Explain 4k (resolution) vs Megapixels (resolution)

Started Nov 21, 2017 | Discussions
orey10m Regular Member • Posts: 223
Explain 4k (resolution) vs Megapixels (resolution)
2

Are both of these terms related to Resolution? (4k vs. 20MP for example)

Can an 8MP camera sensor take a 4k video?

Is it 'proper' to describe a photo as "4k", or is this term only to be used for video resolutions?

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mosswings Veteran Member • Posts: 9,776
Re: Explain 4k (resolution) vs Megapixels (resolution)
6

orey10m wrote:

Are both of these terms related to Resolution? (4k vs. 20MP for example)

Can an 8MP camera sensor take a 4k video?

Is it 'proper' to describe a photo as "4k", or is this term only to be used for video resolutions?

4K refers roughly to the horizontal resolution of an image, but the term is interpreted in various ways. Theoretically, it's an image with 4096 pixels in the horizontal direction. But - a 4K HDTV image has a 16:9 aspect ratio and is 3840 pixels wide by 2160 pixels high, for a total pixel count of 8294000, or 8.3MP. On the horizon are 8K HDTVs, with 7680 x 4320 pixels, or 33.2MP. An intermediate step is 6K HDTV with 18.66MP and 5760 pixels wide by 3240 pixels high.

It's a bit trickier to compare HDTV or cinematic formats with still image formats, because the aspect ratios are different - traditionally, 2:3, or 3:4 in many mirrorless cameras. So a 24MP image in 2:3 ratio is 6000 pixels wide by 4000 pixels high. Obviously, there's a mismatch.

In general, what stills cameras do is use a portion of the frame that makes it easy to extract a video image of the correct aspect ratio. This introduces a problem in that the angle of view reduces proportionately, so unless you are taking a video in the native aspect ratio and resolution of the camera, the video image and stills image differ.

In answer to your question - can an 8MP camera take a 4K video - the answer is no, unless the camera is shooting natively in 16:9 aspect ratio. An 8K 2:3 camera sensor is 3468 x 2312, not quite enough resolution horizontally.

https://www.cnet.com/news/tv-resolution-confusion-1080p-2k-uhd-4k-and-what-they-all-mean/

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xpatUSA
xpatUSA Forum Pro • Posts: 19,491
Re: Explain 4k (resolution) vs Megapixels (resolution)
1

orey10m wrote:

Are both of these terms related to Resolution? (4K vs. 20MP for example)

Oh, oh . . . .

Just kidding, don't shoot me.

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57even Forum Pro • Posts: 15,345
Re: Explain 4k (resolution) vs Megapixels (resolution)

orey10m wrote:

Are both of these terms related to Resolution? (4k vs. 20MP for example)

Can an 8MP camera sensor take a 4k video?

Is it 'proper' to describe a photo as "4k", or is this term only to be used for video resolutions?

As Mosswings said, 4K is 4000 X 2250 pixels in 16:9 video format, or 8.3MP.

A 3:2 ratio image sensor that had 4000 horizontal pixels would have 2667 vertical pixels, or 10.7 megapixels.

So you need an 10.7 + MP camera to shoot native 4K video. Fortunately, that includes just about every modern camera on the market.

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OP orey10m Regular Member • Posts: 223
Re: Explain 4k (resolution) vs Megapixels (resolution)

I checked out that link from cnet - thanks for sending it over and helping to explain.

So 4k is a way of saying "this camera's sensor has enough pixel density (MP) that when it outputs the image later on a screen, it will have/provide data for every pixel on that 4k TV."

But sensors don't really have pixels themselves do they?

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mosswings Veteran Member • Posts: 9,776
Re: Explain 4k (resolution) vs Megapixels (resolution)

orey10m wrote:

I checked out that link from cnet - thanks for sending it over and helping to explain.

So 4k is a way of saying "this camera's sensor has enough pixel density (MP) that when it outputs the image later on a screen, it will have/provide data for every pixel on that 4k TV."

But sensors don't really have pixels themselves do they?

Sensors are essentially nothing but a huge array of photosensitive elements that capture light.  These elements are termed pixels - or sensels to make clear their sensing functions. The term pixel more generally refers to the atomic component of a digital image.

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mosswings Veteran Member • Posts: 9,776
Re: Explain 4k (resolution) vs Megapixels (resolution)
1

57even wrote:

orey10m wrote:

Are both of these terms related to Resolution? (4k vs. 20MP for example)

Can an 8MP camera sensor take a 4k video?

Is it 'proper' to describe a photo as "4k", or is this term only to be used for video resolutions?

As Mosswings said, 4K is 4000 X 2250 pixels in 16:9 video format, or 8.3MP.

To be picky, 4000 x 2250 is 9MP. Standard 4K is 3840 x 2160, 8.3MP. But this just shows how many interpretations of "4K" there are.  It's a marketing term, mostly.

A 3:2 ratio image sensor that had 4000 horizontal pixels would have 2667 vertical pixels, or 10.7 megapixels.

So you need an 10.7 + MP camera to shoot native 4K video. Fortunately, that includes just about every modern camera on the market.

If for some reason you wanted an exact 1:1 crop horizontally, you could make a sensor with 3840 pixels on the long side - then a 2:3 sensor would have 2560 pixels vertically, or 9.83MP. A 4K 1:1 crop compatible u4/3 4:3 sensor would be 3840 by 2880, or 11.1MP.

