Cameras and physics of light

This should be highschool level deduction.

The basics

In short:




In slightly more length:



What this does not mean



What you are being sold

Let’s see what cameras are sold with:




The fix would be:






Imagine taking photos in the light of a fireplace with a cell phone. It has a tiny lens and the images turn out noisy because it can only catch so many photons in the time frame it is doing the exposure (you can’t take a longer exposure because then the photo would be blurred because of movement). Now, some may say the photo is noisy because of the tiny sensor. So, let’s keep the same lens and exposure time, but attach a bigger sensor behind it, further away (we hope the lens can focus that far back). But now, again, we get exactly the same photons at the same respective pixels, and nothing was gained, we still have the exact same noisy image! We could enlargen the sensor to 10 x 10 meters and we still wouldn’t get any more photons (in fact, thermal noise would probably crowd any visible light photon signal at this point).

One can turn this situation around, if one looks at a projector screen setup. Imagine it in reverse, so that the photographic image, like your kid or cat is represented by the projector’s LCD in front of the lamp, and the sensor is the silver screen. Now, if we move the projector further away and keep the image focused so that it fills a larger portion of the silver screen, the image is less intense per square millimeter at the same time. Of course, since the projector only gives out so much light, and it now has to fall on a larger area. That is exactly the same effect as enlarging the sensor but keeping the same optics.

You can also think of a shotgun with a finite amount of pellets. No matter how big the target, it can only catch those pellets.

If someone doesn’t get it and even after reading this text says “large sensor gives good low light performance”, I give up.


Noise, Dynamic Range and Bit Depth in Digital SLRs, Emil Martinec, 2008

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