Smaller Sensor Size = Less Image Noise?

Discussion in 'Open Discussion' started by Archer, Oct 28, 2013.

  1. Archer

    Archer TalkEmount Regular

    Aug 19, 2013
    London, England
    I've always believed everything I heard online about a bigger sensor = less image noise

    But something just occurred to me after shooting over 5,000 images with my NEX 5N at a fashion show this weekend, most of which had a lot more noise then I'd have liked ... How can that be true?

    When I was a kid I'm sure like all of you I took a magnifying glass and concentrated the light from the sun to a tiny point until fire appeared, surely if we have larger sensors we have less concentrated light and therefore more image noise?

    Having a smaller sensor allows us to get the back of the lens closer to the sensor so it has less distance to travel and allows the same size & weight lens to be created which concentrates more light into a give surface area?

    Shouldn't smaller sensor = more light concentration and therefore less noise?
  2. jai

    jai TalkEmount Top Veteran

    Feb 4, 2013
    No, that isn't how it works.

    If you have a F2.8 lens on a Sony 5N and a Canon 5D Full Frame camera, under the same light conditions, then the intensity (light power/area) of the light on the sensor is the same.

    If you take your Canon´s full frame lens, and focus all of the light from that down onto the crop sensor of the Nex (which you can do with a speed booster/lens turbo) then you really can increase the light intensity. This is exactly like your example with a magnifying glass killing ants. But now the lens is no longer acting like an F2.8, it has become an effective F2.0 aperture lens.

    So for an apples to apples comparison, the light intensity is actually the same on big sensors vs small sensors. And because intensity is a measure of power over area, if you increase the pixel area you increase the total light power collected by each pixel.

    More power means a stronger signal, which means a better signal to noise ratio (SNR).
  3. xXx1

    xXx1 TalkEmount All-Pro

    Jan 15, 2013
    It is property of silicon. There are about constant amount of thermal electrons (that is, photons push electrons thru np-barrier but so does temperature) per area. So there is more noise in bigger sensor but noise shows more when you enlarge more. Statistically there is more variation if sensor pixels are small (1 thermal electron per 10 photons is much more than 1 per 100) but this evens out if images are resized to same size.

    It is optically hard to make a small image circle. Typical compact camera has got something like 8 mm sensor so normal objective is also 8 mm. However the foremost lens is about 2 cm in diameter so it should make very fast lens but they are very seldom faster than 1:2.8.
  4. jai

    jai TalkEmount Top Veteran

    Feb 4, 2013
    All this technical talk, basically to say:

    Smaller sensor ≠ More light concentration

    But if you do concentrate more light (as with a wider aperture lens) then you will get less noise.
  5. Gandalf

    Gandalf TalkEmount Regular

    Sep 5, 2013
    The noise isn't a result of light concentration; it's a product of heat. Smaller sensors concentrate the same amount of electrical activity into a smaller area, making it harder to keep cool. As the sensor gets hotter, the noise increases.

    This is why dedicated digital sensors for astrophotography (the better ones) are actively cooled, even to the point of refrigeration. The cooler the sensor, the less noise.