posted Monday, 13 September 2010, 19:43 (+0800), by Martin
This is the third article in a
series on "understanding your camera"
that I am writing as I teach camera basics and camera operation to my children.
Historically, ISO refers to the numerical scale used to measure the sensitivity of photographic film to light. Sensitive film (with a higher ISO rating) required less light to produce an image than less-sensitive film (with a lower ISO rating).
Several other scales were used in times past for measuring film sensitivity, but these were combined into a single standard, ISO 5800:1987, from the International Organization for Standardization (ISO).
The ISO system is now also used to measure the sensitivity of the sensors in digital cameras, but it is now set digitally, with the camera adjusting the signal gain of the sensor to provide similar results as would be achieved with film of the same ISO rating.
This table (taken from the first article in this series on understanding your camera) provides a summary of the effect of varying the ISO:
Doubling the sensitivity of a digital camera's sensor (or doubling the sensitivity of photographic film) equates to doubling the ISO number, and doubling the sensitivity will require half the amount of light to achieve the same exposure.
The ISO ratings on cameras typically starts at ISO 100 (some start at ISO 50), and then continually doubles, as follows:
100, 200, 400, 800, 1600, 3200, 6400, etc
Digital cameras provide higher ISO scales than were possible to achieve with film, and they are improving all the time. For example, the Canon 5D Mark II provides an ISO range from 100 - 6,400, and the Canon 1D Mark IV has an ISO range that reaches 12,800 (and both cameras can be expanded to an even higher ISO range via custom functions).
In general, cameras with smaller sensors typically suffer from more noise and grain than larger sensors, due to the size of the individual pixels on the sensor. A full-frame modern DSLR (such as the Canon 5D Mark II) has better high-ISO performance than a crop body (such as the Canon 50D), while point-and-shoot cameras, with their much smaller sensors, typically provide extremely noisy results when shooting at higher ISOs.
Increasing the ISO of a digital camera makes the camera sensor more sensitive to light, but also makes it more sensitive to noise. As a result, photos taken at higher ISOs typically show what is referred to as noise and grain, as shown in the two images below.
The two images above are small crops at 100% from larger photos, both taken with a Canon 50D. The image on the left was shot at the lowest ISO supported by the 50D, namely ISO 100, and the image on the right was shot at ISO 3200.
For reference, the full images that these 100% crops were taken from are shown below, with the 100% crop section shown with a white square.
When resized from the full 15 megapixels down to a 500 pixel wide image (as shown above), the noise and grain in the second photo above (shot at ISO 3200) isn't all that noticeable. The effect of the noise and grain depends on the camera's sensor size, the sensor performance at higher ISOs, as well as the colours and contrast of the subject material in the photo.
In some cases, it can be desirable to take photos with a lot of noise and grain, for nostalgic reasons, replicating the results you would get when shooting with high ISO film.
However, in most cases, you want to minimise the noise and grain in your photos. This means shooting at the lowest possible ISO that you can, while still retaining an acceptable shutter speed and aperture.
(Remember that aperture, shutter speed and ISO work together to determine the exposure of a photo, and adjusting one variable up or down means a corresponding adjustment up or down in another variable, as discussed in the first article in this series on Exposure Basics.)
However, it's important to remember that it's often better to get a noisy photo (shot with higher ISO) than an unusable photo (no noise or grain due to lower ISO, but unusabe due to blur because the shutter speed was too slow)!
Definition
Historically, ISO refers to the numerical scale used to measure the sensitivity of photographic film to light. Sensitive film (with a higher ISO rating) required less light to produce an image than less-sensitive film (with a lower ISO rating).
Several other scales were used in times past for measuring film sensitivity, but these were combined into a single standard, ISO 5800:1987, from the International Organization for Standardization (ISO).
The ISO system is now also used to measure the sensitivity of the sensors in digital cameras, but it is now set digitally, with the camera adjusting the signal gain of the sensor to provide similar results as would be achieved with film of the same ISO rating.
ISO Scale
This table (taken from the first article in this series on understanding your camera) provides a summary of the effect of varying the ISO:
impact of varying the ISO
Doubling the sensitivity of a digital camera's sensor (or doubling the sensitivity of photographic film) equates to doubling the ISO number, and doubling the sensitivity will require half the amount of light to achieve the same exposure.
The ISO ratings on cameras typically starts at ISO 100 (some start at ISO 50), and then continually doubles, as follows:
100, 200, 400, 800, 1600, 3200, 6400, etc
Improved ISO Sensitivity
Digital cameras provide higher ISO scales than were possible to achieve with film, and they are improving all the time. For example, the Canon 5D Mark II provides an ISO range from 100 - 6,400, and the Canon 1D Mark IV has an ISO range that reaches 12,800 (and both cameras can be expanded to an even higher ISO range via custom functions).
In general, cameras with smaller sensors typically suffer from more noise and grain than larger sensors, due to the size of the individual pixels on the sensor. A full-frame modern DSLR (such as the Canon 5D Mark II) has better high-ISO performance than a crop body (such as the Canon 50D), while point-and-shoot cameras, with their much smaller sensors, typically provide extremely noisy results when shooting at higher ISOs.
Effect of ISO on Image Quality
Increasing the ISO of a digital camera makes the camera sensor more sensitive to light, but also makes it more sensitive to noise. As a result, photos taken at higher ISOs typically show what is referred to as noise and grain, as shown in the two images below.
100% crop at ISO 100
|
100% crop at ISO 3200
|
The two images above are small crops at 100% from larger photos, both taken with a Canon 50D. The image on the left was shot at the lowest ISO supported by the 50D, namely ISO 100, and the image on the right was shot at ISO 3200.
For reference, the full images that these 100% crops were taken from are shown below, with the 100% crop section shown with a white square.
photo taken at ISO 100
Canon EOS 50D, 24-70mm f/2.8L @70mm, 1/250 sec, f/2.8, ISO100
photo taken at ISO 3200
Canon EOS 50D, 24-70mm f/2.8L @70mm, 1/8000 sec, f/2.8, ISO3200
When resized from the full 15 megapixels down to a 500 pixel wide image (as shown above), the noise and grain in the second photo above (shot at ISO 3200) isn't all that noticeable. The effect of the noise and grain depends on the camera's sensor size, the sensor performance at higher ISOs, as well as the colours and contrast of the subject material in the photo.
ISO Recommendations
In some cases, it can be desirable to take photos with a lot of noise and grain, for nostalgic reasons, replicating the results you would get when shooting with high ISO film.
However, in most cases, you want to minimise the noise and grain in your photos. This means shooting at the lowest possible ISO that you can, while still retaining an acceptable shutter speed and aperture.
(Remember that aperture, shutter speed and ISO work together to determine the exposure of a photo, and adjusting one variable up or down means a corresponding adjustment up or down in another variable, as discussed in the first article in this series on Exposure Basics.)
However, it's important to remember that it's often better to get a noisy photo (shot with higher ISO) than an unusable photo (no noise or grain due to lower ISO, but unusabe due to blur because the shutter speed was too slow)!
Wow, what a great and informative blog! Love it.