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Equivalency and auto-focusing

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Jack Hogan said:
1) The size of the imaging sensor has nothing (or very little) to do with phase detection, which is typically performed by separate sensing elements
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I would think the size of the sensor generally governs the size of the mirror box, which in turn governs the (max) size of the AF module. Just as total light is a function sensor size and plays a crucial role in low light performance, I would think the same would be true for PD sensors in the AF module. The larger the PD sensors, the greater their S/N, so it would stand to reason that larger format cameras should generally have better low light PD performance. Of course this is predicated on all else being equal, but for some reason I don't think it is.
 
Andre Affleck said:
Jack Hogan said:
1) The size of the imaging sensor has nothing (or very little) to do with phase detection, which is typically performed by separate sensing elements
Click to expand...
I would think the size of the sensor generally governs the size of the mirror box, which in turn governs the (max) size of the AF module. Just as total light is a function sensor size and plays a crucial role in low light performance, I would think the same would be true for PD sensors in the AF module. The larger the PD sensors, the greater their S/N, so it would stand to reason that larger format cameras should generally have better low light PD performance. Of course this is predicated on all else being equal, but for some reason I don't think it is.
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There are a number of variables. The Nikon D750 uses what Nikon calls the 'Advanced MultiCAM 3500FX II' module. The D4s and D810 use the 'Advanced MultiCAM 3500FX' module. What's the difference? It looks like, from the spread of the focus point, that the 'Advanced MultiCAM 3500FX II' is in fact the 'Advanced MultiCAM 3500DX' module used in the D7100 - presumably selected in order to allow the D750 to be more compact (it's actually pretty tiny for a full-frame DSLR). However, by all accounts, the D750 AFs as well as the others, even if the spread of focus points is a little smaller.
 
His posting patterns suggests he posts to impress, not to learn. The moment this thread left the view of his gang on m4/3rds, he abandoned it.
 
The question is whether the advertised -3 EV is achieved with f/5.6 lenses or if faster lenses are needed (and even making a difference in focus precision)?! Canon states quite clearly that the center AF point is more precise for lenses of f/2.8 and faster and that this is where -3 EV is achieved only. So when a M43 camera can achieve -3 EV (-4 EV for the GH-4) anywhere then the slower FF f/5.6 lens may indeed be at a disadvantage where focusing is concerned.

Especially since I would assume that the extreme limit of -3 EV also means that the camera is faster/better at other low light levels (say 0 EV), because these are now less extreme to begin with!?
 
Timur Born said:
The question is whether the advertised -3 EV is achieved with f/5.6 lenses or if faster lenses are needed (and even making a difference in focus precision)?! Canon states quite clearly that the center AF point is more precise for lenses of f/2.8 and faster and that this is where -3 EV is achieved only. So when a M43 camera can achieve -3 EV (-4 EV for the GH-4) anywhere then the slower FF f/5.6 lens may indeed be at a disadvantage where focusing is concerned.

Especially since I would assume that the extreme limit of -3 EV also means that the camera is faster/better at other low light levels (say 0 EV), because these are now less extreme to begin with!?
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The following image made me believe that fast lenses are necessary not only for higher precision, but also for lower light focusing on the 7D Mark II. It clearly says: "When f/2.8 lens is used."

7d_mkii_feature_03a.jpg


But after going through Canon pages again it seems rather to be that fast lenses are just necessary for the double cross high precision stuff (to get light on the extra vertical strips)?!

Now my question would be if it just happens to coincide that the center AF point is the most light sensitive (-3 EV) as well, or are those additional vertical "high precision" strips needed to achieve that? Or the other way around, will the center AF point of a Canon also focus down to -3 EV with slower lenses (f/5.6) where the extra diagonal strips are not used?

What about the Nikon AF then. Does it focus down to -3 EV with f/5.6 lenses or does it need faster lenses to do so? There are no clear documentations for Nikon as it seems.

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Timur Born said:
The following image made me believe that fast lenses are necessary not only for higher precision, but also for lower light focusing on the 7D Mark II. It clearly says: "When f/2.8 lens is used."
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That's the 7D MkII sensor, which doesn't have much space left on the chip with all those focus points. Based on the feedback coming from early users, it also seems to have 'issues'.
Timur Born said:
7d_mkii_feature_03a.jpg


But after going through Canon pages again it seems rather to be that fast lenses are just necessary for the double cross high precision stuff (to get light on the extra vertical strips)?!
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It's not to do with more light, it's simply that the sensor strips are positioned to read from patches on an f/2.8 exit pupil, if the pupil is any smaller, they're in the dark.
Timur Born said:
Now my question would be if it just happens to coincide that the center AF point is the most light sensitive (-3 EV) as well, or are those additional vertical "high precision" strips needed to achieve that? Or the other way around, will the center AF point of a Canon also focus down to -3 EV with slower lenses (f/5.6) where the extra diagonal strips are not used?
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The centre point is in fact several points overlayed, each (linear) point with a pair of strips. It needs eight separator lenses (for vertical, horizontal and diagonal lines) and the strips positioned relative to those lines to read from a f/2.8, f/4 and f/8 exit pupil. It makes design sense to make the sensitive ones the f/2.8, since it is likely that fast lenses meeting accurate focus will be being used in low light.

In the Nikon sensors, they have less emphasis on cross and double cross points, so more space on the sensor chip and have been able to offer the -3 EV capability with more of the points.
Timur Born said:
What about the Nikon AF then. Does it focus down to -3 EV with f/5.6 lenses or does it need faster lenses to do so? There are no clear documentations for Nikon as it seems.
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The Nikon system doesn't have the diagonal points or the f/2.8 focus capability and confines the cross points to the central block. In practice, it doesn't seem to lose much from that



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