ISO MADE SIMPLE
In photography, ISO describes the sensitivity of a film, or the base sensitivity (and gain) of a digital camera sensor. Together with Aperture and Shutter Speed, ISO effectively completes the third part of the Exposure Triangle.
ISO has both a creative and exposure-related function. Low ISO film/settings are wonderful for bright environments, and when detail and dynamic range are paramount. High ISO film/settings are ideal for boosting exposure in darker environments, but they come at a cost of image noise, reduced detail, and dynamic range. It’s important to note that the loss in detail and dynamic range can also be used for creative effect, which we will discuss later in this article.
Like Aperture and Shutter speed, each “stop” (1 EV) on the ISO scale is measured as either double the brightness, or half the brightness, and the numbers are simple: ISO 100, 200, 400, 800, etc.
In the example below, you can see how image quality changes as a digital camera’s ISO is raised:
Fully understanding ISO can help you capture images that better achieve your creative vision. This article is intended to be the ultimate guide to understanding ISO, and comes from our full-length course Photography 101. However, keep in mind that this guide, like all of the education on SLR Lounge, is not designed to be a technical manual, but rather a field reference tool. We create real-world field guides designed to get you out and shooting as quickly as possible.
Let’s dive into the first part, which is the basic use of ISO to control your image exposure. For those interested, we will include additional technical details and insights at the end of the article.
ISO FOR EXPOSURE CONTROL
On a digital camera, changing the ISO does not actually change the sensitivity of the sensor, it simply boosts the exposure brightness after it has been captured. So, why would you decide to boost your ISO? There are still two main technical reasons for using a higher ISO: Either a scene is too dark, or the proper exposure can’t be reached when a specific shutter speed (or aperture) is desired.
ISO is commonly used in conjunction with Aperture and Shutter Speed, as a “technical aide” for the effect you wish to achieve.
For example, if a fast shutter speed is desired in dim lighting, a photographer might raise their ISO as high as far as they are comfortable raising it. “Comfortable” simply means that any loss of image quality is either negligible or acceptable. You can see how the image quality degrades with each step up in ISO on a Nikon D750 in the animated GIF below.
Don’t start worrying quite yet. Many consumer and most professional cameras today offer exceptionally high-quality images even at ISO 3200 and beyond. In the image below, a faster shutter speed of 1/160th of a second was desired to freeze the motion in the flower petals being thrown into the air. For that reason, the ISO was raised to 3200 in order to arrive at a good exposure at f/2.8.
Oppositely, if a slow shutter speed is required, a low ISO (and/or a small aperture) may be used in order to achieve a correct exposure. Again, this is simply the exposure triangle at work!
Often, both aperture and shutter speed are chosen for their own creative reasons and the only setting left in the exposure triangle is ISO. Thus, it is raised or lowered in order to achieve a desired exposure while accommodating the photographer’s creative vision, even if image quality is reduced at high ISOs.
With that in mind, let’s get into using ISO for more creative purposes!
ISO FOR CREATIVE CONTROL
A beginning photographer might assume that due to the loss of image quality one should never raise the ISO except in extremely dark conditions. However, changing ISO offers creative opportunities in both normal daylight conditions, and certainly in more dim lighting conditions.
There are three creative reasons for you to consider using different ISOs:
- When detail and dynamic range are critical
- When you intentionally wish to create a film effect
- To control motion blur in conjunction with shutter speed
1. USING A LOW ISO FOR MAXIMUM DYNAMIC RANGE
All digital camera sensors offer their best image quality at their native, base ISO, which is the lowest ISO that is not labeled as “LO”. For many types of photography, this offers a creative advantage: The ability to see better detail in both highlights and shadows at the same time in a single exposure.
This is known as dynamic range. It’s the film/sensor’s ability to capture detail in a bright highlight and a dark shadow at the same time, and can be a fantastic creative tool for landscape photographers and other types of photographers who wish to capture such images where very bright highlights and very dark shadows come together in the same frame. (See below)
Sony A7mk2, Sigma 24mm f/1.4 Art, Single Exposure
USING A HIGH ISO FOR FILM EMULATION
As ISO increases, so does image noise. However, since most modern digital cameras still maintain a high level of sharp detail and color at medium-high ISOs, things like image noise can be seen as “digital grain” for a creative look.
One common use of high ISO is to emulate the film grain of a classic, popular portrait film such as Fuji 400H, or Kodak Portra 800. The noise levels are not very damaging to the image quality, yet they can create a much more natural look to grain (noise) compared to when it is added to a perfectly smooth-toned original image in post-production.
