Why the sluggish AFS primes?

Started Jun 19, 2012 | Discussions thread
motobloat Regular Member • Posts: 497
Re: AF Accuracy FX vs. DX

vockman wrote:

szal wrote:

vockman wrote:

[…] FX is more accurate than DX.

Why is that? Can you enlighten us?

I have no idea why, just from experience and especially with fast lenses including 2.8 zooms. The D200 I had a lot of problems with. The D7000 is better. But with the D700 focising is almost always perfect. It might just be the overall DX look; that the subjects is never sharp enough and the background is too sharp and the separation is not as good.

It might also be that the selected focus area on a DX covers a relatively larger area than on FX.

I've always find DX format to be much more challenging to use than FX and maybe demands better technique

You are not comparing apples to apples here. Completely different AF systems, and different pixel counts mean that you would have to downsize images to fairly compare AF accuracy.

The D700 has a 51-point CAM3500 AF system and is 12MP.
The D7000 has a 39-point CAM4500DX AF system and is 16MP.
The D200 has a 9-point CAM1000 AF system and is 10MP.

A fair comparison would be the D300 vs. the D700. Both are 12MP and both have the same 51-point AF system. I think if you make this comparison, you will find that the D300 has "more accurate" autofocus with the same lens. In fact, accuracy is about the same (same lens, same AF system), but due to the larger DOF of the DX sensor, the D300 will be able to hide AF errors better than the D700.

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