a dedicated phase detect unit is simply that it isn't accurate enough to use in open loop mode, so the camera has to resort to contrast detecttake several measurements to fine tune the focus.

The reason for the difference is accuracy is that a dedicated phase detect system has a series of lenses to direct the incoming light to two pixel arrays that are wide apart in order to measure the difference in phase and therefore the angle of incidence which tells you how far out the plane of focus is.

Phase detect on sensor uses a series of special pixels that have no colour filter, but instead have an opaque mask over one half the micro lens blacked out. By measuring the difference in light reading from pixels which have the left side blacked out compared to those which have the right side blacked out you get a measure of the angle of incidence but it's not very accurate as the height of the microlens above the pixel is small, leading to a much shorter baseline. Baseline is key to accuracy, imagine shutting one eye and trying to judge the distance of something by moving your head, if you can only move your head a tiny amount it's very hard.

Traditional phase detect autofocus system runs mainly in an open loop configuration: take measurement, send focus distance to lens. Contrast detect autofocus is closed loop, it has to take a contrast measurement, move the lens, take a contrast measurement, move lens etc. which is clearly much slower.

The reason phase detect on sensor isn't nearly as fast as a dedicated phase detect unit is simply that it isn't accurate enough to use in open loop mode, so the camera has to resort to contrast detect to fine tune the focus.

The reason for the difference is accuracy is that a dedicated phase detect system has a series of lenses to direct the incoming light to two pixel arrays that are wide apart in order to measure the difference in phase and therefore the angle of incidence which tells you how far out the plane of focus is.

Phase detect on sensor uses a series of special pixels that have no colour filter, but instead have half the micro lens blacked out. By measuring the difference in light reading from pixels which have the left side blacked out compared to those which have the right side blacked out you get a measure of the angle of incidence but it's not very accurate as the height of the microlens above the pixel is small, leading to a much shorter baseline. Baseline is key to accuracy, imagine shutting one eye and trying to judge the distance of something by moving your head, if you can only move your head a tiny amount it's very hard.

Traditional phase detect autofocus system runs mainly in an open loop configuration: take measurement, send focus distance to lens. Contrast detect autofocus is closed loop, it has to take a contrast measurement, move the lens, take a contrast measurement, move lens etc. which is clearly much slower.

The reason phase detect on sensor isn't nearly as fast as a dedicated phase detect unit is simply that it isn't accurate enough to use in open loop mode, so the camera has to take several measurements to fine tune the focus.

The reason for the difference is accuracy is that a dedicated phase detect system has a series of lenses to direct the incoming light to two pixel arrays that are wide apart in order to measure the difference in phase and therefore the angle of incidence which tells you how far out the plane of focus is.

Phase detect on sensor uses a series of special pixels that have have an opaque mask over one half. By measuring the difference in light reading from pixels which have the left side blacked out compared to those which have the right side blacked out you get a measure of the angle of incidence but it's not very accurate as the height of the microlens above the pixel is small, leading to a much shorter baseline. Baseline is key to accuracy, imagine shutting one eye and trying to judge the distance of something by moving your head, if you can only move your head a tiny amount it's very hard.

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Traditional phase detect autofocus system runs mainly in an open loop configuration: take measurement, send focus distance to lens. Contrast detect autofocus is closed loop, it has to take a contrast measurement, move the lens, take a contrast measurement, move lens etc. which is clearly much slower.

The reason phase detect on sensor isn't nearly as fast as a dedicated phase detect unit is simply that it isn't accurate enough to use in open loop mode, so the camera has to resort to contrast detect to fine tune the focus.

The reason for the difference is accuracy is that a dedicated phase detect system has a series of lenses to direct the incoming light to two pixel arrays that are wide apart in order to measure the difference in phase and therefore the angle of incidence which tells you how far out the plane of focus is.

Phase detect on sensor uses a series of special pixels that have no colour filter, but instead have half the micro lens blacked out. By measuring the difference in light reading from pixels which have the left side blacked out compared to those which have the right side blacked out you get a measure of the angle of incidence but it's not very accurate as the height of the microlens above the pixel is small, leading to a much shorter baseline. Baseline is key to accuracy, imagine shutting one eye and trying to judge the distance of something by moving your head, if you can only move your head a tiny amount it's very hard.

Traditional phase detect autofocus system runs mainly in an open loop configuration: take measurement, send focus distance to lens. Contrast detect autofocus is closed loop, it has to take a contrast measurement, move the lens, take a contrast measurement, move lens etc.

The reason phase detect on sensor isn't nearly as fast as a dedicated phase detect unit is simply that it isn't accurate enough to use in open loop mode, so the camera has to resort to contrast detect to fine tune the focus.

Traditional phase detect autofocus system runs mainly in an open loop configuration: take measurement, send focus distance to lens. Contrast detect autofocus is closed loop, it has to take a contrast measurement, move the lens, take a contrast measurement, move lens etc. which is clearly much slower.

The reason phase detect on sensor isn't nearly as fast as a dedicated phase detect unit is simply that it isn't accurate enough to use in open loop mode, so the camera has to resort to contrast detect to fine tune the focus.

The reason for the difference is accuracy is that a dedicated phase detect system has a series of lenses to direct the incoming light to two pixel arrays that are wide apart in order to measure the difference in phase and therefore the angle of incidence which tells you how far out the plane of focus is.

Phase detect on sensor uses a series of special pixels that have no colour filter, but instead have half the micro lens blacked out. By measuring the difference in light reading from pixels which have the left side blacked out compared to those which have the right side blacked out you get a measure of the angle of incidence but it's not very accurate as the height of the microlens above the pixel is small, leading to a much shorter baseline. Baseline is key to accuracy, imagine shutting one eye and trying to judge the distance of something by moving your head, if you can only move your head a tiny amount it's very hard.

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Traditional phase detect autofocus system runs mainly in an open loop configuration: take measurement, send focus distance to lens. Contrast detect autofocus is closed loop, it has to take a contrast measurement, move the lens, take a contrast measurement, move lens etc.

The reason phase detect on sensor isn't nearly as fast as a dedicated phase detect unit is simply that it isn't accurate enough to use in open loop mode, so the camera has to resort to contrast detect to fine tune the focus.