20D focus technology

[wibbly]

My G3 uses contrast detection. This is one of the reasons claimed behind its slow focus speed (it has to hunt). Has the marginal advantage I can use linearly (cheaper) polarised polarising filters.

"Phase-detection" based focusing systems, I'm told, drive the lens to the focus position, which is one reason they're much faster/accurate.

1. Is there a FAQ that described the difference between the systems?

2. Does the 20D definately use phase-detection? (I'll need to junk my old filter ;-)). The manual talks of holizontal-line and vertical-line and cross sensors. Does this imply a contrast based system?

W

[roanjohn]

............I think the 20D still would need a contrast between two objects to focus accurately.............it will have difficulty focusing on a sky or a blank wall for example............

.........but let me assure you, the focus speed is much faster than that of the G3............even without the technical mumbo-jumbo.

Ro1

[Jon]

The 20D needs circular polarizers not so much because of the type of autofocus it has, but because it uses partially-reflective mirrors, and those don't get along well with linear polarizers. With a linear polarizer, the light exiting the filter is all vibrating in one plane; with circular polarizers, although one component of the light has been filtered out, a secondary layer of the filter "de-polarizes" the remaining light so the sensorts on both sides of the partially-reflective mirrors wil see the same light. Note - partially reflective mirrors are also used in metering.