WHAT IS ISO?
ISO stands for International Organization for Standardization, and in terms of photography, designates an exposure index system devised by that organization. In essence, ISO is an exposure rating index that is used to tell you how sensitive (sometimes referred as fast) the film (or a digital sensor) is to light.
This system was created for film and carried over into digital. It replaced the older ASA (American Standards Association) and DIN (Deutsches Industrie Norm) exposure index ratings in order to maintain consistency across the field of photography internationally.
ISO is measured in a series of numerical settings (50, 100, 200, e.t.c.). Each rating number doubles as it increases. The higher the ISO speed number, the more sensitive the film or sensor is to light, and therefore generates faster exposure times. Higher ISO settings are easier for low light situations, however it also introduces image noise or film grain into the photograph. The higher the ISO speed number you select, the less light you need to take a photo with correct exposure, however you are likely to compromise the quality of your overall image.
The lower the ISO number the more time the film or sensor needs to be exposed.
The faster (higher) the ISO speed, the less exposure time is needed.
Grain (film) or image noise (digital) may look great in some situations, yet in professional beauty, product and some other types of commercial photography, it is highly undesirable. Digital images suffer most in this regard, whereas film retains some interest as long as the grain is not too prominent.
ISO and Digital Photography
In digital photography ISO 100 is considered normal ISO speed, which provides crisp, detailed images with little to no image noise. It is a good practice to set your camera to ISO speed 100 – 200 and raise it only when necessary. When taking your photograph, you can mitigate some of the exposure time lost with a low ISO speed by introducing strobe lighting, lowering the shutter speed, or adjusting aperture settings. It is wise to determine a cap on your ISO speed based on what you are shooting and the technology in the camera you are using. For example, a crop-frame camera will need more time to collect light than the more light sensitive sensor on a full-frame. Likewise, some camera models are rated more light sensitive than others. Identify your specific camera’s light sensitivity and tailor your shooting style to maximize its strengths and overcome its weaknesses.
RELATED: Introduction to Digital Imaging
ISO in digital is a way for the technology to ‘trick’ the sensor into being more sensitive. You may notice the image noise produced by a high ISO image is very similar to the image noise that can be created in post-production by increasing the exposure too much. The reason for this is that the technology is using similar methods to bring out more information. Unlike analogue, where the light sensitivity is actually different based on the ISO roll of film you are using, you are not changing out the sensor on the DSLR camera. It is as sensitive as it will ever be. When you increase the ISO past a certain point, you are telling the programing to ‘make’ the sensor see more light to improve the exposure.
How to Choose the Correct ISO Speed
- When there is plenty of light, like a sunny day, use the lowest ISO setting.
- When working in low light situations outdoors, open up your aperture (lower f-stop) or use slower shutter speed (not lower than 1/80 to avoid camera shake and motion blur) before bumping up your ISO.
- When shooting low light indoors, introduce artificial light in the form of flashes or strobes. If using artificial light is not an option, open your aperture, use a slower shutter speed, and increase your ISO speed in increments until you find the lowest ISO setting possible to obtain proper exposure.
- The different films for analogue cameras determine the ISO; so always keep a variety of rolls around. I usually have 100, 200, and 400 on hand whenever I shoot with film. Just keep in mind, you need to use the full roll before switching and adjust your ISO setting on the camera so your white balance is not thrown off.
ISO and Everyday Scenarios
The lowest ISO setting will always provide the cleaner, higher quality images. There will be times you may need to play around with the 3 main settings that control exposure – ISO, Aperture, and Shutter Speed – to find the perfect balance. These three are also known as the Exposure Triangle.
- Bright Sunny Daylight: 100 ISO – in this scenario you are likely to need to address overexposure than underexposure. A good rule of thumb (from film days) is ISO 100, the aperture and shutter speed should be based on the desired depth of field and movement of the subject.
- Overcast or Indoor: 100 ISO with lower shutter-speed and opening aperture. If those methods still do not work, raise your aperture in increments to 400 – 800 ISO based on the ambient light. Introducing reflectors, strobes, or flashes will help add additional light when possible.
- Low Light (Outdoor/Indoor) with no additional light source: 800 ISO and above.
The following photographs were all taken around the same time, (4pm on a bright day) with the same amount of ambient light (indoors near a south-facing window). You can check the camera settings in the top left corner of each image. The lens used was a Canon 24-70mm on a full-frame body.
Aperture f/2.8 – Shutter Speed 1/80 – ISO 100
This is the base example, with aperture wide open and shutter at the lowest hand-held speed (without internal shake) and the normal ISO setting.
Aperture f/2.8 – Shutter Speed – 1/50 – ISO 100
The slower shutter speed allows more ambient light to reach the sensor, however the image is still underexposed. It also introduces a little camera shake.
Aperture f/2.8 – Shutter Speed – 1/160 – ISO 800
Overall exposure is improved, but image noise from the higher ISO speed may or may not present a challenge; depending on what the photograph will be used for.
Aperture f/7.1 – Shutter Speed – 1/160 – ISO 6400
A change in depth of field has occurred and more area is in focus due to smaller aperture (f/7.1) The image noise in shadows have increase dramatically due to the high (6400) ISO speed.
A Few More Thoughts
Many of the newer professional DSLR cameras perform better at high ISO settings, often introducing dramatically less image noise than their predecessors. Be sure to keep in mind, they will still introduce noise. If you are shooting for large prints or for a client, remember where they will be seen and if image quality will be scrutinized.
Consider these options before raising the ISO speed:
- Strobes are your friend: use artificial light whenever possible in low light.
- Camera Shake: use a tripod to minimize camera movement. Really low shutter speeds can be used for still-life images. Don’t have one yet? Grab a bag of rice and use your timer or remote. Even the act of pressing the shutter button will cause camera shake when the shutter speed is lower than 1/80.
- Depth of Field: If a shallow depth of field (less in focus) is an option, open up your aperture to increase exposure.
- The Film Grain Effect: It is easier to get away with noise or grain in B&W images. It’s the chroma (color) noise, that is the most unsightly. Also, people are accustomed to the vintage look of film and it can add another dimension of interest to your photograph that would have otherwise been an eyesore.
RELATED: Image noise is most often related directly to use of high ISO. To learn more about Image Noise and some ways to address it in your images, read Understanding Image Noise.
- Higher the ISO, the more sensitive the film/sensor is to light.
- Lower the ISO, the more crisp and clean the image will appear.
- ISO speed relate to allowed aperture and shutter speed combinations. It is one of the three main factors that affect exposure in an image.
- Higher the ISO, more noise or grain in the photo, especially in the black / shadow areas of the image.
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