Northumberland Camera Club: The benefits of getting used to IOS

Long night-time exposures require careful noise control.Long night-time exposures require careful noise control.
Long night-time exposures require careful noise control.
ISO is one of those adjustments that a lot of people leave on auto. But it something that you would do well to get used to.

Cameras perform best at their base ISO. This setting is usually either 100 or 200. Raising the ISO is often thought of as making the sensor more sensitive to light. That is a useful analogy, but not strictly true.

ISO adjustments are akin to turning up the volume on a hi-fi amplifier. If you have ever done that with the music turned off, then you will know that the background hissing and hum on the speakers gets louder. Play music and the noise is still there but it is lost behind the music.

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Sensor noise manifests not as audible interference but at graininess on the photo. The noise is always there but at low ISOs the signal from the light hitting the sensor drowns it out. At higher ISOs, fewer light signals hit the sensor while the noise level stays the same; the signal to noise ratio becomes smaller and so the noise more noticeable.

Usually try to keep your camera at its base ISO so other exposure settings allow maximum light to hit the sensor. But sometimes you may want to increase it. For example, photographing birds often requires a fast shutter to stop movement and shooting handheld in low light where camera shake is an issue are examples where a higher ISO may be needed.

There is another source of digital noise. When shooting long exposures, the electronic circuitry of the sensor heats up, emitting electro-magnetic radiation. The sensor picks up these unwanted signals and they appear on the image as noise.

This begs a question. For long exposures, is it better to have a higher ISO and a shorter shutter value or use the base ISO and a longer exposure? Higher ISOs also reduce the dynamic range of a photo. For images with bright highlights and dark shadows, where you want to bring out the detail in both, start with a lower ISO. However, increasing the ISO to the highest level where there is acceptable noise and thus allowing a shorter shutter value may otherwise be the better option with lower contrast shots.

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Different cameras have different ISOs at which noise becomes unacceptable. As technology improves, this is getting ever higher. My first ever DSLR could only manage to produce suitably clean images up to ISO 400. Contemporary cameras can produce low noise shots at ISO 3200 and above.

One technique many photographers use for reducing noise at higher ISOs is to slightly over-expose, though not so much to blow out the highlights. Called ‘shooting to the right’ because the histogram moves towards the white end of its scale, this method means that there is a greater ratio of light to noise. Correcting the exposure in post processing helps hide that noise in the shadows, where it is most visible. However, increasing extra exposure in camera may well slow the shutter down, maybe defeating the point of increasing the ISO to start with.

Because pixels are more closely packed together—a higher pixel density—large pixel-count cameras have smaller photosites (light detecting cells on the sensor). This creates more noise because of the lack of heat dissipation. There are of course some types of photography where greater resolution is necessary, such as astrophotography. But contemporary cameras with lower pixel densities and larger photosites perform better in both noise and dynamic range.

Noise can be reduced in software. I find On1 and Nik Dfine 2 and Topaz Denoise work well.