Phone megapixels and file size relationship

[NomoneyNohoney]

I have a couple of phones, which have a Zeiss PureView 41 megapixel camera built in.
Using the camera app, I take two separate jpg pictures simultaneously. One is approx 1.5mB in size, and the other -called Pro.jpg - is about 8 - 10 mB or so. One is meant for emailing around, and low-key stuff, while the higher resolution picture is the 'quality' copy.
What I'm confused by is this. If the camera is 41 megapixel, why are my hi-res pictures about 10mB? Is there a correlation between size in megapixels and size in actual jpg size?

Having typed this out, I realise that I'm getting a bit distracted by the 41 megapixel size, which doesn't signify the disk file size, hence my question about, if there's a correlation. Anyone got a simple explanation for my tired brain please?

[dionaeamuscipula]

I have a couple of phones, which have a Zeiss PureView 41 megapixel camera built in.
Using the camera app, I take two separate jpg pictures simultaneously. One is approx 1.5mB in size, and the other -called Pro.jpg - is about 8 - 10 mB or so. One is meant for emailing around, and low-key stuff, while the higher resolution picture is the 'quality' copy.
What I'm confused by is this. If the camera is 41 megapixel, why are my hi-res pictures about 10mB? Is there a correlation between size in megapixels and size in actual jpg size?

Having typed this out, I realise that I'm getting a bit distracted by the 41 megapixel size, which doesn't signify the disk file size, hence my question about, if there's a correlation. Anyone got a simple explanation for my tired brain please?

The lower resolution shot has less information (less pixels) in it so the file size is smaller.

DM

[UncleEbenezer]

JPEG is a compressed format, designed to store photograph-like images efficiently. The file size depends on many things: the compression will save very different amounts on different images. If you have an image editing tool - and perhaps even in the settings of your phone camera - you can adjust many things, including a general tradeoff between image quality and size.

Megapixels are a marketing term.

[Bminusrob]

Megapixels are a marketing term.

This is not correct. Megapixels refers to the number of pixels in the camea's sensor, so a 1920x1080 image (HDTV size) will be 2 megapixels; a "4k TV" image will be 3840x2160, or 8 megapixels. When a picture is taken, it contans this number of pixels. In order to convert to JPEG, the image is broken down into 8x8 pixel blocks. Each of these block is converted using a discreet cosine transform, into "frequency space". This space is highly susceptible to data reduction, and hence the JPEG quality levels. If you could reduce the JPEG quality threshold to zero, you would be left with one pixel from each of the original 8x8 blocks.

Cheers, Rob

[tsr2]

Having typed this out, I realise that I'm getting a bit distracted by the 41 megapixel size, which doesn't signify the disk file size, hence my question about, if there's a correlation. Anyone got a simple explanation for my tired brain please?

I'm not sure about simple, but I will try to explain.

If your camera can be set to capture "raw" images then that stores the image exactly as it comes from the sensors, so the size correlates exactly with the number of (mega) pixels.

The next thing to understand is that when you compress data, the amount it can be compressed by depends on a number of different factors, so the final size after compression may vary considerably.

A simple example is that if a row of 8 pixels is exactly the same colour you can represent that in a simple compression scheme as "repeat red 8 times", whereas another 8 pixel row that is made up of 3 colours may be represented as "repeat green 3 times, repeat yellow 4 times, red once", which would take up roughly 3 times the space.

Lower quality does not necessarily mean fewer pixels. Compression algorithms come in two types, lossy and lossless. A lossless compression algorithm allows you to get the original image back, exactly as it was. A lossy algorithm will not normally allow you to get back to exactly how it looked at the start, e.g. it might store 3 or 4 similar shades of red as exactly the same. JPEG uses lossy algorithms, so picking different quality levels means that you can get higher compression by losing some of the definition in the picture.

[tsr2]

Megapixels are a marketing term.

This is not correct. Megapixels refers to the number of pixels in the camera's sensor, so a 1920x1080 image (HDTV size) will be 2 megapixels; a "4k TV" image will be 3840x2160, or 8 megapixels.

That is true, but not the whole story. Megapixels are a precise technical term, not a marketing term, but like many precise technical terms they can be abused. Megapixels are one, and only one, element of picture quality. A 40 Megapixel phone camera may take lower quality pictures than a 10 Megapixel DSLR, primarily because more Megapixels and fancy software can't disguise the limitations of cramming optics into a phone case.

There are a bunch of other factors involved, but I think optics and sensor resolution (Megapixels) are probably the biggest.