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02-22-2015, 02:43 PM   #1
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What means 'f=2.8 AF points'?

I feel confused with the description of AF points: linear, cross, f=2.8
Does it mean other AF points are not so much helpful with 2.8 lenses?
Or opposite, this f=2.8 AF points work only with fast glass?
Actually, with manual focus SLRs the faster the lens (i.e.: 1.8, 1.7, 1.4) the much easier focussing was
Are DSLRs 'optimized' for f=4.0 - 5.6 glass?

02-22-2015, 03:07 PM   #2
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On the K-3, the three middle AF points are phase-detect precise to f/2.8 focus accuracy. The others not quite so accurate - rated at f/5.6. It isn't a matter of how fast the lens is. The slower lenses probably perform similarly no matter which focus point confirms. At no point would you expect the other focus points to be more precise than the three rated at f/2.8.

Does it mean you are likely to see less accuracy as you go faster than f/2.8? Most often, yes. Can CDAF be more accurate - yes, especially in difficult light. Keep in mind that most fast lenses (especially longer ones) also tend to have a bit of focus shift. Are there workarounds? Yes, stop down and/or use manual focus so you are in control of exactly where the lens is focused.

The good news - AFAIK - is that this both the quickest and most accurate PDAF focusing we've gotten from Pentax. In my experience, it seems to be the most significant improvement Pentax made compared to the other recent models.
02-22-2015, 07:31 PM   #3
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QuoteOriginally posted by Prakticant Quote
Does it mean other AF points are not so much helpful with 2.8 lenses?
Or opposite, this f=2.8 AF points work only with fast glass?
The first is closest to the truth. The concept is called focus sensitivity and is expressed as f-number. The optical physics are a bit complicated, but it is enough to say that the general standard for PDAF is a focus sensitivity of f/5.6.* What that means is that the focus precision (chance of actually attaining accurate focus) is no better at wider apertures than at f/5.6. There are huge implications since the DOF is much deeper at f/5.6 than at say, f/2.8 meaning that with faster glass, missed focus is much more likely at that maximum aperture than with slower lenses. Note that this nothing to do with the amount of light present, the precision limit is the same for both bright and dim conditions. The focus sensitivity is driven by the aperture size alone.

PDAF systems with one or more f/2.8 sensors are pretty much the domain of upper end cameras. Both the K-5 (one at center) and the K-3 (3 at center column) have f/2.8 sensors. The f/2.8 sensors have the same focus precision at f/2.8 as at wider apertures and and still work with slower glass. With lenses faster than f/2.8 there is still a loss of precision relative to a split-image or Canon S-type focus screen and potential for missed focus, but that is just the way it is.

As noted above, CDAF (liveview AF) can be more accurate, but is slower and may fail to attain focus in dim light. My K-3 requires the AF assist lamp for CDAF in light where the PDAF system functions fine without it. As for focus shift on stop-down...yes, it exists, but it is usually offset to a degree by increased DOF. The lenses I own that shift have about 3-5mm change at 1m distance from f/1.4 to f/5.6. In practice, the impact of focus shift for AF lenses is the same as for legacy manual focus lenses. As always YYMV.


Steve

* I have long felt that the general scarcity of fast AF glass in the Pentax line was intentional and due to the limitations of the AF system. Note that this is not just a Pentax problem. Back in the day, most cameras shipped with a f/2 or faster 50mm lens and a viewfinder that was equal to the task. Current cameras ship with f/3.5-5.6 zooms and viewfinders that are no better than the AF system.

Last edited by stevebrot; 02-23-2015 at 02:57 AM.
02-23-2015, 01:54 PM   #4
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QuoteOriginally posted by stevebrot Quote

Steve

* I have long felt that the general scarcity of fast AF glass in the Pentax line was intentional and due to the limitations of the AF system. Note that this is not just a Pentax problem. Back in the day, most cameras shipped with a f/2 or faster 50mm lens and a viewfinder that was equal to the task. Current cameras ship with f/3.5-5.6 zooms and viewfinders that are no better than the AF system.
That was an excellent, fuller explanation you gave up above, Steve.

I believe the scarcity of fast AF glass due to AF accuracy as you mention is one of only three significant factors in that decision. A second factor is the overall historical Pentax philosophy emphasizing compactness (and to a lesser extent more economical offerings). The third factor, not generally well understood but possibly most important relates to the limits of digital sensors: they do a poor job (to varying degrees) of converting light at wide apertures - especially beyond f/2. For instance an f/2.8 lens might have a t-value of 3.0 at the sensor (actual light received). If that lens were to be an f/1.4, you might then expect it to have a 1.5 t-value; instead, it more likely will come in at 1.9 or worse; further, you tend to get blooming issues at those wider apertures that aren't apparent on film. I'm listing light acceptance along with blooming - both resulting in the failure of a lens to perform optimally at wider apertures as a single factor. Possibly light acceptance failure and blooming are somewhat intertwined.

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