2 ms·

Cross-type has nothing to do with low light or low contrast. Sorry, I don't understand what point you want to make. There are dozens of things that affect focus

by ksk 6y ago

Cross-type has nothing to do with low light or low contrast. Sorry, I don't understand what point you want to make. There are dozens of things that affect focusing, or to be more specific - focusing speed, focusing accuracy, focusing repeatibility, etc. Many of these factors are tied to the lens. For e.g. Outer focusing points on a poorly made "consumer" lenses, generally, are not very accurate when you're close to MFD, and the focusing system tends to hunt a lot. STM focus motors are not designed for focusing speed, but they are designed for repeatibility. Also, most low light focusing and low-contrast focusing is hampered because of the high read-noise from the line sensors. I have the A9II and it has the best focusing system I've ever used. I thought the 1DX II was accurate, but this blows it away. Haven't tried the 1DXIII though.

arghwhat 6y ago
> Cross-type has nothing to do with low light or low contrast. Of course it does. PDAF needs to observe a feature (a phase difference) in the image, which requires light and contrast. In strong light, there are usually plenty of features available, making PDAF seem borderline magic, especially for something like DPAF where almost every single pixel can be used as a focus point. In lower light and contrast, fewer features are available. Subjects are rarely perfectly symmetric, so there are different features are available across the different sensor axis (e.g. a striped shirt). As subjects are not symmetric, the chances of you running out of features to focus on due to reduced contrast along two axis is much lower than running out along just one. Cross-type allow both axis to be used simultaneously, while non-cross can only use one. This means that, with the same sensitivity, cross-type is much more likely to be able to detect a feature in an area than non-cross, as there are more features available to it. As features turn to noise and AF becomes difficult, this leads to increased focus ability at lower light. For non-cross, a 90 degree rotation of the camera is needed to reveal those features. > For e.g. Outer focusing points on a poorly made "consumer" lenses, generally, are not very accurate when you're close to MFD, and the focusing system tends to hunt a lot. I see what you're trying to say ("inner focus point is not just cross-type, it's also the sharpest point on the lens"), but that's not really relevant here. I'm making the observation on a camera without cross-type at all, dead-center or thereabout. One can also use the zebra MF assist feature to highlight this, at least for DPAF cameras. > I have the A9II and it has the best focusing system I've ever used. I thought the 1DX II was accurate, but this blows it away. Haven't tried the 1DXIII though. This is the beauty of on-sensor focus - you measure exactly what you see, instead of measuring something else entirely and hoping that the calibration data is good enough...
- ksk 6y ago
  Sorry, I don't agree with you and there is no point going around in circles. You're arguing "in theory", I'm arguing "in practice". Anyway, good luck.

by ksk 6y ago

arghwhat 6y ago

> Cross-type has nothing to do with low light or low contrast. Of course it does. PDAF needs to observe a feature (a phase difference) in the image, which requires light and contrast. In strong light, there are usually plenty of features available, making PDAF seem borderline magic, especially for something like DPAF where almost every single pixel can be used as a focus point. In lower light and contrast, fewer features are available. Subjects are rarely perfectly symmetric, so there are different features are available across the different sensor axis (e.g. a striped shirt). As subjects are not symmetric, the chances of you running out of features to focus on due to reduced contrast along two axis is much lower than running out along just one. Cross-type allow both axis to be used simultaneously, while non-cross can only use one. This means that, with the same sensitivity, cross-type is much more likely to be able to detect a feature in an area than non-cross, as there are more features available to it. As features turn to noise and AF becomes difficult, this leads to increased focus ability at lower light. For non-cross, a 90 degree rotation of the camera is needed to reveal those features. > For e.g. Outer focusing points on a poorly made "consumer" lenses, generally, are not very accurate when you're close to MFD, and the focusing system tends to hunt a lot. I see what you're trying to say ("inner focus point is not just cross-type, it's also the sharpest point on the lens"), but that's not really relevant here. I'm making the observation on a camera without cross-type at all, dead-center or thereabout. One can also use the zebra MF assist feature to highlight this, at least for DPAF cameras. > I have the A9II and it has the best focusing system I've ever used. I thought the 1DX II was accurate, but this blows it away. Haven't tried the 1DXIII though. This is the beauty of on-sensor focus - you measure exactly what you see, instead of measuring something else entirely and hoping that the calibration data is good enough...

ksk 6y ago
Sorry, I don't agree with you and there is no point going around in circles. You're arguing "in theory", I'm arguing "in practice". Anyway, good luck.