This is really a continuation of the previous thread ‘Comparing Low Light Performance of Different Cameras’, but as this is the interesting and useful part, I thought a new thread may be appropriate. The cameras under comparison are a Nikon D80, an Olympus E30, and a Finepix S5600.
Strictly speaking, the method used here predicts the ‘light gathering ability’ of the cameras, with the lenses that would typically be supplied. That is, with the aperture wide open, how much light (how many photons) is delivered to the sensor for a given shutter speed? As previously discussed previously, the signal-to-noise ratio is set primarily by the absolute number of detected photons.
In practice, smaller sensors with a higher proportion of dead area will perform slightly worse, because more photons are lost to the dead areas. Unfortunately I don’t have data comparing sensor performance which takes this and other second order effects into account, though as previously explained it should not be difficult to measure. However, I’ll give the smaller sensor in the S5600 the benefit of the doubt, and assume that the proportion of dead area is the same for all sensors, in which case the comparison essentially just gets down to comparing the total amount of light delivered to the sensor by the lens.
Nothing beats a real life example.
Let’s make a prediction for how much more light the Nikon D80 SLR can potentially collect and throw at the sensor, compared to the Finepix S5600, with an Olymus E30 thrown in as well just for fun.
For all lenses, the intensity of light at the sensor is inversely proportional to the square of the F-number. Therefore :-
The total light striking sensor is proportional to A/(FxF)
A is the sensor area, eg in mm^2
F is the lens F-number
Here are the specs and results for these cameras :-
Fuji Finepix S5600
Focal length 6.3 – 63 mm (38 to 380mm equivalent)
F3.2 - 3.5
Sensor area = 5.75x4.31 = 24.8 mm^2
Amount of light on sensor = 24.8/(3.2x3.2) = 2.42 (at L=38mm)
Amount of light on sensor = 24.8/(3.5x3.5) = 2.02 (at L=380mm)
Nikon D80 (with standard lens)
Focal length 18 – 135mm (27 to 202.5mm equivalent
F3.5 – 5.6
Sensor area = 23.6x15.8 = 373 mm^2 (APS-C / Nikon DX size)
Amount of light on sensor = 373/(3.5x3.5) = 30.4 (at L=27)
Amount of light on sensor = 373/(5.6*5.6) = 11.9 (at L=202)
Olympus E30 (with standard lens)
Focal length 14 – 54mm (28 to 108mm equivalent
F2.8 – 3.5
Sensor area = 17.3x13 = 225 mm^2 (4/3 size)
Amount of light on sensor = 225/(2.8*2.8) = 28.7 (at L=28)
Amount of light on sensor = 225/(3.5*3.5) = 18.4 (at L=108)
OK. That was pretty easy, and the results are interesting. The calculated amount of light on the sensor is in arbitrary units, which is fine as it is only the comparison between cameras that interests us, not the absolute values. This comparison is with the lens apertures wide open.
Fortunately all three lenses have about the same 35mm equivalent focal length at full wide angle, making for a neat comparison. The two SLRs are roughly the same, with 30 and 29 light units striking the sensors. However, the point and shoot S5600 is not in the running, and can only muster 2.4 units of light, a whopping 12 times less than the SLR cameras! Let’s get a feel for what that means in the real world. For example, in a situation where the S5600 was flat out to get enough light, forcing the user to select a long exposure time of 1/10 second, the SLRs would do it in around 1/120 seconds. However, there is no avoiding the fact that the depth of field will be lower whenever that extra aperture is actually used.
In comparing the D80 with the E30, the D80 with this particular lens is overall a better performer. Over the Olympus zoom range of 28 to 108mm (35mm equivalent) the Nikon is slightly better, but there is not much in it. However, the Nikon has a much greater zoom range out to 202mm, making it a lot more useful, AND still maintaining a slight edge in low light performance. Impressive. However, the simple fact of the matter is that you get what you pay for, and what you are prepared to carry, and the Nikon performance comes at the expense of greater size and weight, and that’s how it always is and will always be.
The S5600 is very substantially worse because the lens is physically smaller, because the large zoom range (10:1) nearly always throws away light, and generally because it is a much cheaper lens.
OK guys. Was that simple, useful and real world? I don’t claim these predictions will be super accurate, but as far as I can see they should be more-or-less right, and good enough to be useful. A disclaimer is necessary though, as my expertise is more with science, and I know very little about photography. As always, comments from the gurus here are encouraged. I would be interested to check these predictions by measurement, but I only own one camera, and it is not even an SLR. I am considering the purchase of a new camera though, which is the reason why I took the trouble to think about this topic, so if and when I do I might take some measurements.
One last point. The interpretation is slightly more wooly when the number of megapixels is different. That is a topic in itself, but what it amounts to is that with post processing (or in camera noise reduction) the extra megapixels do not cause much more visible noise if the noise reduction is applied to the extent that all images contain about the same detail. In other words, it doesn’t greatly affect the above conclusions, and I don’t want to confuse matters by talking about it. In addition, larger sensors have superior dynamic range, and can provide lower noise images at low ISO than is possible with a smaller sensor even when there is light to burn. However, these are minor advantages for many of us, remembering that noise at low ISO is not usually a problem anyway. Of course, it goes without saying that there are many, many other factors to consider when choosing a camera.
If anyone would like to repeat the above calculation for their camera(s) of interest, could you post you calculation in this thread, preferably in the same format that I used that clearly shows the lens specs as well as the calculation, so we may all benefit?