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D7100: AF focuses on background a lot?


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doppler_fto

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"RE: D7100: AF focuses on background a lot?"

doppler_fto Registered since 23rd Mar 2013
Thu 09-May-13 06:12 PM

Just to be clear, I am absolutely in agreement that a repeatable test would both be even more convincing and would also help qualify what exactly is happening.

>Let me put it another way. You made available 6 NEF files.
>In at least 5 of the 6, your subject of interest is obviously
>moving, and you shot these at a shutter speed very susceptible
>to subject motion

The point of my previous response was to demonstrate that I have sufficient dexterity that hand-holding at 1/80s is no big deal. The self-portrait was 80mm at 1/15s with no VR and was fine.

Given that the post was 5 years ago, it should also suggest that, as I typically shoot at these shutter speeds, I am very familiar with how well or poorly a camera should focus at these shutter speeds. The D300 would knock 'em out all day long.

>In the 6th image (701_1009), focus was not where it should
>have been- the subject's face.

I'm not an idiot. I wasn't expecting the camera to focus on the guy's face when the focus point was clearly on his abdomen. I was expecting the camera to focus on his abdomen, though, which it did not.

>None of the images are razor sharp anywhere. You did a
>credible job (with one exception) under the circumstances but
>I don't think it's "good enough" for these purposes,
>simply due to the circumstances and challenges of that venue.

I think the disconnect here is that I see no real link between the quality of the final image and the AF performance during the capture of that image. Yes, I've been posting images that aren't very good. And? The fact of the matter is that the images are poorly-focused in such a way that the AF system is clearly focusing on something that isn't the intended subject.

Think about it from this perspective: The D7100 AF system is rated to operate down to -2 EV. ISO only affects the image sensor, and so high-ISO noise or noise-reduction blurring do not impact the AF system whatsoever. Moreover, all of the sample images were taken at well above that minimum light level. Even in cases where the subject is moving, AF-C should be tracking that moving subject. I mean, that's why I picked AF-C instead of AF-S. That's why I paid for two of the fastest-focusing lenses in the Nikkor lineup. And I can assure you that my D300 behaved as expected — plenty of noise, but the focus was dead on.

So the point is that I don't see how anyone could look at 0517, or 3898, and conclude that the AF system is doing the right thing. The shutter speed that I choose doesn't magically cause the AF system to focus on the background. (And even if it did, 3898 is in bright sunlight, and the central focus point is above an _extremely_ high-contrast target).

>That is NOT an assessment of your skills. It is simply a
>statement that this is not a good set of images for the
>purpose of supporting your claims.
>
>I don't think you got the point I tried to make about
>"extraordinary claims" and "extraordinary
>evidence". If you want to find agreement with your
>claims, you need to eliminate all doubt on our part, and you
>cannot expect others, via forum discussions, to thread fine
>needles while assessing a problem like this. You need
>slam-dunk impeccable images. That simply cannot be done hand
>held, in a poorly lit room, of obviously moving subjects, at
>1/100s and 80mm.

I am a mathematician and an engineer. I understand your point. Moreover, I write test-related software for a living, so I also understand the concept of a minimal test case. But I also understand that a sufficient burden of proof can demonstrate that a problem exists, even if it doesn't fully narrow down the cause of the problem. I believe that the test images (and associated context from my explanations) demonstrate that something is misbehaving with the AF systems on my D7100 cameras, although I don't yet know what it might be.

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  of moments
    that people love.

This is a hot topic  It has been locked by a team member D7100: AF focuses on background a lot? [View all] , doppler_fto , Fri 03-May-13 05:19 AM
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