It is extremely hard to take good pictures without a good understanding of how ISO works and what it does. Camera ISO is one of the three pillars of photography (the other two being Aperture and Shutter Speed) and every photographer should thoroughly understand it, to get the most out of their equipment.

What is ISO?

Your ISO allows you to take pictures in low light situations.

It is basically a measure of your digital sensor’s sensitivity to light. The higher the number, the more sensitive to light your sensor becomes.

ISO Speed & Noise

However, all this increase in sensitivity does not come free. There is a price to pay with your image appearing more noisy.

See, when you boost the sensitivity of your image sensor by selecting a higher ISO, the image sensor is now able to record a fainter light signal. However, it is also true now that it will record fainter noise, where noise is any signal that is not attributed to the light from your subject. Remember that an image sensor is still an analog device and it generates its own noise, too! The increased sensitivity allows the image sensor to record more light signal and more noise. The ratio of light signal to noise (S/N ratio) determines the “noise” in your resultant image.

An image sensor is usually calibrated so that it gives the best image quality (greatest S/N ratio) at its lowest possible ISO speed. For most consumer digital cameras, this value will be expressed as ISO 50, ISO 64 or ISO 100. A few digital cameras use ISO 200 as their lowest ISO speed.

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The lower the ISO the less noise you will see. As your ISO increases, the noise level does as well. This noise level most likely will not become significant until your ISO reaches numbers of 800 or higher (depending on your camera).

The aperture stop of a photographic lens can be adjusted to control the amount of light reaching the film or image sensor. In combination with variation of shutter speed, the aperture size will regulate the film’s degree of exposure to light. Typically, a fast shutter speed will require a larger aperture to ensure sufficient light exposure, and a slow shutter speed will require a smaller aperture to avoid excessive exposure.

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The basics

The pupil is the aperture of your eye and, like your pupil, the aperture of a camera lens can be varied in size to let more or less light in. The first basic concept of aperture to get your head around is that the smaller the number [f/1.4 for example] the bigger the aperture, or ‘hole’.

Diagram of decreasing aperture sizes (increasing f-numbers) for “full stop” increments (factor of two aperture area per stop)

A device called a diaphragm usually serves as the aperture stop, and controls the aperture. The diaphragm functions much like the iris of the eye – it controls the effective diameter of the lens opening. Reducing the aperture size increases the depth of field, which describes the extent to which subject matter lying closer than or farther from the actual plane of focus appears to be in focus. In general, the smaller the aperture (the larger the number), the greater the distance from the plane of focus the subject matter may be while still appearing in focus.

The lens aperture is usually specified as an f-number, the ratio of focal length to effective aperture diameter. A lens typically has a set of marked “f-stops” that the f-number can be set to. A lower f-number denotes a greater aperture opening which allows more light to reach the film or image sensor. The photography term “one f-stop” refers to a factor of √2 (approx. 1.41) change in f-number, which in turn corresponds to a factor of 2 change in light intensity.

In the old days, cameras had a ring on the lens to change aperture. That ring was marked off in ‘one stop’ gradations:

1.8 – 2 – 2.8 – 4 – 5.6 – 8 – 11 – 16 – 22 – 32 and so on, but usually not much past f/32 on even the best lenses.

Aperture priority is a semi-automatic shooting mode used in cameras. It allows the photographer to choose an aperture setting and allow the camera to decide the shutter speed and sometimes ISO sensitivity for the correct exposure. This is sometimes referred to as Aperture Priority Auto Exposure, A mode, Av mode, or semi-auto mode.4

Photography gear and how to buy it:

1. What is a DSLR?
2. DSLR vs Point and Shoot
3. How to Buy a DSLR Camera
4. First Nikon DSLR and Lens
5. First Nikon Lens?

Understanding basics of photography:

1. Understanding ISO – A Beginner’s Guide
2. Understanding Aperture – A Beginner’s Guide
3. Understanding Shutter Speed – A Beginner’s Guide
4. Understanding Digital Camera Modes
5. Understanding ISO, Shutter Speed and Aperture – A Beginner’s Guide

Other photography tips:

1. What is Bokeh?
2. How to Take Good Pictures
3. How to Take Sharp Photos
4. Nikon Lens Bokeh Performance
5. How to Photograph Children
6. How to Photograph Birds
7. How to Photograph Textures
8. How to Photograph the Moon
9. How to Photograph a Lightning
10. Astrophotography: taking pictures of stars
11. Focal length comparison on Nikon DX (1.5 crop factor) cameras

Software and post-processing:

1. What is EXIF Data?
2. How to remove EXIF Data?
3. Nikon RAW (NEF) Codec for 64 bit Vista or Windows 7
4. How to add frames to photos in Photoshop