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Forum: Pentax K-1 & K-1 II
09-21-2018, 09:16 AM
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Well, have you really read my explanation:
So, your 5.6 lens works just fine with the central focus point. But they do not take advantage of the higher precision offered by these points.
This says exactly the same thing as I explained above, but in three pages instead of a few sentences. The goal here wasn'.t to explain in depth all the technical intrecacies of a modern PDAF system. It was rather to explain in a concise an simple way that focusing points aren't all equals by design. Some are 2.8, some are 5.6, and some are f8 (although not on Pentax bodies). The f2.8 ones are more accurate by design and it's normal and expected that the OP get more shots in focus with these f2.8 points than the 5.6 ones.
Yes it is. There are plenty of articles on the web and a few thread on PF discussing this topic in depth, if people want to know how it works under the hood. But for this specific thread, there's no need for the OP to go to know more than there are 2.8 points and 5.6 points and that 2.8 are more precise and accurate than 5.6 when using a large aperture lens.
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Forum: Pentax K-1 & K-1 II
09-21-2018, 07:12 AM
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Light level has nothing to do with it. The points all have the same sensitivity to light, they're all rated at -3 EV. The diiference is simply because the 2.8 points are more precise than the 5.6 ones.
To make it simple, the way a phase detect autofocus system work is by splitting the image in two slits. When the two images are the same (in phase), it means the image is in focus. If the image in one slit is blurry relative to the other it means the focus is too far or too near. Now, the farther the slits are separated, the more accurate the system is. In a 2.8 point, the slit are a distance calibrated to give proper focus based on an aperture of 2.8. In a 5.6 point, the slits are nearer and calibrated for an aperture of 5.6. Why then not all points are calibrated for 2.8 ? It's because the larger distance between teh slits require a larger aperture to cover both. Thus, in a 2.8 point, you need at least a lens with an aperture of 2.8 to cover both slits. Since many lenses aren't 2.8 or larger, this would mean the AF system would not work with many lenses. Thus 5.6 is kind of the lowest common denominator since it probably covers 99% of lenses. The center points also have both pair of slits: 2.8 and 5.6. So, if a lens slower than 2.8 is used, they still work but fall back to the regular 5.6 slits and are then no better than any other points. On the other hand, if a lens with an aperture of 2.8 or larger is used, the center points will use their 2.8 slits to give more precise and accurate focus. And this is exactly what you observe with your 1.4 lens: the center 2.8 AF points gives you more precise and accurate focus than the other 5.6 ones. This is to be expected and what the 2.8 points have been designed to do.
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Forum: Pentax K-1 & K-1 II
09-21-2018, 06:36 AM
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It's certainly written somewhere in the K-1 manual. I don't have a K-1, thus I can't give you the exact page number but here's a quote from the K-1 review on PF, p. 18:
As on the K-3, the center point and the two points directly above/below it are able to take better advantage of faster lenses (up to F2.8).
The default for all other points is f5.6. True not only for Pentax but for Canon and Nikon as well, focus points are f5.6 unless specified otherwise. There's even f8 points on some camera, not very accurate but still working with slow lens with TC.
They have the same sensitivity in terms of light level, but they don't have the same accuracy. The f5.6 points will be in focus according to the DOF the image will have a 5.6. If you use 1.4, it means the focus might be off because the DOF is much thinner, but the focus would have been somewhere in the plane of focus at 5.6... The same is true for the 2.8 points. They're significantly more accurate than the 5.6 ones, but might not be 100% accurate when using a 1.4 aperture.
So, what you experienced is a fundamental limitation of the PDAF system as implemented today in DSLR. This is where CDAF, in Liveview, or manual focus (particularly in LV with focus peaking) will give you better results since they will work based on the DOF of the final picture.
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Forum: Pentax K-1 & K-1 II
09-21-2018, 05:52 AM
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For one thing, only the central points are f2.8. All the others are f5.6 points. They're OK for general shooting but expecting them to give accurate and reproducible results with a f1.4 lens is a stretch... The're certainly not the best way to get accurate focus in thin DOF situation with a large aperture lens. So, there's nothing wrong with your results. If you close down to f5.6, the rated aperture of non central focus points, I'm pretty sure everything will be sharp.
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