The use of ISO in digital photography

First published on: Tuesday, 18 December 2007

I’ve been asked why there is a need for an ISO function on digital cameras when exposure can be easily set using a combination of shutter speed and aperture. What are the reasons to increase ISO?

This post is a collection of my thoughts and experience on the matter. There are also links to related articles and photographs.

Rather than re-defining what ISO is, let me point a few links to you:

  1. What is ISO, at Photoxels

  2. Red channel noise at ISO 100 vs ISO 800, at DPReview

  3. When ISO grain is desirable, at IanTalbot

Base ISO

The base ISO of a digital camera can be found in the specifications. The Nikon D300, for instance, has a base ISO of 200, but can go as low as ISO 100. Some digital compact cameras such as the Canon Powershot G9 have a base ISO of 80. The Canon EOS 40D has a base ISO of 100.

Maximum ISO

The maximum ISO on the D300 is 6400, while the Canon 40D goes up to 3200. The Canon G9 can reach ISO 1600.

When to use high ISO

Photographers traditionally try to avoid the use of high ISO due to the following reasons:

  • Image noise increases

  • Colors become desaturated

  • Dynamic range decreases

Although we would like to use our cameras at the base ISO, this is not always feasible. We end up using high ISO values when we encounter the following situations:

  1. To freeze motion when the light levels are low. If you’re photographing sports in the outdoors such as football or soccer, you would like to maintain a sufficiently fast shutter speed in order to freeze motion, otherwise the legs of a running athlete would end up looking blurred. This becomes difficult when the weather condition is not optimal. You will find that shutter speeds start dropping and you’re already at the maximum wide open aperture of your lens. The shutter speed cannot stay high because at the chosen aperture, shutter speed has to decrease in order to maintain a correct exposure.

  2. To reduce image blur when shooting while handholding the camera. There is a general rule when it comes to handholding your digital camera. To capture relatively sharp images without the use of a tripod, the shutter speed, in seconds, should be a minimum of 1 / (focal length), or if it’s a crop digital SLR, 1 / (focal length x 1.5). When shooting subjects up close, the requirements for a fast shutter speed becomes greater. Again, depending on the light conditions and your chosen aperture, the desired shutter speed sometimes just cannot be achieved. If the subject is relatively static, using a tripod, a lens or camera with vibration reduction (or image stabilization) helps with reducing (but not entirely eliminate) the impact of handshake. The other alternative is to increase ISO to the point where the shutter speed is fast enough.

  3. Flash photography. When photographing a subject indoors or when taking a night portrait, you’ll notice that the photographs will come out with the main subject properly exposed, but the background is dark or almost black. The reason is because the flash on the camera only outputs enough light to illuminate the main subject which is nearer to the camera. The background, which is further away, will not get the benefit of the light from the single flash. In order for a brighter background, we will need the help of what little ambient light there is available. You can’t increase the exposure time too much, because your main subject cannot remain still for long. For general event photography, and depending on the selected aperture, focal length and amount of ambient light, it’s normal to select ISO 400, 800 or even 1600.

Choose the right digital camera, lens, lighting equipment and postprocessing workflow

ISO cannot be increased to whatever value we desire. For instance, ISO 1600 is the upper limit for most modern day digital SLR cameras as far as usable images are concerned — any higher and there will be too much noise in the image to deal with effectively. Some higher end digital SLRs such as the Nikon D3 is able to handle up to ISO 6400 and give you images of reasonable quality.

The final print size will also influence your decision as to how high you should go with the ISO on your camera. I sometimes use ISO 3200 or 6400 on my Nikon D300 if I know that the images will not be printed larger than 4 x 6 or 5 x 7. You’ll need to experiment with printing photos from your camera to ascertain the relationship between print sizes and maximum ISO the camera can go to which gives reasonable quality at that print size.

If you are shooting indoor sports such as badminton, basketball or ice hockey, then a fast lens is mandatory. Fast lenses have maximum wide open apertures of f/2.8 or greater (say f/2, f/1.8 or f/1.4) and give you greater opportunities for faster shutter speeds at relatively lower ISO. Two issues with fast lenses are their relatively high price (except for the 50mm f/1.8 lenses — they tend to be the cheapest of all lenses) and the narrow depth of field. Photographers on a tight budget typically have a 50mm f/1.8 or 85mm f/1.8 to shoot such sports. Those with more money get the 70-200mm f/2.8 zoom lenses. If you have more money, get the 200mm f/1.8 or f/2 prime lenses.

If you are shooting interiors or are the main photographer at an indoor event, you can arrange for multiple flash units or other lighting equipment to be set up at the location. This gives you the advantage of better lighting conditions so you are able to get the required shutter speed at the desired aperture and ISO.

Often, we just have to use high ISOs and then deal with noise reduction in the post processing stage. Here we use the noise reduction software tools that are available either in the default programs provided by the camera manufacturer, or commercially available software such as Noise Ninja, Neat Image, Dfine or the Adobe Photoshop line products. I personally stick to the software provided by the manufacturers for my day-to-day image noise reduction needs.

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