Also, the slight loss of color saturation and dynamic range can create a much more natural, film-like look that can bring the image closer to the creative vision with less editing.
USING ISO TO CONTROL MOTION BLUR
Using a higher ISO even in normal daylight conditions offers you an obvious way to control your shutter speed creatively, without having to adjust your aperture.
For example, if you are shooting an action portrait of someone running or jumping, yet you also wish to use a smaller aperture for greater depth of field, then raising your ISO to 400 or 800 will allow you to use a shutter speed that is 2-3 stops faster. If this is the difference between 1/100 sec and 1/800 sec, it could be the difference between a slightly blurred subject and a tack-sharp subject!
Opposingly, if a landscape photographer needs to shoot at an aperture of f/8 or f/11 for deep depth of field, yet is in a location where tripods are not allowed, they may decide to raise their ISO to 400 or 800 in order to achieve sharp results at a hand-holdable shutter speed such as 1/15 sec.
Last but not least, if motion blur is the intended creative effect, using your lowest ISO is the best creative choice. In fact if blowing out highlights is not a risk, then using a “LO” ISO is a great creative option for allowing an even slower shutter speed in-camera. (See below for an explanation of “LO” and “HI” ISO numbers)
Each creative decision that you make about your ISO and exposure can have benefits and/or drawbacks. Whether you choose a no-compromises approach or opt to experiment, the most important thing is to understand exactly how ISO works, what the different ISO’s on your own camera look like when correctly exposed, (or poorly exposed!), and how this influences images for achieving your creative vision.
COMMON ISO TERMS/SETTINGS
Common “whole stop” ISO’s include: 100, 200, 400, 800, 1600, 3200, 6400, and higher, in double/halve increments. However, ISO can also be controlled in 1/3 or 1/2 stop increments if desired, such as ISO 125 or 160.
Usually, depending on which camera you have, whole ISO stop settings may have slightly better image quality than intermediate ISO stop settings. This is less of a problem in the latest cameras compared to digital cameras from 10+ years ago, however, it is still important to test your own camera’s optimal ISO settings if you want to get the most out of your camera.
ADDITIONAL TECHNICAL INSIGHT ABOUT ISO
HOW IS ISO MEASURED?
They way ISO is measured, as per the International Organization for Standardization itself, (ISO 12232:2006) is simply a specific level of brightness or exposure.
This brightness level is, visually, 18% grey. Does this mean that ISO 100, 200, 400, and others all correspond to fixed brightness levels, such as lumens or EV? Unfortunately, no. A given ISO does not correspond to a specific brightness until it is given the context of the rest of the exposure triangle.
In other words, every way that ISO is referenced on charts and graphs, it is simply used as a corresponding brightness based on your shutter speed and aperture. (Again, The Exposure Triangle)
For example, ISO 100, 1 second, and f/1.0 correspond to the fixed brightness “EV 0”. However, that same brightness level could also be achieved by changing the aperture and shutter speed together. Also, ISO 100 could result in the fixed brightness of “EV 0” if entirely different shutter speeds or apertures were used.
So, that’s the scientific reply to the question. What about the real world? Indeed, ISO 100 is the same brightness on all cameras. Whether you have a Canon, Sony, Nikon, Pentax, Fuji, Panasonic, etc. camera, ISO 100 should be ISO 100, in theory. (For the truth, scroll down to Actual ISO Versus Stated ISO.)
HOW DOES DIGITAL ISO WORK: SENSITIVITY VERSUS GAIN
The biggest mistake that many photographers make when explaining ISO is the use of the term “sensitivity”. As in, “raising your ISO raises the sensitivity of the sensor.” This is simply NOT TRUE.
When a digital camera’s ISO is raised from 100 to 200, the sensor is NOT actually becoming more sensitive to light. In order to understand why, we have to learn how a sensor works.
A digital image sensor is made up of individual pixels, and each individual pixel “collects” individual photons of light. After the image is fully exposed, the sensor’s electronics convert those photons into electrical signals. And whether you set your camera’s ISO to 100, or 800, or 3200, the number of photons that the sensor notices (collects) is still the same.
When you raise your ISO, the sensor is amplifying (increasing) the electronic signal that was created when the pixels collected/counted their photons, making the image appear brighter. To simplify, it is taking the limited photons that were captured, (in dark conditions, for example) and multiplying the numbers by 200, 400, 800, etc.
