Question re Nikon AF location

adbsgicom

Does anyone know why the Nikon system seem bent on keeping the cross-type AF points all centered? It is one of the few things about Nikon that doesn't seem well-thought out (or I haven't figured out the wisdom in). Do the D3/4 series not do this? Poking around, the Canon D1 X has them spread over the full range of the AF zone.

ziggy53

Both Canon and Nikon have very competent AF systems. The more cross-type sensors, the more AF data has to be accumulated and processed. Nikon has, for a long time, also processed color data from the exposure sensor, adding to the complexity. (The last couple of Canon generations also process color data, although somewhat differently from Nikon.)

More data means more processing requirements for a given camera throughput, meaning a more powerful image processor is required, meaning more manufacturing costs.

For reasons that are beyond my research it is increasingly harder to place increasingly more complex AF sensors further from the center axis of a lens. I suspect that it partly has to do with the center being "balanced" in terms of the digital rangefinder, and then increasingly more "unbalanced" as you move to samples towards the edges and corners, resulting in more processor compensation in processing the 2 data streams, in the case of a cross-sensor, versus a single data stream in a linear AF sensor. Small aperture lenses appear to compound the problems. (Dual cross sensors are yet another level of complexity.)

All camera manufacturers try to strike a balance of speed, accuracy, utility and cost. I believe that those are the primary determinants in the decisions for AF sensor design and selection.

The most complicated AF sensors, the Area-SIR type sensors, are reserved for the single-digit Nikon and Canon cameras, partly because they are also the most sensitive type of sensor, but they also seem to be the most accurate, although even Area-SIR uses cross-type and linear regions to form the actual data samples used for focus determination.

Remember that all of these systems relate to "phase differential" data, in which both the distance and the direction of defocus are sampled, compared to to the simpler edge detection used in simpler contrast-detect AF systems, like those used in most mirrorless designs. There is nothing too simple about phase-detect AF analysis.

The most cogent document regarding the concepts involved is here:

http://dougkerr.net/pumpkin/articles/Split_Prism.pdf

adbsgicom

Thanks, Ziggy!

Matthew Saville

In layman's terms, yeah Canon has more cross-type AF points to the left and right, compared to Nikon's central clump of AF points. Nikon's D2-series used to have cross-type points that were more spread out, but the D3 changed things.

But what Nikon sacrifices in edge focus accuracy, they make up for in central focus accuracy. The D3 and D3s are renowned as the champions of low-light focus tracking, and even the D700 is an amazing performer in extremely low light.

Hopefully the next generation of Nikons will have improved AF off-center, even though the D4 / D800 still have a very similar focus point spread pattern. Personally, I've shot with both the 1Ds / 1D series, (though not the 1DX yet) and I have to say that I don't really feel a serious advantage in the edge AF points. The bottom line is that no matter what, the center point is always going to be your best bet in extremely low light.

=Matt=

adbsgicom

Thanks. It is just something I noticed in my move from Canon to Nikon. Occassionally, using the edge AF, things don't focus/focus fast, especially in portrait orientation. Generally not a problem for doing portraits and getting focus on the eye, but doing kids moving around and wanting to have the focus well off-center is sometimes a bit annoying. Still glad I made the switch; just one of those odd differences.

Matthew Saville

adbsgicom wrote: »

Thanks. It is just something I noticed in my move from Canon to Nikon. Occassionally, using the edge AF, things don't focus/focus fast, especially in portrait orientation. Generally not a problem for doing portraits and getting focus on the eye, but doing kids moving around and wanting to have the focus well off-center is sometimes a bit annoying. Still glad I made the switch; just one of those odd differences.

Which cameras are you using, by the way? The 7D's edge points might be better than the D300's, for example, but on a D700 I hardly notice a difference compared to a 1Ds...

Although on full-frame, the "edge" focus points aren't nearly close enough to the edges anyways. :-(

=Matt=

adbsgicom

I went from the 5Dmk2 -> D700. I've never tried a 1D series. Given I didn't have both bodies at the same time for long, I never did a real side-by-side so some of it may be mis-remembered...

Matthew Saville

adbsgicom wrote: »

I went from the 5Dmk2 -> D700. I've never tried a 1D series. Given I didn't have both bodies at the same time for long, I never did a real side-by-side so some of it may be mis-remembered...

Indeed the 5D mk2 is one of the WORST cameras for edge focus point use, they're not cross type of course, only the 7D and 1D-series have off-center cross-type AF points if I am not mistaken. (Maybe the 40D / 50D?) Either way, yeah full-frame's one disadvantage in focusing is focus point spread.

Luckily, if you REALLY value off-center focusing, the DX D400 should be coming soon and although the low-light performance and DOF won't be the same as full-frame, it will have GREAT focus point spread. The edge focus points are still going to be non cross-type, but with the right focusing mode and technique, you'll be quite pleased. I've always found that my D300 + D700 combination serves me very well in that respect... :-)

=Matt=

ziggy53

Matthew Saville wrote: »

Indeed the 5D mk2 is one of the WORST cameras for edge focus point use, they're not cross type of course, only the 7D and 1D-series have off-center cross-type AF points if I am not mistaken. (Maybe the 40D / 50D?) ...

The Canon 40D/50D/60D have 9-point AF, all cross-type at f5.6 or faster. The center AF point is also a high-precision sensor with lenses of aperture f2.8 or faster.