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On some of Sony's new 4K cams they claim the following:

The new model incorporates a 25x optical zoom lens to cover a wide focal range for multiple shooting requirements, with three independent lens rings to support intuitive operation. Using optical zoom lens with Digital Extender in an HD mode, a 50x zoom equivalent image without any picture degradation can be captured thanks to its 4K resolution sensor.
Sony Z190

Basically, Sony claims that this "digital extender" feature can digitally double your zoom, but still send full HD resolution.

I'm skeptical there's no significant difference. I would think that using only a portion of the sensor with the extender means less light and color information goes into making the image compared to down scaling the 4K image. Also, I'm not a physicist, but my understanding is that it's physically not the same as more optical zoom with more exposure.

Am I right? What are the differences I should notice? I actually do own this cam and personally think there is a loss in clarity, but can't explain why that would be. Exposure and color look the same to me.

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    I don't know about this camera/lens specifically, but a few years back I saw a demo at NAB that electronically panned an HD image out of a 4K raster. There was no noticeable quality drop compared to a native HD image. – Michael Liebman Feb 9 at 3:39
  • @MichaelLiebman I'm very much intrested in the name of any available softwares that do that. I'm also intrested if the software can auto track motion within the frame. Do you member what this after was called with a company that makes it? – user24601 Apr 12 at 21:08
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    Conveniently enough, at NAB this week, Panasonic demo'd extracting a 4K crop from an 8K sensor. And like you were hoping, they are using computer vision techniques to track faces, including in profile, and key objects. The example they gave was being able to keep a tennis player who is serving, and their racquet, properly framed. Unfortunately, I can't find an product info, but I'm pretty sure it is going to be an add-on to their PTZ cameras. – Michael Liebman Apr 13 at 0:41
  • @MichaelLiebman I think I need to start attending NAB. Is this the same thing or a lesser event or something else? – user24601 Apr 13 at 1:09
  • By the way, I heard rumor that the Hollywood red carpet events are managed with a massive sized sensor camera and they capture multiple full HD scenes from the one cam, all along the red carpet. Or maybe a series of three or four, but still, impressive if true. – user24601 Apr 13 at 1:11
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I've shot 4k footage since 2014, and when mastering in 1080p, I often use digital zoom in post. There are multiple factors I would look at.

  • Cameras that use a single sensor will display debeyering artefacts when examining the direct pixel readout (mushier detail and reduced colour information). This issue is nicely improved when downsampling 4k to 1080p, but is revealed when digitally zooming. The Z190, however, uses a 3-chip design. If each chip is full 4k (which the marketing copy certainly implies), the quality difference between downsampled and a direct pixel crop should be significantly less noticeable than on single-chip competitors.
  • Lens resolution can be a contributing factor as well. If the lens barely keeps up with the 4k resolution of the sensors, or the three-chip design introduces optical issues, then downsampling will likely hide minor softness. Digital zoom will reveal these optical shortcomings simply by magnifying what's being projected onto the sensor. On the upside, lens performance is typically best near the centre, so a centre crop is at least likely to be optically stronger than the edges.
  • While exposure and colour should look identical because nothing is changing (aperture, shutter speed, gain, pixel pitch), you're right that the exposure performance will change, as measured by the amount or type of visible grain. From user reports I've read, this is best demonstrated in the Sony A7 series of cameras where similar light sensitivity (based on the amount of visual noise) can be seen in direct pixel readouts from the A7s (giant pixels, lower resolution) and downsampled readouts from the A7r (smaller pixels, higher resolution, but the subsequent higher noise is reduced during downsampling). Digital zoom will reveal the per-pixel noise that downsampling would have averaged out, which can impact perceptual sharpness.
  • If you decide to record in 4k and downsample to 1080p afterward, zooming as needed in post, compression may come into play as well. While the Z190 records 4:2:2 10-bit, digital zoom can reveal minor compression softness in colour detail that would have been reduced during downsampling. However, I would expect this to apply only to downsampling/zooming 4k footage in post. If the camera is recording a 1080p 4:2:2 image directly, switching between a downsampled full sensor readout and a crop of the centre pixels should have little to no impact on compression results.
  • Some cameras may simply process things slightly differently when performing downsampling versus direct pixel readout.

When tracking down the source of the softness, I'd recommend starting by recording 4k footage and cropping it in post, then comparing it to footage shot using the 1080p zoom mode. Though highly unlikely, if poor camera processing of 1080p footage is to blame you'll see the difference here.

It's more likely a combination of lens resolution, optical performance of the three-chip design, and perceptual sharpness improvements from reduced noise when downsampling. But by all means, try some locked down resolution tests first! After tracking down the source(s) of the softness, then it's a question of how to combat it.

My approach has always been to shoot in 4k and rely on "digital zoom" in post while never reaching a full 2x zoom (direct pixel crop). This ensures the footage is always downsampled to some extent, maintaining the higher relative quality we're used to seeing now.

  • Giving the selection, but I'll report back if I ever get around to some resolution testing. – user24601 Apr 11 at 2:12

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