28

In actuality, a cross type AF sensor is just two "normal" AF sensors, one with the lines oriented in a vertical direction and the other with the lines oriented in a horizontal direction, that are superimposed over the same area. Non cross-type AF sensors can be either vertical, horizontal, or even diagonal. Some very early AF film cameras with a single ...


15

The fact that Phase-Detection stops at the focus-distance rather than exceeding and then coming back has a big impact (as Stan mentioned in his answer) on video since this motion is visible in the recording. Another impact is that Phase-Detection requires dedicated sensors in fixed positions which is why the number of AF points is relatively small. Contrast-...


10

The question insists on a practical, not a technical answer. Phase detection is better at focusing on moving subjects, or subjects where contrast is not available because of low-light, for example, when shooting sports. Contrast-detection can be faster than phase detection where subjects are not moving or where contrast is plentiful, for example, in a ...


9

Phase detection AF is better suited to: fast acquisition of focus - mostly open loop system, i.e. it takes a measurement and moves the lens. moving subjects - measurements can be taken very quickly allowing it to track motion. large/heavy lenses - fewer lens movements required. film cameras - requires only a low res sensor which doesn't block film plane. ...


7

Applying gain by increasing the ISO setting generally only serves to reduce read noise by amplifying the analogue signal before readout and digitization. Signal to noise ratio in the highlight remains unchanged. The increase in image brightness that also results would be irrelevant (besides any reduction in quantization noise) to the AF sensor, as it ...


6

In general, contrast-based autofocus systems as found in mirrorless cameras and DSLRs in live view are identical, and have the potential to be the most accurate. Phase detection tells the system which direction to go, though, so it can be very fast — contrast-based systems have to seek around to find out. The main downside comes from using a separate sensor,...


4

You linked to that other question, stating that AF systems do not work as one thinks. Oddly, you come to the conclusion that you should buy another camera. Let me quote from the accepted answer: The Canon 7D is not unique in the way multiple focus points share parts of the same lines on an AF array. Most of the pro grade bodies and many of the pro-sumer ...


4

For phase detection AF to work properly, the camera needs more than just a clear glass replacement for the removed filter, it needs a replacement of a different thickness. IR rays don't focus on the same point as visible light, which is why lenses have (used to have?) a red dot to tell you how to adjust focus from the visible light distance to the correct ...


4

Hybrid AutoFocus combines these two AutoFocus Technologies - Contrast and Phase Detection. In Hybrid initially Phase Detection is used, and then Contrast Detection is used. In Hybrid AF, Contrast and Phase Detection complements each other. Phase Detection Advantage => Speed. Lacks => Accuracy. The reason is, in Phase Detection the camera knows which ...


3

Okay, so, the fundamental problem, as discussed in the other question you've linked, is that focus "points" actually have to be areas, not literally points, and the camera can't know what you're actually interested in with that point. However, if you're particularly interested in in-focus faces, there is a solution. That is: use a camera with contrast-...


3

There are several reasons for this: It is quicker as the name implies and as you figured out. "Why not use the viewfinder instead if focusing speed is important?" Because you cannot. :) At least for me, this happens mostly when I shot with my dSLR above the head (it happens in photo journalism) and hence in order to frame I use the Live View. Also, when you ...


3

Is AFMA per-lens or a global adjustment? In theory, it could be either depending on how the camera designer approaches it, but the usual case is per-lens for reasons I'll go into below. Making it global would be a ham-fisted way to go about it unless the camera only has one lens. Can AFMA actually fully correct issues in poor-focusing lens+body ...


3

How does on-sensor phase detection AF not impact the image? It does. But the effect is usually so miniscule as to not be noticeable. With early "hybrid" sensors, only a few thousand of the millions of photosites on the imaging sensor were used in this way. If a sensor contains 20 million photosites (pixels), and 20,000 of them are used for sensor based ...


2

At least as a statement about how autofocus cameras work in general, I believe the quote is basically wrong. Older System Let's start at the beginning of AF: the Minolta Maxxum/Dynax/Alpha 7000 from 19851. In the original Minolta lineup, there are lenses in two varieties: some send 32 bytes of data, and others 45 bytes of data to the camera. This does ...


2

As a first-order approximation: Contrast-detection autofocus is more accurate and more flexible Phase-detection autofocus is much faster Contrast-detection always requires "hunting" back and forth to find the best focus; in ideal conditions phase-detect moves certainly to the right point Contrast-detection which uses the main sensor is likely to hurt ...


2

1600, aperture 5.6, shutter speed 1/64 is 'low light' Not dark by any means, but roughly 300 lux if I didn't do something wrong (no guarantee). (1 x 1600/100 x (5.6/1)^2 x 64/100 ~= 320) The 1/64th shutter speed may be marginal shake-wise depending on what your actual (unstated) focal length is and whether you are using an (unstated) tripod. Phase ...


2

Contrast AF in live view, Phase detection in all other modes. Explanation: Once you use live view, the light doesn't reach the AF phase detector in the prism anymore since the mirror is locked up. Nikon does not yet incorporate phase detection on the imaging sensor.


1

I agree small birds in flight are extremely challenging, but I don't think the main challenge is camera's autofocus system. The main challenge is that it's hard to spot small birds (I once went to a natural park to photograph birds -- I heard a huge amount of birds all around me, but rarely saw them and on the rare occasion I saw one, it flied 10 meters and ...


1

One category of focus error isn't handled by any manufacturer, except in the case of a few mirrorless models. Most high-speed lenses (e.g. f/1.4 or faster) have spherical aberration. When the lens aperture is stopped down, the focus shifts. Since most cameras do all focusing operations (contrast detect or phase-shift) with the aperture wide-open, they ...


1

AFMA works by applying an offset to the calculated amount of lens movement needed as computed by a phase detection AF system. It is used to compensate for several possible focus errors: Differences in optical distance from a lens to a dedicated AF sensor compared to the optical distance from a lens to the imaging sensor¹ Differences in the exact amount a ...


1

Lenses with heavy lens groups don't work well with contrast AF, because they can't be moved fast enough with the many tiny steps required for contrast AF. Thus, such lenses are usually not offered for systems with contrast AF. They can be adapted (e.g. FT lenses for mFT), but then they do not work well. So the practical limitation is that such lenses will ...


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