The ISO is one of the three components of the Exposure Triangle. The ISO rating is used to measure the media sensitivity to the light whether you use film or a sensor (in a Digital camera).
Ability to change your ISO is a blessingIf there is one major feature that came from the Digital camera revolution, it has to be the ISO setting that enables you to change the sensor sensitivity at will, after every single shot. As a matter of a fact, before the 1990s all cameras used a roll of film of an unchangeable ISO rating (i.e. a 100 ISO roll). Should one needed to use 400 ISO for a particular situation, he/she would have needed to remove the roll of film located int eh camera and in that process any part of the film that was unused at that time (i.e only 14 shots taken out of the 36 available) would be unusable forever. Suffice to say that when one put a roll of film in a camera, it was critical to wonder what situation was going to be photographed. I’d rather mention this straight away: if I use the past tense for film photography, it is not to imply the days of film photography are over. On the contrary, there are many amazing photographers out there who loves shooting and develop film and should not be booed for it. It all comes down to personal preferences. Most point and shoot cameras on the market today offer you the possibility to choose an ISO value from 100 ISO to 1600 ISO. For DSLR and professional camera, you can extend the ISO to 12800 ISO or even 25600 ISO. *I know some Nikon camera offer you 200 ISO as the lower ISO available and some professional camera let you even choose 50 ISO. Here is the ISO scale: 100, 200, 400, 800, 1600, 3200, 6400, 12800, 25600. The jump from 100 ISO to 200 ISO (or from 400 to 800) is called a full stop. Some cameras let you use 1/3 stops for more flexibility with your exposure. Each time you increase the ISO value by one full stop, you increase the camera sensor sensitivity, so it captures twice more information from the light. The same logic is applied when reducing the ISO value by a full stop ( the sensor being less sensitive will capture half the information coming from the light). Samples shot with my Canon 60D. These were shot in small JPEG and not post-processed.
What ISO should one use?Well, it all depends on of the situation that is being photographed. Here are a few rules of thumb that should not be seen as restrictive:
- Sunny, overcast day: 100 ISO.
- In the shade: 200 ISO or 400 ISO.
- In the evening or dark places: 800 ISO and above.