Film Speed - ISO
ISO determines how sensitive the image sensor is to light. The more sensitive the sensor is, the better it can capture images in low light levels. Wikipedia has a very technical explanation about what film speed is, but honestly I don’t think you will ever need that much info (but if you’re curious, you can read about it here.
All you need to know is that ISO determines the sensitivity of your sensor.
If a combination of shutter speed and aperture don't deliver enough light, you can boost your ISO. The penalty is red and green spots (noise) over your image.
The most common ISO speeds are: 100, 200, 400 and 800, but they can go lower (80, 50) or higher (1600, 3200, 6400, etc.) depending on your camera model. Similar to shutter speed, it also correlates 1:1 with how much the exposure increases or decreases.
So ISO 200 is 2x brighter, and ISO 400 is 4x brighter than ISO 100. But unlike aperture and shutter speed, a lower ISO speed is almost always desirable, because high ISO speeds increase image noise.
Image noise is always present. Both light and the circuits around the image sensor produce interference, which is then picked up by the sensor as image noise. But when the ISO is low, you won’t really see it. When you amplify the sensitivity of your sensor (from ISO 100 to 200, 400, etc.), it also amplifies the noise. The higher the ISO number, the more sensitive the sensor will be, but the more noise you are introducing in the picture. Also darker regions of an image will contain more noise than the brighter regions.
As a result, ISO speed is usually only increased from its minimum value if the desired aperture and shutter speed aren't otherwise obtainable.
A low ISO setting (50–400) generally produces cleaner images, but the pictures can be dark if not shot in bright sunlight or with a good flash. Images are also more susceptible to blurring because of the slower shutter speeds required when using a low ISO setting. This is a slow ISO (slow film).
Higher ISO settings (800–1600) can produce better exposed pictures in low light but also introduce more electronic noise. This is called fast ISO (fast film).
Never cameras have better sensors, so less interference is picked up. As a result you can shoot at high ISO and still get clean images.
So how do you use ISO speed?
- Use ISO 100 or 200 when taking photographs outside in sunny conditions.
- If the sky is overcast or it is evening time, then use an ISO within the range of 400 to 800.
- Night time or in cases of low light you might need to set your digital camera ISO to 1600. If you don't then your photo will appear too dark (and probably blurry because of the low shutter speed).
Example:
Let’s say a correct exposure is f/8 at 1/100sec ISO 100.
Increasing the ISO to 200 will make your image 2x brighter. That will allow you to stop down the aperture to f/11 for more depth of field, or to change the shutter speed to 1/200 second to stop motion.
Increase the ISO to 400, and you can stop down the aperture to f/16 or change the shutter speed to 1/400 second.
Some more examples of when to use higher ISO settings:
- If you want to take a photograph in dark situations and not use a tripod.
This way, your camera will automatically keep a fast enough shutter speed for you to hand hold your camera.
- If you want to take photographs in darker situations other than outdoors, without the use of a flash.
For pictures in a museum, theatre, and other places where a flash is prohibited or would ruin the atmosphere.
- If you want to take a photograph indoors of a moving subject.
Higher ISO will allow a faster shutter speed.
