It is an observed fact that SLR cameras using relatively large sensors perform substantially better than point-and-shoot cameras in low light situations. The most common explanation is that the much larger sensors in SLR style cameras gather more light in proportion to their larger area – typically 4 times that of the smaller sensors found in point and shoot cameras.
Careful thought suggests this explanation is a wrong – one of those myths that if repeated often enough comes to be regarded as fact. The reason that SLR cameras perform better in low light situations is almost entirely due to their larger lenses, which gather more light. The larger area sensor does not inherently have anything to do with it, except for a slight second order benefit due to the fact that the dead area of a larger sensor is a slightly lesser proportion of the total sensor area.
Before justifying my statement above, I need to define clearly what I mean by the aperture of a lens. In this discussion, aperture means the effective diameter of the iris that controls how much light passes through the lens. Forget about ‘f-numbers’ in this discussion – I will refer only to aperture diameter, which of course is expressed in length units such as millimeters.
It is essential to state clearly what constitutes a fair image noise comparison between two cameras that are identical in all respects except sensor size. Firstly, the number of pixels must be the same. The shutter speed must also be the same. Both cameras must be at the same distance from the same object, and the image of this (identically sized) object must exactly fill both sensors, which of course means that the focal length of the lenses will be set differently for each camera. Further, both lenses must be set to the same aperture, or otherwise it is perfectly obvious that one lens will gather more light, which would not make for a fair comparison between the different sizes of sensor. With identical apertures set for both cameras, the depth of field and diffraction limited resolution will also be the same for both cameras, again making for a fair and equal comparison. Correct exposure for both cameras being compared is obtained by adjusting the camera ISO setting. The numeric ISO reading thus obtained is irrelevant. Thus we have a truly fair ‘apples versus apples’ comparison between two cameras with different sized sensors, totally relevant to the real world. Both cameras are ‘doing the best they can’ under identical conditions and constraints, with the only variable being the size of the sensor. I contend that the total amount of light striking the sensor will be the same for both cameras, despite one sensor having a greater area, and that the image noise performance will therefore also be essentially the same.
This conclusion makes good sense. All else equal, the amount of light gathered by the lens is proportional to the area of the lens aperture, and this same amount of light is delivered to the sensor, regardless of sensor size. In other words, for a larger sensor, the same amount of light is simply spread over a larger area. The electrical signal level registered by the sensor is proportional to the total number of photons arriving at the sensor which, as discussed, is independent of sensor size in a properly structured comparison. This is not rocket science. This is common sense.
Note that the superior low light performance of SLR style cameras is not in dispute – what is in dispute is the reason for this superiority, and the reason is mainly because SLR style cameras have physically large lenses which gather more light. It may be correctly deduced from this discussion that compact point-and-shoot cameras will always have inherently poor low light performance, not for the most part because they use a small sensor, but because compact cameras must necessarily use a physically small lens. Judging from the statements that I commonly read in camera reviews, implying that low light performance of point and shoot cameras could be greatly improved if only the sensor was much larger, this point does not seem to be well understood.
The described method of comparing cameras with different sensor sizes is lens independent, as it should be when the object of the test is to compare only the effect of sensor size. That is, the test is genuinely measuring the signal to noise of different sized sensors, with the lens in both cases delivering the same total number of photons to the sensor. If such testing is performed in the real world, I would expect larger sensors to produce only a slightly better result (less noise in image) on account of a lesser proportion of ‘dead’ sensor area, with diminishing returns beyond a certain sensor size. A camera comparison of this type would be meaningful and revealing, yet unfortunately I have not seen camera comparisons performed in this way.
One final point. The comparison method described above gives no credit for the ‘speed’ of the lens, which is appropriate when the object of the test is to compare only the effect of sensor size. However, if the object of the comparison is to compare the low light and image noise performance of the camera as a whole, including lens, then the lens aperture should be fully open on both cameras under comparison, that is, both cameras are doing their absolute best to gather as much light as possible. As previously, correct exposure for both cameras being compared is obtained by adjusting the camera ISO settings. Needless to say, in this case superior low light performance will usually be accompanied by a poor depth of field, as expected from a ‘fast’ lens operating at a large aperture. If testing is done in this way, I would expect SLRs to perform very significantly better (lower image noise) than a point and shoot camera, NOT because of the larger sensor, but because of the larger aperture of the SLR lens, which collects more light.
In summary, it is true that SLR style cameras generally outperform compact point and shoot cameras in low light conditions. However, despite popular belief, the reason for this superiority has little to do with sensor size, but is due to the larger lenses on SLR style camera that gather more light.