A digital sensor, in fact, has a fixed sensitivity, known as its signal-to-noise ratio. This is the sensor’s fixed ability to accurately count the number of light photons that are hitting the sensor’s pixels. This is known as the quantum efficiency of the sensor.
Again, this does not mean that shooting at higher ISOs is pointless, or necessarily a bad thing, for numerous reasons described above and below.
WHAT IS A NATIVE ISO OR A BASE ISO?
The term “native ISO” commonly refers to the range of ISOs that are not listed on the camera as either “Hi-1” or “Lo-1”. (Not to be confused with “whole stop” ISOs, which refer to all ISO’s in 1 EV increments often starting from ISO 100 or 200.)
Any ISO that is listed as “Hi-1” or “Hi-2” etc. is indicated in this manner because it is deemed unreliable, both in overall image quality, (extremely high noise, loss of color and sharp detail) and also in actual brightness value. This is because a sensor’s amplification capability becomes unreliable at a certain point, and cannot achieve a precise “stop” that qualifies as a specific ISO. The camera specifies “ISO Hi-1”, to note that you will no longer be getting an accurate exposure. (Nor will you get very good image quality!)
Geeky Fact: Contrary to popular belief, “Hi” ISO’s do not always correspond with the exact point at which a digital sensor switches from performing its gain amplification natively, (during the analog, photons-to-electrons conversion) to “fake” digital post-production brightening. For example, a camera may switch from analog gain to digital gain for ISO’s higher than 3200, yet still list ISO 6400 and ISO 12800 as if they were native ISOs, before going to “HI-1” to denote a sub-standard ISO 25600 image, simply because the camera is delivering good enough image quality at those ISOs.
WHAT IS DUAL-GAIN ISO?
Dual gain ISO is a technology used in some of the newest sensors like that of the Sony A7Riii and Nikon D850, to change the amplification of a sensor’s image data at an earlier stage in the analog-to-digital conversion of photons to electrons. (ADC) It is still not an actual change in the sensitivity of the sensor, but merely a “cleaner” way to amplify the image data, which results in a slight boost in overall image quality when jumping from the next-lowest ISO.
On most cameras which currently offer dual gain, this happens in the vicinity of ISO 400 or 800. In addition to a notable drop in ISO noise, an increase in dynamic range is often noticeable.
WHAT IS ISO INVARIANCE?
Okay, now that we’ve tackled the myth about sensitivity versus gain, let’s talk about an even more controversial subject, ISO invariance! What does this term mean? Simply, ISO invariance refers to a camera’s ability to shoot an under-exposed image at a lower ISO, and then recover that exposure in post-production with (hopefully) minimal loss of image quality.
Say, for example, you capture one image at ISO 100 that is two stops under-exposed and one image at ISO 400 that is correctly exposed. Will the under-exposed ISO 100 image look the same as the ISO 400 image, when its exposure is increased by 2 stops in post-production? If yes, the camera sensor is ISO invariant up to ISO 400.
See in the example below, using a Nikon D750, a demonstration of how the highlight noise levels remain similar until ISO 3200, and the shadow noise levels remain pretty similar until ISO 800.
Thanks to advances in modern image sensor technology many cameras are actually ISO invariant up to ISO 400, 800, or even 1600 or higher. However, remember that dynamic range and color saturation/accuracy are all significantly affected by increasing your ISO. So although noise levels in mid-tones or highlights may remain invariant when under-exposed at lower ISO’s, it is almost always still better to use the right ISO for a “correct” exposure.
ACTUAL ISO VERSUS STATED ISO
A question that many photographers have is, are ISO’s always exactly the same actual brightness from camera to camera?
In theory, any given ISO number should produce an exposure brightness that is exactly the same on all camera sensors as well as all films. For example, any camera that uses ISO 100, 1/100, and f/16 should correctly expose a sunlit landscape exactly the same way.
Unfortunately, however, not all camera sensors achieve such precision; there is room for error in the actual brightness of a digital sensor’s ISO. In other words, what is listed as ISO 400 on a certain camera may actually appear as bright as ISO 500.
Thankfully, generally speaking, these discrepancies are now rarely any greater than 1/6 or 1/3 of a stop.
There are many misconceptions about how ISO works, and negative ideas about its creative use. However, if you have gotten to this point then you have a mastery of both technical and creative understanding, which can improve your photography both in quality and artistry.
So, get out and take photos, practice, and experiment with new techniques, then share your photos with our community! SLR Lounge’s Critique Page and Facebook Group are both excellent places to share the latest creative imagery you’ve made.
Written by Pye Jirsa and Matthew Saville