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:

(Click to enlarge!)

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.

If you haven’t already, be sure to check out our official guides to Aperture and Shutter Speed, which are of course the other two parts of the Exposure Triangle.

Let’s dive into the first part, which is the basic use of ISO to control your image exposure.

For those super-geeks who are interested in “getting the facts straight” when it comes to how ISO actually works, we will include additional technical details and insights towards the end of this article. We will explain it in a more simplified way first, however, because that is simply the best way to grasp the concept of ISO in practical, real-world terms.


On a digital camera, changing the ISO does not actually change the sensitivity of the sensor, it simply boosts an image’s 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.

Canon 5Dmk3, Canon 16-35mm f/2.8 L,
ISO 3200, f/2.8, 13 sec

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.

Camera used: Nikon D750

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.

Canon 5DmkIV, Canon 24-70mm f/2.8 L mk2
ISO 3200, f/2.8, 1/160 sec.

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!

Check Out Photography 101 to learn about the Exposure Triangle

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 the 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!


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:

  1. When detail and dynamic range are critical (low ISO)
  2. When you intentionally wish to imitate a grainy film (medium/high ISO)
  3. To control motion blur with faster shutter speeds (medium/high ISO)


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” on the camera. For many types of photography, this offers a creative advantage: The ability to see better detail in both highlights and shadows, in a single exposure.

This is known as “good dynamic range”, and is the film/sensor’s ability to capture detail in a bright highlight and a dark shadow at the same time. This can be a fantastic creative tool for landscape photographers and many others who wish to capture 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


As ISO increases, so does image noise. However, since most modern digital cameras still maintain a high level of sharp detail and vibrant colors at medium-high ISOs, image noise can be seen as “digital grain” for a creative look.

Nikon D750, Nikon 70-200mm f/2.8 VR
ISO 400, f/2.8, 1/320 sec
SLR Lounge Preset System – Kodak Portra 800, Medium Film Grain

So, 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 if it is added 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. Obviously this “look” isn’t for everybody, however, it’s still a valid and unique way to get creative with photography.

Canon 5D mk3, Sigma 24mm f/1.4 Art
ISO 800, f/1.4, 1/200 sec, SLR Lounge Preset: Kodak Portra 800 – Muted


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!

Nikon D300, Nikon 24mm f/2.8 AI-S
ISO 400, f/5.6, 1/3000 sec

Oppositely, if a landscape photographer needs to shoot at an aperture of f/8 or f/11 for good 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.

(Scroll further below for an explanation of “LO” and “HI” ISO numbers)

Canon 5DmkIV, Canon 24-70mm f/2.8 mk2
1/20 sec @ f/14 & ISO 100

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 “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 recent 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.



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 200 or ISO 400 could still result in the total brightness of “EV 0” if entirely different shutter speeds and apertures were used to once again maintain the exposure triangle.

So, that’s the scientific reply to the question. What about the real world? Indeed, ISO 100 is supposed to be 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.

This is only in theory, however. For the truth, scroll down to Actual ISO Versus Stated ISO.


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. That was indeed what would happen on film, if you switched from an ISO 100 film to an ISO 400 film, however, digital cameras just don’t work that way. 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” photons of light. After the image is fully exposed, the sensor’s electronics convert those photons into electrical signals. The number of photons that the sensor collected would still the same, whether you set your camera’s ISO to 100, or 800, or 3200.

When you raise your ISO, the sensor is merely amplifying (increasing) the electronic signal that was measured when the pixels collected their photons. In other words, the sensor is taking the limited number of photons that were captured, (in dark conditions, for example) and multiplying that number whenever you set the camera to a higher ISO.

Simply put, a digital sensor has a fixed sensitivity, known as its signal-to-noise ratio. This is the sensor’s inherent ability to accurately count the number of photons that hit the sensor. This is known as the quantum efficiency of the sensor.

This does not mean that shooting at higher ISOs is pointless, however, or necessarily a bad thing, for numerous reasons described above and below. (See Native ISO and ISO Invariance, below.)


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 at a certain point the sensor’s amplification capability becomes unreliable, and cannot achieve a precise “whole stop”. The camera specifies “ISO Hi-1”, to note that you will no longer be getting an accurate exposure. Generally speaking, it also is an indication that you will you get very good image quality!

Nikon D750

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. This is simply because the camera is delivering good enough image quality at those native (in name only) ISOs.

Canon 5DmkIV, Canon 24mm f/1.4 L mk2
ISO 50 (LO-1), f/11, 1/200 sec


Dual gain ISO is a sensor technology used in some of the newest cameras like the Sony A7Riii and Nikon D850. It changes 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.

In other words, during the creation of a raw (or JPG) image file, there are two main points at which an image’s brightness data can be boosted: First, at the inital moment when photons are converted into electrons, and/or second, after the image has been fully converted to electronic data, and is being finalized as a RAW/JPG file. If a camera has the capability for dual-gain ISO, this means it has two different points on the total ISO scale at which the photons-to-electrons conversion can be magnified with as clean image quality as possible.

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 another potential benefit.


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 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 mostly two stops under-exposed, yet preserves one very bright highlight, and another image at ISO 400 that is mostly correctly exposed, except the one bright highlight is clipped. Will the ISO 100 image look the same as the ISO 400 image when its exposure is increased by 2 stops in post-production? If yes, that 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.

Nikon D750

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, especially if you are not clipping any highlights at that higher ISO, and/or if you have any extremely dark shadows in which you would rather not see too much noise when boosting them in post-production.


A question that many photographers have is, “are ISO’s always exactly the same brightness, from camera to camera?”

In theory, any given ISO number should produce an exposure brightness that is identical on all camera sensors and films. For example, any camera that uses ISO 100, 1/100, and f/16 should expose a sunlit landscape exactly the same way.

Unfortunately, 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 as per official ISO measurements.

Thankfully, generally speaking, these discrepancies are now rarely any greater than 1/6 or 1/3 of a stop, or 2/3 EV at worst.

However, it is important to know that not all camera sensors are created equally. If a sensor claims that an ISO is 1600, when in reality it is actually more like ISO 1000, this will do two things: First, you will have to use a slower shutter speed to achieve the same exposure. Second, if you don’t notice this need for a slower shutter speed, you may be led to believe that your camera is actually much better in low light than it really is.

This is why it is always important to pay attention to the other settings in the exposure triangle whenever you are comparing two sensors’ ISO performance; one camera may be 1/3 or even 2/3 EV “worse” than it is claiming, if the same shutter speed and aperture are used.

Canon 5D mk3, Sigma 24mm f/1.4 Art
ISO 200, f/1.4, 1/500 sec


There are many misconceptions about how ISO works, and arguments about its  technical or creative uses. However, if you’ve gotten to this point then you have a mastery of both technical and creative understanding! This can improve your photography in many ways.

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