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I was searching to buy a camera, but I saw in specs they wrote:

2MP(full hd) video recording
5MP image size

Why the vido size is 2MP but image size is 5MP? How they calculate these numbers?

If I had a camera that could shooting in 4K, how can I find it's photo's size in MegaPixel?

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    \$\begingroup\$ What does this have to do with electrical engineering? \$\endgroup\$ – pipe Jan 6 '18 at 6:54
  • \$\begingroup\$ @pipe: Where should I ask this? May you transport it to right place? \$\endgroup\$ – user8663682 Jan 6 '18 at 7:00
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The numbers given in pixels or megapixels tell us the number of pixels (or, sensors) that the hardware can process.

A photo frame is a matrix of data grabbed from sensors. A 5MP photo contains nearly 5 millions of pixels that are processed and saved by the hardware. As you know, it is done in a single shot and the duration of a single shot is a few milliseconds.

Since a video can be thought of as a series collection of photo frames, the hardware may not be fast as it was on processing photo frames. I mean, it may not be able to process those 5MP frames in 60 (or even 25) times per second (Yup, the frame rate). In other words, if a camera can record maximum 1920x1080 (i.e. 2MP) video at 60FPS, then the hardware is capable of processing 1920x1080x60=124.4 millions of pixels in only one second. So, that is the limit.

To your last question: I have no idea about if there is a direct relation between those two. But, as I explained above, the limitation comes from the hardware itself.

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  • \$\begingroup\$ Thank you, nice answer! I have other question: If a camera has 5MP cmos sensor, but it records 2MP videos, is that mean, it doesn't use all of it's CMOS sensor's area(border pixels) while recording videos? \$\endgroup\$ – user8663682 Jan 6 '18 at 8:02
  • \$\begingroup\$ Also, shouldn't you multiply the 1920*1080*60 by 3 or 4 respect to the image type(I think RGB colored images have 3 or 4 channels per pixel) \$\endgroup\$ – user8663682 Jan 6 '18 at 8:14
  • \$\begingroup\$ @user8663682 Actually I don't know but probably you are true. A 5MP sensor is a matrix of 2592×1944. So, for 1920x1080, 336 pixels of left and right, 432 pixels of top and bottom are not used. So yeah, outer contours of sensors are disabled. But, I say again, I am not sure. But sounds believable. \$\endgroup\$ – Rohat Kılıç Jan 6 '18 at 8:23
  • \$\begingroup\$ Multiplying by 3 or 4 is just about RGB or RGBA (or RGBW) as you stated. I only talk about "pixels" (not sub-pixels or channels) which are a combination of RGB or RGBx. So the number of "pixels" processed in a second is 1920x1080x60. \$\endgroup\$ – Rohat Kılıç Jan 6 '18 at 8:24

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