Sensor "pixels" count vs digital image output pixels count

[fog37]

TL;DR Summary

sensels vs pixels

Hello,

• I did some reading and wanted to check with you to make sure I have the correct information: in general, a digital camera is rated with so many Mega pixels which indicate the number of pixels of the largest image (mm x mm) that the camera can produce. The camera sensor (CCD or CMOS) has "pixels" too which are more properly called sensels. The number of sensels and the number of pixels of the digital image are generally not the same. "In general", the number of sensels is larger, correct?
• The larger the camera sensor, the larger the light collecting area, the larger the sensels will be. The camera resolution is determined by the sensel's size and the magnification (which depends on the lens).
• In general, and surprisingly for me, a larger sensor with larger sensels is a technically better choice but the price is higher. I tend to think that the more sensels the better because that would increase image resolution...

Thank you!

 

[Drakkith]

The camera sensor (CCD or CMOS) has "pixels" too which are more properly called sensels. The number of sensels and the number of pixels of the digital image are generally not the same. "In general", the number of sensels is larger, correct?

Is it? I thought the number of photosites (sensels I guess, which is the first time I've ever heard of this term) was generally equal to the pixels in the resulting image.

The larger the camera sensor, the larger the light collecting area, the larger the sensels will be.

No, the physical size of the sensor and the physical size of the sensels are not necessarily related. One can have a very small sensor with very small sensels (for a large density) or a very large sensor with very large sensels (for a low density), or anything in between.

In general, and surprisingly for me, a larger sensor with larger sensels is a technically better choice but the price is higher. I tend to think that the more sensels the better because that would increase image resolution...

It depends. Two sensors of identical physical size but with different size sensels will have different resolutions. But a larger sensor can have more sensels yet have the same resolution if the sensels are the same physical size as a smaller sensor.

 

[Baluncore]

If each pixel has three "sensels", one for each colour, does that explain it?

 

[Drakkith]

If each pixel has three "sensels", one for each colour, does that explain it?

My understanding was that each pixel corresponded to one sensel, even in color sensors. There is just some processing going on to get the color data from the surrounding sensels.

 

[pbuk]

The word "sensel" is pedantically correct (it is a contraction of "sensor element") but does not refer to anything different than a "pixel", which because it is a contraction of "picture element" should strictly speaking only refer to displays, not sensors.

 

[DrJohn]

Re light gathering and image quality.
Imagine two cameras, each giving a 24 Megapixel image. One camera has a large sensor, say 1inch by 1.5inch, and the other has a sensor 1cm by 1.5cm. Each has the same number of sensels to give the resulting 24megapixel image. So the sensels on the 1cm by 1.5cm sensor must be much smaller than on the 1inch by 1.5inch sensor.
So the larger sensor can gather more light at any given exposure, the signals from the sensels need less amplification to be processed and the result is a better quality image. Because the amplification require in the camera with smaller sensels can introduce more random noise. And thus its harder for it to give a good image (lots of signal amplification required) in low light conditions.

That's my understanding of it anyway.

 

[Baluncore]

So the larger sensor can gather more light at any given exposure, the signals from the sensels need less amplification to be processed and the result is a better quality image.

That assumes they employ the same optics, but the optical magnification used would be different.
The available light energy is a function of the objective lens area or aperture, spread over the same number of sensels.

 

[Drakkith]

So the larger sensor can gather more light at any given exposure, the signals from the sensels need less amplification to be processed and the result is a better quality image. Because the amplification require in the camera with smaller sensels can introduce more random noise. And thus its harder for it to give a good image (lots of signal amplification required) in low light conditions.

Assuming the same optical setup, yes. The smaller sensor will also have better 'resolution' in the sense that it resolves finer details if the optical system allows. The larger sensor has less resolution but a much larger field of view.

 

[sophiecentaur]

Assuming the same optical setup, yes.

The f number of the lens is relevant too. A wide aperture lens will admit more light (obvs) but its focal length will govern the illumination of a sensor element.