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Author Topic: Low light focus ability of z7 vs d850  (Read 517 times)

bwana

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Low light focus ability of z7 vs d850
« on: November 13, 2019, 10:32:15 pm »

One of the reasons for fast glass on an slr is to maximize light capture to allow focus. But with a mirrorless, focusing is achieved with the same sensor that captures the image. Aside from eliminating the need to do ‘af tuning’, doesn’t this also allow a mirrorless to focus in much dimmer light (since it can amplify the signal more and better off a large sensor rather than the smaller/fewer sensors  near the prism facing the mirror)? Shouldnt focussing on a mirrorless be orders of magnitude more sensitive than an SLR (lower EV)?

But from what i've read, the d850 focusses worse in live view. why is that?
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Aram Hăvărneanu

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Re: Low light focus ability of z7 vs d850
« Reply #1 on: November 14, 2019, 06:26:39 am »

Phase-detect focusing requires a fast aperture not because it needs lots of light, but because essentially it is measuring parallax. There has to be enough of a geometric difference in the light paths from a point to the sensors in order to work. This is true for both DSLRs and mirroless cameras, there is no fundamental difference here.

The d850 focuses slower (not worse, if anything, better) in live view because it uses contrast-detect focusing. The Z7 uses a combination of both phase and contract-detect.

The amount of light per unit area is the same for DSLR and mirroless for a given f-stop, so there's no difference there either. Also the sensors can be amplified just as much.

In principle, contrast detect could work in dimmer EV levels (and smaller apertures) compared to phase-detect at the expense of speed, not sure in practice if that is true or not (probably not).
« Last Edit: November 14, 2019, 06:35:33 am by Aram Hăvărneanu »
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bwana

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Re: Low light focus ability of z7 vs d850
« Reply #2 on: November 14, 2019, 09:26:58 am »

Thank you. I guess then the only worry with using focus points at the edges is the blurriness caused by the lens and the light falloff.
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RPark

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Re: Low light focus ability of z7 vs d850
« Reply #3 on: November 18, 2019, 04:50:40 pm »

Without getting into the technical side of things (and I own a D800, not D850) I have never had much of an issue with my Z6 focussing. I did have a problem with backlit subjects once but that seems much improved with firmware 2.0 and 2.1.

I haven't tested every possible combination of subjects in low light but, again, I just haven't seen the kind of problems hashed out endlessly in Facebook groups. I suspect some of that chatter is from inexperience or deliberate disruption.

In short, I'm very happy with the focussing performance of the Z6 for stills and video.

Incidentally, my introduction to mirrorless came via the Fujifilm X-Pro2 (traded 2 Nikon bodies and several f-mount lenses) which I bought in April 2016. I recently made a video comparing in-camera 4K video output from both cameras.
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BernardLanguillier

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Re: Low light focus ability of z7 vs d850
« Reply #4 on: November 18, 2019, 07:40:34 pm »

In my experience the Z6 is significantly better at focusing than the Z7.

Just like the a9II is significantly better than the a7rIV.

Cheers,
Bernard

D Fuller

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Re: Low light focus ability of z7 vs d850
« Reply #5 on: November 18, 2019, 08:52:36 pm »

In my experience the Z6 is significantly better at focusing than the Z7.

Just like the a9II is significantly better than the a7rIV.

Cheers,
Bernard

I think it’s significant that every “best-in-class” focusing camera I know of is in the 24Mpix range. I suspect that the combination of the sensitivity of larger photo sites and less data-moving overhead makes that so.
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BernardLanguillier

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Re: Low light focus ability of z7 vs d850
« Reply #6 on: November 18, 2019, 09:07:49 pm »

I think it’s significant that every “best-in-class” focusing camera I know of is in the 24Mpix range. I suspect that the combination of the sensitivity of larger photo sites and less data-moving overhead makes that so.

Yes, indeed.

It's probably impacted by:
- EVF refresh rate
- Sensor read-out speed
- light collecting ability of AF sensels (better with low res)
- power optimization scheme
- availability or not of dedicated processors
- ...

Cheers,
Bernard
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