I am always baffled when people talk about noise and high ISO use. Yes, there is a possibility that your images will be more noisy at ISO800 than say at ISO100. However, it is also important to know that digital exposure is not quite the same as looking at the photo on the LCD and judging the exposure that way. There is a useful tool, and it’s called the histogram. This tool tells you what tones you have recorded in every single image. Due to the sensors’ characteristics the signal to noise ratio is what defines the final image quality; thus level of noise. Signal-to-noise ratio is defined as the power ratio between a signal (which is the meaningful information) and the background noise (which is the unwanted signal).
Speaking digitally, this means that we need to use a method of exposure, which effectively reduces this unwanted signal; the noise. So the principle of shoot-to-the right was born. This simply means that when you take a frame, you review the histogram, which shows the image tonalities and push the graph towards the right (overexposure) until you hit the dreaded blinkies (clipped highlights). Then accept that as the correct digital exposure. This shoot-to-the-right method maximizes the amount of good data captured in the right side of the histogram.
I will now demonstrate with some images I took of one of my trusty dogs – Biscuit – on a dreary winter afternoon when it was dark and cloudy. I deliberately chose to shoot all images at ISO1600 with my Canon EOS 1DmkIIn camera body. Sure it’s a 1D series, but the principle remains the same for any digital SLR camera and should yield very similar results.
I took shots at five exposure correction settings in Av mode, using evaluative metering, which is my standard daytime operating method (EC = exposure compensation).
This is how the images look in Digital Photo Professional. See exposure compensation applied written in RED over each image. That is compensation applied in camera, using the quick control dial on the back, at the time of capturing the RAW images.
This was shot at -1EC. Check how underexposed it looks, but the histogram tells you all you need to know. It’s very badly underexposed. A serious candidate for ugly noise when corrected.
Now compare the below two shots that essentially look the same at first glance, but one is very noisy and the other, well not at all.
The first is the one taken with an exposure compensation of -1, which was then boosted up +1 stop in Digital Photo Professional. It is very ugly with noise. Then compare it to the second shot, which was captured as a RAW file too, with +1 2/3 stops of exposure compensation applied. That is a total of 2 2/3 stops of light. It makes a HUGE difference and the proof is in the pudding.
If you learn to expose properly, using the histogram, you can minimize noise in your RAW files and shoot at relatively high ISO values. It’s about knowing the way to bring out the best results from your equipment.
Take care and happy shooting.