Welcome to Lesson Three: Understanding ISO and Your Camera’s Sensor
If this is your first month joining us for #AdoramaU, we strongly recommend beginning with our first two lessons on aperture and shutter speed, which builds on some of the foundational concepts discussed below.
By now you should be comfortable with two key aspects of exposure control. Your lens’ aperture, which controls the depth of field and amount of light received by your sensor, and shutter speed, which determines the length of time that light is reaching your sensor, both combining to create an exposed image. These results are consistent and repeatable, assuming all other factors are equal, which brings us to the third most important variable for creating an exposure.
Fujifilm X-Pro1 & XF 18mm Lens
Image Settings: f/5.0 @ 1/1600, ISO 200. At the X-Pro1’s base ISO of 200, this image will be grain free and maintain the highest dynamic range for shadow and highlight detail.
Introduction to ISO
For all practical purposes, ISO could otherwise be known as the “Sensitivity of Your Sensor”, but something about SOYS just doesn’t have the same ring as ISO. In fact, ISO isn’t even an acronym, but a shortname for “International Organization for Standardization”, which was established and first published for digital camerasin Geneva, Switzerland in 1998. ISO’s are presented as linear and logarithmic numbers, and as the name implies, a standard measure of your sensor’s sensitivity to light. Expect to find this number ranging anywhere from ISO 50 up to the current (commercially available) low-light allstar, the Sony A7S’ ISO 409,600.
The ISO setting is one of the 3 pillars of the exposure triangle—aperture, shutter speed, and sensor sensitivity. For this reason, the ISO setting is one of the most important controls on your camera and knowing how to use it effectively is absolutely essential. Fortunately this scale corresponds to the same full stop sequence as aperture and shutter speed, so a setting of ISO 200 means that the sensor is twice as sensitive to light (one f/stop) as one rated at ISO 100, and a sensor set at ISO 400 is four times (two f/stops) more sensitive than one set to ISO 100, and so on. This full stop sequence of ISO ratings is depicted in Figure 3.1.
Taken at ISO 200 this image displays excellent tonality and accurate color rendition.
(Image by Fahad Jafarullah)
Film ISO vs. Digital ISO
While the convenience and image quality of digital cameras have rocketed past 35mm and medium format film quality, film still has its place and will always be revered and respected in the photography community. But today we’ll hit on those two main points: convenience and image quality. One of the most obvious and appreciated advantages of digital photography is that you can fluctuate between ISOs in-camera depending on the lighting conditions and quality considerations. Film wasn’t nearly as forgiving for photographers; anyone shooting 35mm would choose their film speed, load their camera and they were stuck with it, so to speak. Many would carry two camera bodies with two different ISO-rated films. Some would shoot with professional medium format cameras that allowed you to rotate out the film backs, keeping two or three different film backs in their bag to anticipate a variety of lighting conditions.
Another profound achievement of digital cameras is that sensors can produce significantly cleaner images at higher ISOs than film at the same rating. Much like film, higher ISO settings produce noisier images. This textured pattern of “digital grain” is generally considered less attractive than film grain but doesn’t render as poorly until significantly higher ISOs. The good news is that most modern digital cameras, especially DSLRs and mirrorless interchangeable-lens cameras, have relatively large sensors with “native ISO ratings” of around ISO 100 or 200, meaning that this is the speed at which they will deliver optimum image quality and the widest tonal range. Setting higher ISOs does not alter the inherent sensitivity of the sensor—it simply increases the gain on the image-capture circuitry. Like using a faster film, this lets you set smaller apertures, higher shutter speeds, or capture proper exposures in less light.
Because of the incredible advances in image processing technology over the last few years, a typical late model digital camera with an image sensor having a native ISO of 100 will still yield images of outstanding quality when set to ISO 800. Any loss of image quality, color saturation or increase in noise will only be noticeable at very high magnification. Indeed, many current full-frame (24 x 36mm-format), APS-C-format, and 1-inch-sensor cameras are capable of capturing sharp, relatively noise-free images even at settings of ISO 1600 and 3200 providing you don’t view them at close distances or make huge enlargements. This far exceeds the capabilities of film where even full-frame 35mm images shot at ISO 1600 exhibit noticeable graininess (which may still have its charm for artistic purposes!).
Original Image: Canon 5D Mark III & Sigma 35mm f/1.4 Art Lens.
Image Settings: f/2.5 @ 1/80, ISO 3200
100% crop: At ISO 3200, most cameras begin to display heavy digital noise. The Canon 5D Mark III is a low light all-star and does an excellent job of controlling noise and still maintaining shadow detail.
