Xiaomi Mi 9 and Redmi Note 7 Pro have the same camera sensor — Sony IMX586 Exmor RS. Does this mean both smartphone have same image quality output when pictures are taken?
Note: This is a simplified answer. It does not go into detail, and therefore, please do not take it as authoritative answer on specifics about the exact processes that take place inside a digital camera.
The same sensor does not necessarily mean equal pictures. The sensor is one component of the whole process - the others are the analog-to-dicital-conversion (ADC) and the digital signalling processor (DSP) with its algorithms.
The sensor is 'just' a converter - it converts incoming light into electricity. The ADC then takes that electric signal and interprets according to a look-up table (i.e. a curve), as sensors are not perfectly linear, so there are dents and spikes in the signal that need to be corrected in order to get a picture that is close to the expectation of the user. The DSP then does de-noising and certain 'image enhancements'.
If any of these components change, the picture might look different.
Also note that one of the most important factors is the lens: focal length is a very important factor, and image quality can suffer greatly from an (extremely) bad lens.
To take a digital picture you need:
- Image processing (CPU and algorithms)
As you see, sensor is just a one link in a chain, and the chain is as weak as it's weakest link. Currently, we are experiencing great technological progress in the third stage. Compared to DSLRs, smartphones have abysmal both optics and sensors - however the extraordinary image processing allows some smartphones to almost close this huge gap under favorable conditions. It is notable that one of the highest photographically ranked phones (namely Pixel) doesn't even have the best optics nor sensor, yet it outdoes it's better-equipped competitors by a large margin. Even using different camera app can drastically alter image quality, as shown by Pixel camera app being hacked into running on non-Pixel phones.
If sensor quality would be the most important to final image quality, then Sony smartphones would be the best - as they make the best sensors, proven by everyone else using Sony sensors. But they're not, Sony smartphones are outdone by competition running Sony sensors.
So, to answer your question:
Does this mean both smartphone have same image quality output when pictures are taken?
No. But as they come from one manufacturer, it's possible that other components are exactly same.
Probably the same image quality, but not necessarily.
It's not only the sensor that takes the picture. It's also the lens. The sensor doesn't have an integral lens, it's the camera manufacturer that selects the lens.
It is for example possible that the field of view on both cameras is slightly different due to slightly different focal lengths (although typically camera phones have normal focal lengths so the difference would be small). The aperture also changes the depth of field, although quite frankly with these small sensors the DoF is nearly infinite.
Additionally, it's possible that one camera uses higher quality optics than the other. If this is the case, the camera with higher quality optics wins.
See if you can find information about the optics: focal length, aperture, manufacturer of the lens. I'd also advise that:
- If the camera feature is important to you, find some reviews of both phones with camera tests
- Phone cameras typically only have digital zoom, so zooming in reduces picture quality
- Remember always that a phone camera with a small sensor is not equivalent to a "real" camera, i.e. DSLR or mirrorless.
They can have hugely different image quality also depending on the post processing. A great example of this is the Google Pixel's "Night Sight" mode, which allows Google's phones to take some pretty impressive photos in very poorly lit areas, without any modifications to their camera's sensor or lenses.
This article has some comparison pictures and some more details about using that mode https://www.droid-life.com/2018/10/23/google-pixel3-night-sight-camera-download/
These two pictures aren't even different phones, they're the same exact phone, the only difference being the post processing of the images.