Today’s dip into my photo library brings me back to Melbourne, and a photograph I took in St. Patrick’s cathedral while following the Lonely Planet walking tour of the city. It’s a photograph which highlights one of the limitations of small sensors on most compact digital cameras.
I took this photo with a Fuji Finepix s5000, a camera which served me well and which I was very impressed with right up until I upgraded it to my first digital SLR. The s5000′s selling point was its 10x optical zoom, although I wasn’t taking advantage of it when I took this photograph at the widest zoom of 5.7mm.
A focal length of 5.7mm sounds like it is incredibly wide – for instance I’ve demonstrated before how wide a 10.5mm focal length is on my Nikon D300 – but the field of view of a lens at a particular focal length is dependent on the size of the sensor being used to capture the image. The sensor on the Finepix s5000 is (or rather was – it is no longer sold) at most a third of the size of the sensor on my D300, and so that 5.7mm focal length of the s5000 lens would be equivalent to around an 18mm focal length on my SLR.
So the s5000 was a capable camera with a large focal range, but the small sensor size caused some issues with noise in low light. Small sensors mean small pixels, which are less tolerant to digital noise. In low light, to get proper exposures, you typically either need to increase the ISO (which effectively amplifies the signal from each pixel) or use longer shutter speeds. If you amplify the signal, you also amplify the noise, and so higher ISOs lead to noisier images. If you use longer shutter speeds, you increase the time for that noise to build up and effect the image being captured by the sensor, and so also lead to noisier images. Add in a small sensor with less noise-tolerant pixels, and noise becomes a problem in all low light situations for compact cameras, no matter how low the ISO.
The shot above is taken at ISO 200, which is low, but required a 2 second exposure, which is slow. You can see the impact of the noise most notably in the arches over the altar.
Noise aside, the camera handled the exposure quite well, although I have pushed the final image quite a bit in post processing to bring out detail in the highlights and the shadows. And when you try to bring out shadow detail, you reveal more of the noise in the image, as it tends to be most noticeable in the shadow areas.
In the days of film, low light photographers had to deal with a similar phenomenon called grain, and in many cases was quite aesthetic – the same can be said for certain types of digital noise, but colour noise, which seems to be a uniquely digital characteristic, is less aesthetic. Where colour noise is a big problem, a monochrome conversion of an image can sometimes help and give it a more grainy feel, along the lines of that achieved from high ISO film.
In almost all cases though it’s best to minimize noise in digital photographs, which means using low ISOs, short exposures, large sensors, and where not all of these are possible at once, noise reduction algorithms, either in-camera as my Nikon D300 can do, or later in post processing.
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