Z7 auto ISO seems seriously flawed

Started 4 months ago | Discussions thread
FingerPainter Veteran Member • Posts: 7,966
What do you want? Where can you get it?
9

j2scriba wrote:

The auto ISO setting has two adjustable parameters:
"minimum shutter speed" and "maximum sensitivity"
One would expect the algorithm to select ISO as low as possible to achieve the mimimum shutter speed in automatic modes and increase ISO if light is too low to do that, until the maximum sensitivity is reached.

However, this is not quite what is happening.

Yes, it is what is happening. The question is what should be expected to happen when both the ISO and shutter thresholds have been reached and light drops even further. What does happen is exactly as documented.

If maximum ISO is reached, the A-mode increases shutter speed regardless of the mimim shutter speed.

That is how it is documented to work.

At first I thought there was some hidden option I had overlooked, but Nikon support (Germany) confirmed that this is how it's supposed to work. The reasoning behind it was explained like this: "Since A, S, P are automatic modes, the camera does whatever it takes to achieve good exposure."

OK, so you have reached minimium shutter and maximum ISO, you are in an autoexposure mode, and it get darker. What do you want the camera to do? You would seem to have three theoretical options:

  1. exceed the shutter speed boundary
  2. exceed the ISO boundary
  3. stop adjusting settings and accept a darker image. I.e. get out of autoexposure mode.

Nikon chose the first option. You don't like that choice.

Which of the other two options do you prefer?

If you choose the second option, you can get the exact same behaviour by choosing a higher max ISO with the existing implementation. So there is no need for Nikon to make any change. Why don't you do that? What do you see as the downside?

No camera that I know of has a mode where it is in autoexposure only within a certain EV range. You are either in an autoexposure mode, which means the camera will adjust settings if it can to maintain a targetted lightness, or you are in a manual mode and it is up to you to adjust settings.

This may be an acceptable design paradigm for a point-and-shoot,

How many point and shoots do you know of that let you set a minimums shutter speed for auto-ISO configuration?

but for a presumably professional target group this seems bizarre.

A pro would know that if his shooting envelope is so wide that he is likely top hit both his max ISO and min aperture, that he should decide in advance which of these he is willing to adjust.

For me, this feature is useless in this implementation.

Can you point to a camera that has an implementation you can use?

Here's the scenario I'm looking at: I need to shoot from a moving car and need a minimum shutter speed of 1/250s.

And presumably you never want to adjust shutter manually yourself and part of the time there will be so much light that you would need faster than 1/250 at base ISO, so you can't just use M mode and Auto-ISO.

I want to shoot wide open to get windscreen dirt out of focus so I select A-mode at open aperture.

OK

The camera adjusts shutter speeds shorter than 1/250s. When things get dark, e.g. in tunnels, ISO is increased. When the scene gets really dark and there's not much to see, there's no point in turning ISO all the way up to max and blow out whatever light patches there might be to reveal only noisy shadows, so I set a max ISO of, say 8000.

So you want the camera to start producing darker than metered images rather than exceed either the max ISO or the min shutter. You want its autoexposure mode to stop being an autoexposure when both thresholds have been reached. Do you know of any camera that does this?

Have you tried highlight-weighted metering as a way to protect them in cases like this?

With the strange auto-ISO design the camera will disregard minimum shutter speed and motion-blurred pictures.

I don't think the design Nikon chose is as strange as one where an Auto-exposure mode would stop being one. That's not to deny that there would seem to be an application where such an option could be useful. But Nikon's design decision is not at all strange. There are few stills-taking situations where you'd need what you describe. A stills camera user would adjust settings himself when needed.

You are running into this problem partly because you are shooting video from a moving platform. This mean you get extreme changes in light in a short time with no opportunity to change settings. Nikon doesn't really think in video, and the Z7 is a stills camera that happens to be able to take some video. It is not a video camera. I'd be curious to know if there is an actual video camera that behaves the way you would like.

As everybody knows, ISO is mostly irrelevant in digital cameras (after hitting a camera-specific optimum ISO) and post processing RAW files to boost shadows will produce very similar or better results to high ISO during exposure, as we have better control over noise

no you don't

and much better preserved highlights. So headline-grabbing extreme-ISO shouldn't be used for serious work, anyway.

So sometimes you want to be able to accept images that darker than targeted. Is  there a camera that lets you automagically shift between doing this and getting the targeted lightness?

The strange disregard for minimum shutter speed pretty much spoils this feature.

It spoils if for you peculiar use case which seems even stranger to me, from a stills-shooting perspective.

Underexposure can be fixed in post,

No it can't. Exposure is fixed at capture time. Under-lightness can be fixed in post, but you still have to deal with the other effect of underexposure: low SNR and, as a result, have to choose between losing detail with "noise-reduction" or accepting noise.

blurred pictures cannot.

Right so you have to prioritise shutter speed over "correct" lightness, which you are not doing by being in A mode or any other autoexposure mode.

Am I the only one who thinks that "No" (as in "No longer exposure than I specified") should really mean "No" in this setting?

Probably not the only one, but I don't know how many people would agree that what the camera should do is switch out of being in an autoexposure mode when both the shutter and ISO thresholds have been reached. Personally I think it is an intriguing idea, but since I know of no camera that offers it, it doesn't seem right to single out the Z7 for your criticism. What's really going on is that you misunderstood how the mode worked.

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