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Forum: Pentax DSLR Discussion 03-25-2019, 12:36 PM  
How does Pentax AF-C compare to other brands?
Posted By Frater
Replies: 113
Views: 10,514
Agree, that 1% is a bit to harsh. The sensitive, effective AF areas are a bit more than 5 times as big as the viewfinder square, in each dimension: at least 5x5 = 25x, but the odd T and L shapes take away a bit from the sensitive AF area again, so let't settle on: between 10x and 20x mismatch between viewfinder indicated size and actual (effective) AF area size.

Whereas for Nikon, it is a pretty close match. And they don't have these odd polygonal shapes, but are straight rectangular (or cross).

I assume, that Pentax has to cheat via AF area size to maintain AF sensitivity claims, so that it doesn't fall behind Nikon in that respect too much. Wheras Nikon can do AF sensors which are sensitive naturally, so that Nikon can keep them as small as indicated in the viewfinder, and still have -3 EV or whatever.

The Problem is though, that the Pentax cheat approach causes AF to fail on threedimensional smaller targets not only in darker situations, but all light situations suffer from that, as a collateral damage.

This was for the K-5, newer models may be better or worse (if to catch up with the sensitivity claims further raised by Nikon).

But for two dimensional stuff (landscapes) (everything at infinity), the actual size of the sensitive area is irrelevant anyway ;) Actually for landscape, you don't need AF alltogether.
Forum: Pentax DSLR Discussion 03-25-2019, 11:49 AM  
How does Pentax AF-C compare to other brands?
Posted By Frater
Replies: 113
Views: 10,514
I wouldn't be obsessed too much with tracking, unless you photograph dogs running towards you, or cars or whatever else is approaching at a constant, easily predictable speed. All cameras are comparably pretty good at this easy task.
However, what makes Pentax AF often useless for any serious people photography is not related to tracking. But instead to two unrelated flaws:

1- huge AF areas, each covering a big portion of the motiv, sometimes (outside the center spot) in awkward big T or L shaped areas.
Mind that the tiny squares in the view finder are only about 1 to 10% of the true area size of the AF regions.

For the German test, where they used a two dimentional flat "wall" to approach the camera, this is very favourable for Pentax.
However in real life situation, e.g. heads of persons (or animals) being a bit further away, thus appearing small to the camera, huge AF areas tend to fail.

A Nikon D7200 would fare well, because its AF points are really only just AF points, rather than huge oddly shaped AF regions as in Pentax cameras.

2. Pentax is bad in detecting (and tracking) small, erratic, random movements, as typical for e.g portrait photography with shallow DOF.

It is a pity that the German test had only a very simplistic test setup, only simulating a flat wall coming perpendicularly towards you in absolute constant speed.

In a better setup simulating real life problems, let's expect that the K-3 would have failed a lot, confirming the bad reputation of Pentax AF implementations out in the market.
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