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05-18-2013, 12:14 AM   #1
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Need help understaning imrovements in K-5 II AF system

Hi,

I did some research but still some things remain opaque. From K-5 II(s) news-release:
QuoteQuote:
The K-5 II features a newly developed, highly sensitive SAFOX X AF module. Thanks to the unitís high-sensitivity AF sensor, which performs brilliantly in low-light conditions, this new module offers a broad AF working EV range (-3 EV to +18 EV). SAFOX X can work with luminous flux based on F2.8 levels in addition to that of F5.6. This increase the AF accuracy drastically when using with very fast lenses coupled with an upgraded AF algorithm, it delivers exceptional AF performance, including such useful features as a select-area expansion function, which automatically tracks the subject even when it moves away from a pre-assigned select point by assessing distance data collected by neighboring focus sensors.
1. new module offers a broad AF working EV range (-3 EV to +18 EV)

- Does the range depend on the brightness of lens used? I mean, having 1.4 or 4.0 lens which one will focus at -3EV? Another question is T-value. If, for example, one is has T/2.9 and another is T/3.2 (both marked as f/2.8) will the first one focus better in dim light?

2. SAFOX X can work with luminous flux based on F2.8 levels in addition to that of F5.6.

- as I understand, new, bigger and brighter lenses are used in new AF module. Is it true? If not what does it mean physically? Why does it mean that only 2.8 and faster lenses will gain some benefit (I read it more than once on the forum)? Wouldn't all lenses gain some benifits because now AF system uses more light (or more sensetive sensors)? Will it increase the range of usable apertures from f/8 to, say f/11? Which AF points got the improvement (center, 5, 11) compared to K-5 AF?

3. including such useful features as a select-area expansion function, which automatically tracks the subject even when it moves away from a pre-assigned select point by assessing distance data collected by neighboring focus sensors

- is that feature for AF-C only? Say, if I use AF-C with automatic AF point selection, why if 'subject moves away form pre-assigned' don't just use another AF point? Am I right in understanding that the feature works only for AF-C plus single AF point mode?

I do believe that the new AF system is better in practice, what I want is understanding of technical details and help in decrypting that quote from news-release.

Thanks in advance.

05-18-2013, 04:30 AM   #2
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1. yes the EV range depends on the T-stop of the lens.
T stop is how much light a lens let through, F stop is simply a ratio between lens opening and focal length.

2. the F/2.8 of the AF sensor has nothing to do with the lens of the AF sensor.
It means that lenses faster then f2.8 focus more accurately.
Af spot consist of 2 light sensativev strip, with f/2.8 these strips are further apart then with f.5,6
It's only the center spot that has f/2.8 but it also has f/5.6


3. no idea.
05-18-2013, 06:00 AM   #3
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Yes tracking works only on AFC
05-18-2013, 07:51 AM   #4
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So what I've been wondering is: will it focus better (than K-5 and previous) with slower lenses or with teleconverters (that allow AF)? Once you get past an effective aperture of 5.6-6.3 on the K-5, it will hunt like crazy. Does the II do better here?

05-18-2013, 10:27 AM   #5
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QuoteOriginally posted by disya2 Quote
Hi,

I did some research but still some things remain opaque. From K-5 II(s) news-release:

2. SAFOX X can work with luminous flux based on F2.8 levels in addition to that of F5.6.

- as I understand, new, bigger and brighter lenses are used in new AF module. Is it true? If not what does it mean physically? Why does it mean that only 2.8 and faster lenses will gain some benefit (I read it more than once on the forum)? Wouldn't all lenses gain some benifits because now AF system uses more light (or more sensetive sensors)? Will it increase the range of usable apertures from f/8 to, say f/11? Which AF points got the improvement (center, 5, 11) compared to K-5 AF?
The AF lenses are spaced farther apart which improves AF accuracy. As you move the AF lenses farther apart you need a wider aperture (at least F/2.8) to take advantage of the wider spacing. It is like vision. The farther apart your eyes the better your depth perception. You are triangulating distance. The farther apart the sensors the more accurate the triangulation. Lenses need to be at least F/2.8 or faster or the aperture blocks the wider sensors and the camera defaults to the 2 narrower sensors that will work with any lens at F/5.6 or faster. If you have 85mm F/1.4 you will see a lot better focus accuracy at wider apertures and under lower light.
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