What really makes smaller sensors have more noise?

Started Apr 20, 2013 | Discussions thread
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What really makes smaller sensors have more noise?
Apr 20, 2013

The other thread filled up and I didn't get a good chance to discuss my theories due to an admittedly poor post title.

Now if we try to create an image with a few different sensor sizes, we have to try and hold all things equal for an image (FOV, output size, framing, shooting distance, lighting, ISO, SS, etc) But there are a few things that cannot be equal.   One is the focal length, which has to be adjusted to keep FOV and framing equal.  The next is the enlargement factor - which is how much bigger the print/screen view is than the actual sensor that recorded it.  And the final, which is often posited by some to be the main factor, is the total amount of light hitting the sensor.

Now I'd like to argue that the enlargement factor is the main determinant of the noise difference.  If you enlarge the size of an image farther, you keep making the noise larger and easier to perceive.  This is easily seen just by changing the size of a viewing window, or viewing an image on a cell phone and then a large monitor.    As further proof, I will submit that if you cut a print from any sensor in half, you effectively have an image that was taken with a  sensor half the size and at the same enlargement factor with half the total light hitting it.  Does it now appear more noisy? No it does not.

And finally, if we take similar sensors (like Sony's recent ones) of different sizes and use, say OM-D, and D800 at the same enlargement factor, the noise appears fairly consistent, with differences like explained by each companies processing preferences.

The D7000 and D800 both use recent Sony sensors and their native resolutions give almost an equal enlargement factor at 100% crop. Note how similar the noise is at all ISOs.

So to me, this means that at a given enlargement factor (i.e., smaller prints/screens for smaller sensors) noise is going to be pretty much constant for a given sensor tech.   If we use the same size prints to keep everything equal, then we've used different enlargement factors resulting in larger noise for the smaller sensors.

So where does pixel density fit in?  It effects the enlargement factor of 100% crops and encourages larger printing which means larger enlargement factor.

I can't find how "total light" fits in.   Longer focal length required for larger sensors definitely changes DOF (as does enlargement factor), but i don't see how it can result in more light or less noise, either.

Feel free to provide intelligent rebuttal if you think I am incorrect.

Nikon D7000 Nikon D800
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