Pixel Size, Not Pixel Count!
Contrary to popular belief, the prime determinant of overall image quality in digital photography is not system resolution, also known as the camera’s megapixel (MP) count, but pixel size. While other factors come into play, most notably dynamic range and bit depth, the reason a 20MP sensor in a full-frame camera can take sharper far less noisy images than a 20 MP point-and-shoot at a high ISO is pixel size. Physically smaller sized pixels are required to squeeze in the same MP count found in the full-frame sensor camera. Needless to say, larger pixels are much more efficient in capturing the full image signal and transmitting it to the analog-to-digital (A/D) converter that produces the final captured image.
All things being equal, a camera with a lower pixel density and larger pixels will always be able to capture images with much lower noise, better color reproduction, and higher quality, especially at higher ISO settings. Obviously, a larger sensor also has more room to accommodate these larger pixels. With the release of the Sony A7 series, Sony is able to perfectly illustrate this point with outwardly similar but dramatically different cameras. The Sony Alpha a7S (12.2MP) delivers astonishing image quality even at ISO 12800 while the Sony Alpha a7R (36.4MP) is noted for degrading image quality at about ISO 1600. The monstrous 36.4MP sensor just isn’t capable of collecting as much light and suffers with poor high-ISO performance.
Canon 5D Mark III & Sigma 35mm f/1.4 Art Lens
Image Settings: f/1.4 @ 1/30, ISO 2500. Taken well before sunrise, these image settings are clear indicator of the low lighting conditions.
Practical Advice for Digital Camera Buyers and Shooters
- When buying a new digital camera, favor those with larger sensors, especially if you shoot in a lot of low light because they will deliver superior image quality at higher ISO settings. Pros and serious enthusiasts should consider full-frame models, but the latest models with APS-C and 1-inch sensors are capable of excellent image quality at ISO settings up to 1600 and beyond. Recently introduced cameras are also likely to incorporate the latest image processing systems, which will yield enhanced responsiveness and better high-ISO image quality.
- Select a camera with a convenient and intuitive ISO-setting system. Some cameras bury the ISO settings in the menus but some of these have the capability of dedicating a button to ISO setting. Familiarize yourself with the ISO setting on your camera and find out how to do that so you can make quick ISO settings on the fly.
- Use the lowest ISO setting that is consistent with shooting at handheld shutter speeds. Utilize the handheld rule discussed in Lesson Two on shutter speed as a reference—the equation is one over the focal length of the lens you’re using (e.g. 1/200 sec when the focal length is 200mm) in order to minimize the blurring effects of camera shake. If you notice your shutter speed is too low select the next highest ISO until you get a speed that can be handheld.
- Use your camera’s Auto ISO setting when shooting action subjects and/or in rapidly changing light. The most important thing is getting the shot, and you don’t want to miss a picture opportunity while fiddling with your camera’s ISO control. Most of today’s cameras provide an Auto ISO setting that lets the camera choose the ISO automatically as the light or subject changes, and you can almost always specify the maximum ISO the camera will set. If you plan to make large prints or display the images on a large screen you may want to limit the maximum ISO setting to 800, 1200 or 1600 to avoid excessive noise or loss of image quality.
- Assess your camera’s performance at various ISO settings and determining the highest ISO setting that will yield images that satisfy you. Place your camera on a tripod and shoot a series of pictures of a distant subject in good light with lots of high- and low-contrast detail. Shoot a picture starting at your camera’s lowest ISO working your way up in full stop increments. Compare the results at high magnification on your computer or monitor and determine for yourself what is acceptable ISO performance of your camera.
- Experiment! Don’t be afraid to use a variety of ISO settings with the same subject when shooting handheld to see how various combinations of shutter speeds and apertures impact the results. Think of the ISO-setting control of your camera as just another creative tool in your arsenal and the gateway to a fun learning experience and you’ll go a long way toward taking your photography to the next level!
To give you some practice with ISO and reward your participation, check back each week in April for our weekly #AdoramaU assignments.
- Week 1: Use a low ISO to capture animage outdoors. Set ISO to 50, 100 or 200.
- Week 2:Use a mid-level ISO to capture an image in shadows. Set your ISO to 400-800.
- Week 3: Use a high ISO to capture an image indoors or at night. Set your ISO to 800 or higher.
- Week 4: Capture fast action with a high ISO and fast shutter speed. Set your ISO to 400 or higher.
