High ISO Shooting

You often want to capture the best images you can with the light you have available. Most of the time a tripod isn’t practical, and even when it is, a long exposure can be undesirable. When there isn’t much light, setting a high ISO allows for fast shutter speeds in low light, but with increased image noise and less detail in the highlights and shadows. Remember the golden rule for eliminating camera shake:

Shutter speed must equal double lens focal length (mm) as a fraction of a second
This means that with a 50mm lens, you’ll need a shutter speed of 1/100sec or faster. Image stabilising lenses help things a lot, but just remember this rule and you’ll be fine.

 

Why not simply use a high ISO all the time?

If a fast shutter speed eliminates camera shake, and a high ISO allows for a fast shutter speed, why not simply always shoot at a high ISO? Well, there are two main reasons, the most visible of which is loss of detail. When working out how much perceived detail is lost at high ISOs, I find it helpful to think of my usable image size halving each stop from 3200 upwards, at least when shooting jpegs. A bit more detail can be recovered from RAW files.

I’d say these figures are a rough guide to how much detail you retain (in megapixels) at various ISOs; the first figure is for jpeg, the second is raw –
ISO 100 = 20mp (at max quality)
ISO 3200/6400 = 10mp
ISO 6400/12800 = 5mp
ISO 12800/25600 = 2.5mp

Going by the above information, the drop off is fairly steep at high ISOs. However, even a photo shot at 25600 has enough detail recorded for use at screen resolutions, so is fine for photos taken for a website. Just be aware that you won’t have enough detail to be able to print the photos at a decent size or crop them by a significant amount.

 

Be aware of dynamic range loss

With detail loss being the most visible effect of high ISO shooting, many photographers don’t realise that at very high ISOs (6400+), dynamic range is lost, too. At the extremes, the camera cannot resolve any extra detail, and it simply stretches the white point, brightening the picture and increasing the contrast in order to clip dimmer colours as white.

At low ISOs (up to ISO 800), most DSLR cameras can record around 11 stops or dynamic range, meaning the brightest highlight is 2000x lighter than the darkest shadow. This drops to 9 stops by 6400, and decreases linearly for each further bump in ISO. This means that by 25600, you only have 7 stops of range (the brightest recorded detail is only 125x lighter than the darkest), which results in burned out highlights and loss of detail in the shadows. Exposure is an exponential scale, so that 4 stops of lost range means that your camera is working within 1/16 of its full tonal range when stretched to the highest ISOs.

 

Overexpose for best results

High ISO shooting is particularly well suited to high-key photography. Burning out all the highlights in a high key, high ISO, image means you haven’t really lost much dynamic range – you’ve simply clipped detail that would have been present in the whites. Pushing the ISO up and over exposing when taking a shot will result in a lower signal-to-noise ratio. A bright high ISO image always beats a dim low ISO image for the amount of detail captured (up to 6400 – any higher than that is not necessary if shooting raw, just brighten the images in post-production).

When shooting Jpeg, ISO 6400 is fine providing you have a nice bright image (this is due to noise being much more visible in darker areas of an image). A well exposed – or even ‘overexposed’ – image shot at 6400 will look better than an underexposed one shot at 3200. Even when shooting raw, you can usually pull an overexposed image back by a stop without worrying too much about having clipped the highlights.

So, to summarise, if you’re not on a tripod, bump up the ISO. You will end up with a greater percentage of usable pictures, many will be higher quality overall, and a bit of grain didn’t matter throughout the many decades of 35mm film, so why should it matter now?