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The problem with modern cameras is that they don't usually have a 1:1 crop in video mode, so they do all sorts of bad things to get video at maximum possible crop - dropping lines, pixel binning, you name it.  It's definitely a forced marriage.

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xpatUSA
xpatUSA Forum Pro • Posts: 19,491
Re: Explain 4k (resolution) vs Megapixels (resolution)

mosswings wrote:

orey10m wrote:

I checked out that link from cnet - thanks for sending it over and helping to explain.

So 4k is a way of saying "this camera's sensor has enough pixel density (MP) that when it outputs the image later on a screen, it will have/provide data for every pixel on that 4k TV."

But sensors don't really have pixels themselves do they?

Sensors are essentially nothing but a huge array of photosensitive elements that capture light. These elements are termed pixels - or sensels to make clear their sensing functions. The term pixel more generally refers to the atomic component of a digital image.

You beat me to it!

I was a little suspicious of the statement's intent but here's more from a Foveon Scientist, if @orey10m really meant it:

http://kronometric.org/phot/sensor/fov/ABriefHistoryofPixel2.pdf

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mosswings Veteran Member • Posts: 9,776
Re: Explain 4k (resolution) vs Megapixels (resolution)
1

xpatUSA wrote:

mosswings wrote:

orey10m wrote:

I checked out that link from cnet - thanks for sending it over and helping to explain.

So 4k is a way of saying "this camera's sensor has enough pixel density (MP) that when it outputs the image later on a screen, it will have/provide data for every pixel on that 4k TV."

But sensors don't really have pixels themselves do they?

Sensors are essentially nothing but a huge array of photosensitive elements that capture light. These elements are termed pixels - or sensels to make clear their sensing functions. The term pixel more generally refers to the atomic component of a digital image.

You beat me to it!

I was a little suspicious of the statement's intent but here's more from a Foveon Scientist, if @orey10m really meant it:

http://kronometric.org/phot/sensor/fov/ABriefHistoryofPixel2.pdf

Ah, yes. And if we choose, we can now head down the rabbit hole of luminance resolution vs. chrominance resolution in the various sensor types.
We haven't even begun to discuss that a 4K HDTV, with 8.3MP stated resolution, is actually talking about a TRIAD of display elements - R, G, and B, for a total of 24.9 addressable sub-pixels (yes, that's a term),  The Bayer filters used in conventional sensors create an image file with an effective resolution about 2.3-2.4 times lower than their advertised resolution. Yes, a 24MP sensor generates a file with 24MP in it, but it's not able to resolve color to more than 10MP or so.  That's not been a problem, because the eye is more sensitive to luminance, and doesn't see the reduced color resolution as much.

Bottom line - if you want a 4K HDTV image with full panchromatic resolution of 8.3MP, you have to start with a Bayer filtered sensor with 2.4 times the pixels we've been quoting here.

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MrBrightSide
MrBrightSide Contributing Member • Posts: 926
Re: Explain 4k (resolution) vs Megapixels (resolution)

So then would it be safe to say that the 3-sensor 4K cameras from Sharp and Sony will provide video image quality that single-sensor 4K cameras can't match, lenses, illumination, and processing being equal (even though they have smaller 2/3" sensors)?

The Bayer filters used in conventional sensors create an image file with an effective resolution about 2.3-2.4 times lower than their advertised resolution.

mosswings Veteran Member • Posts: 9,776
Re: Explain 4k (resolution) vs Megapixels (resolution)
1

MrBrightSide wrote:

So then would it be safe to say that the 3-sensor 4K cameras from Sharp and Sony will provide video image quality that single-sensor 4K cameras can't match, lenses, illumination, and processing being equal (even though they have smaller 2/3" sensors)?

The Bayer filters used in conventional sensors create an image file with an effective resolution about 2.3-2.4 times lower than their advertised resolution.

Well, there is a reason why there are still 3-sensor videocameras, but I wouldn't attribute it to resolution - rather, to increasing the effective light-gathering of the R and B channels.  It would seem that a single sensor videocam would have to be much larger than a 3-sensor system to get the same effect, and the larger image circle would make for a much bulkier optical system. A rough calculation suggests that a u4/3 sensor wouldn't be quite big enough, but an APS-C sensor would.

Also, if I didn't include an important qualifier, I should have - the effective color resolution of a Bayer sensor is lower than its advertised resolution.  The luminance resolution is closer to the advertised resolution.

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alanr0 Senior Member • Posts: 2,429
Re: Explain 4k (resolution) vs Megapixels (resolution)

MrBrightSide wrote:

So then would it be safe to say that the 3-sensor 4K cameras from Sharp and Sony will provide video image quality that single-sensor 4K cameras can't match, lenses, illumination, and processing being equal (even though they have smaller 2/3" sensors)?

The Bayer filters used in conventional sensors create an image file with an effective resolution about 2.3-2.4 times lower than their advertised resolution.

Depends what you mean by a "4k single sensor camera", and "processing being equal". 4K characterises the display resolution.

Is it an 8-10 Mp Bayer sensor which needs to interpolate and de-mosaic the colour channels, a 24-32 Mp sensor with full RGB information for every 4k pixel, or something different?

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