ISO (or International Standards Organisation) is the name by which different films (or these days digital sensors) sensitivity is measured. I?m going to tailor this article to digital cameras more than film but much of it goes for film in the same way.
Every camera has a base ISO level (usually ISO100 but can be ISO50 or even ISO200) the actual name of this level really doesn't matter it could be ISO1 it makes no odds, what is important to know is that it is this base level that is the least sensitive to light. It is also this base level that will give you the best image quality and the least noise (in film it would have been grain).
When you increase your ISO (step it up) you increase the sensors sensitivity to light (decreasing ISO decreases sensitivity). This increase in sensitivity is double the previous ISOs level. This is why the steps are usually ISO100, ISO200, ISO400,600 on up to 3200-6400 or even higher in top end cameras (This level is likely to increase over time with new technology so may well be out of date). What this means is that if it took 1/25 of a second to correctly expose an image at ISO100 then it would take 1/50 of a second at ISO 200 and 1/100 at ISO400 (you increase shutter speed with each increase of ISO).
The issue with increased ISO and the reason we don?t all always shoot at the highest ISO we have available (not always wanting fast shutter speeds aside) is that with the increase in sensitivity comes the problem of digital noise (that random pattern that can affect the image). Digital noise is the result of electrical interference between the photosites, more commonly called pixels, the more pixels you have on a sensor surface the more risk there is of noise (one of the reasons point and shoot cameras with small sensors suffer more than SLR?s with larger sensors). Better spaced pixels have less chance of interference so larger sensors (or lower pixel counts) usually produce less noise (although technology is improving the noise filters of cameras all the time).
Lower ISO means less sensitivity to light but better image quality (IQ), higher ISO means greater sensitivity to light but at the risk of more noise.
I hope this was informative, I know it was shorter than many of my basic articles while at the same time being slightly more technical. I advise reading this article in conjunction with our other basic articles ? Understanding Aperture, Understanding Shutter Speed and Understanding Exposure. I have not included details of when to use different ISO?s as I personally think it is up to you to know what works best, I would not presume to know what you wish to achieve with your photos.
If you found this article of use please share it with your friends, and let me know what you liked. If you found any mistakes or have any advice I should include please contact me via the feedback link at the top of this page.
Ashley Beolens Sorry no photos at the moment I may add some to show noise at a later date
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