What is ISO?
ISO stands for International Standards Organization (kind of a non-descriptive name to me, but there you go). The ISO number corresponds to the sensors (initially the films) sensitivity to light, and is a standardized scale used throughout the photography industry (so ISO 400 should be the same for all cameras more or less).
How does ISO effect your image?
Each camera has a base ISO, which has the best image quality. This is generally ISO 100. As you increase your ISO, your camera becomes more sensitive to light. ISO 200 is twice as sensitive to light as ISO 100, ISO 400 is twice as sensitive as ISO 200 and so on (this are refereed to as stops of light reviewed here). The more sensitive to light your sensor is, the more light it can detect and you can take images in darker conditions, like inside events, the night sky or pictures of leopards in the fading light of the day! :)
But nothing in life is free, and by increasing the sensitivity of your sensor, you end up reducing the image quality. Increasing the ISO of you camera leads to:
- Reducing the dynamic range of your image
- Reducing image sharpness
- Introducing and increasing image noise
Which ISO should I use?
This is, in my opinion, easy to answer: Choose the lowest ISO you can that will give you the shutter speed and/or aperture you want.
Now, that being said, you can generally expect to increase your ISO with less light, and lower your ISO in better lighting conditions. Particulalry with wildlife photography, where we are facing constantly changing lighting conditions, it is important to keep an eye on your ISO and change it according the conditions.
Now there reaches a point where you might consider changing your shutter speed or aperture as the ISO required for perfect settings would result in an image with an unacceptably low image quality (a point of diminishing returns). I can't tell you what this limit is as every camera has different ISO capabilities and you will also have different levels of what you think is acceptable.
Different cameras have different ISO capabilities
As technology is getting better, sensors are getting a lot better in terms of their ISO performance. It is a safe bet to say that the quality of an image taken at ISO 1600 on a 8 year old camera is not as good as an image taken at ISO 1600 on a new one. So it is important for you to be aware of your specific cameras capabilities in term of ISO performance in order for you to know it's limits.
One of the biggest factors that determines the image quality at higher ISO's is the pixel sizes on the sensor. The larger the pixel size the better it is at collecting the light (this is a VERY watered down version, if you are interested in the physics of this, google it). Likewise, the larger the sensor itself, the more larger the pixels can be and still have a high mega-pixel count. This generally means that full-frame cameras have a better ISO performance than the APS-C cropped sensor cameras.
Don't be afraid to push the limit of your cameras ISO
I have covered the subject of high-ISO images before (see links bellow), so I will highlight the main point here. An image taken at high-ISO is better than no image at all. If I had to choose between a sharp image taken at high-ISO, or a blurry image (camera shake) taken at a more desirable ISO, I would take the first option every time!
Read these for more on this subject! :)
