Dynamic Range
In most circumstances the Z7's image quality will be remarkably similar to that of the D850 (as you might expect). However, it's not quite fair to say it's the same.
Key takeaways:
- In principle, dynamic range is comparable to that of the D850: one of the best performances we've seen.
- However, striping and banding (caused by the addition of on-sensor PDAF) means the usable dynamic range is reduced.
Dynamic range is the range of tonal values that a camera can represent, from the brightest recorded tone to the darkest usable value. There is, of course, an element of personal taste to this.
Rather than just quoting numbers, we try to show the photographic impact. We do this in two ways: first, our Exposure Latitude test shows the effect of lifting the shadows of images shot with increasingly low exposure (as you would if you were trying to capture/retain more highlight information).
However, reducing the exposure increases the noise level, which makes it difficult to tell what's happening at the sensor level. This is where our 'ISO Invariance' test comes in. Here we shoot different ISO settings using the same exposure values. This means that any difference in noise must come from the camera. Between the two tests, we can see how tolerant the camera's files are to having addition tonal information pulled up from the shadows into the visible parts of the image.
Exposure Latitude
In many respects the Z7 performs comparably with the D850, something confirmed quantitatively in our collaboration with Bill Claff. This means it stands up well against the best of its immediate peers, and for the most part better than some other full frame rivals, with a caveat we'll get to below. There is, as you'd expect, a small noise benefit to shooting at ISO 64, rather than 100, simply because the camera can collect more total light at lower ISOs.
Look closely, though and beyond a 4EV push, a banding pattern starts to appear. You may not think that you push images by 4EV, but lifting the shadows or adding too much contrast, if you're trying to brighten the foreground of a sunset image for example, may have the same effect. And it may even be visible at print size, as the banding is easily visible in even lower resolution (8 MP) downsized images.
ISO Invariance
In terms of ISO Invariance, there's a visible noise difference between shooting at ISO 64 and brightening, vs natively shooting at higher ISO. This isn't because there's a lot of noise being added at ISO 64 (the result is at least as good, or noticeably better than its immediate peers). Instead it's because the higher ISO performance is even better.
The sensor uses a 'dual gain' design that at higher ISO uses a lower-noise readout method in each pixel (but at the cost of capacity, hence it not being used in lower ISO modes). From ISO 400 upwards, once in this higher gain mode, there's little benefit to increasing the ISO setting (you may as well keep the ISO at 400 then brighten your images when processing the Raws).
Even then, on the D850 the difference was small enough that there are times it's preferable to shoot at ISO 64, rather than 400, since this means you retain around two and two-thirds stops of extra highlight information. This benefit might outweigh the additional noise cost were it not for...
PDAF Banding and Striping
As you'll see in some of the images above, a banding pattern can appear if you try to pull darker tones into the image. The frequency of these stripes leads us to believe it's a result of the way the phase-detection rows of pixels are incorporated into the image (something supported by our collaboration with Bill Claff).
In addition to the banding in shadows, we've also occasionally - albeit very rarely - seen the same pattern in images with flare caused by a bright light source (a striping problem that we've seen on other cameras with on-sensor phase detection elements).
How much, if any, impact this has for you will depend on your photography. It doesn't happen in every image with a bright light source and, based on our experience with other cameras, may be lens-dependent. You also may not encounter it if you only make minor adjustments to your Raw files. However, it is something to be aware of and which shows the Z7 shouldn't be assumed to have 'D850 image quality.'
*This image was shot using a pre-production camera. However we conducted all other testing and confirmed the result using a final 'production spec' camera before publishing.
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