What is ISO and how do I use it for my Photography?
By: Frithjof Moritzen
Tags: Post Processing, Photography, Exposure Triangle, Iso, Exposure

BASIC PHOTOGRAPHY SERIES: EXPOSURE TRIANGLE PART III

What is ISO and how do I use it for my Photography? Frithjof Moritzen Follow Nov 23 · 10 min read

Make some noise!

Photo by Markus Spiske on Unsplash

If you are as old as I am and grew up in Germany (as I did), then you would come across something called DIN, which film manufacturer used to describe how light sensitive their films were. Other countries used ASA (American Standards Association) and other, now extinct, measurements for light sensitivity of films.

ISO is the common successor, and, who would have guessed that, is a single world-wide used standard for describing … well, what exactly? That’s what I am going to explore with you in this article. You will learn about

what does ISO stand for

what is ISO measuring?

what is the native ISO setting?

how do I use ISO in my photography?

where do I find the ISO settings

what is auto-ISO

find out how far you can go with high-ISO settings on your camera

ISO as part of the exposure triangle

That is a lot of stuff to cover, let’s dig right in! As this is an article for mostly beginners in photography, I will not cover the more advanced scenarios where your camera ISO settings play a role, e.g. in flash photography. Also note that for the sake of simplicity I’ll cover ISO used for digital cameras, not for film cameras. They are very similar, but not the same.

What does ISO stand for?

ISO is short for “International Organization for Standards”. That’s odd. Should be IOS, but for convenience you could see ISO as ‘International Standards Organisation). That fits better, so we stick with that.

Knowing what it stands for does not give us any sign about what that setting does, as the International Organization for Standards is publishing a LOT of other standards not even closely related to photography. So why did the camera manufacturers use ‘ISO’ as the term to describe this camera setting? It is not completely unrelated to what the ISO setting does.

ISO gives the camera manufactures a standardized way of measuring how sensitive a camera sensor is to the incoming light. That’s the simple way to describe what that ISO setting means for a camera. It is important that the camera manufacturers all use the same way to measure that, to make sure that a camera with the same settings of aperture, shutter speed and ISO setting, using the same focal length lens, sensor size and illuminated scene all produce the same ‘output’, e.g. a jpg image with the same brightness. Independent of camera models.

So what does an ISO setting of 100 or 200 mean? I think it is best to think about these numbers as a relative measurement of how light sensitive a sensor is set to. A setting of 200 means the sensor is now twice as sensitive to light as setting 100. It is a linear scale. 400 is double as sensitive as 200. 50 is half as sensitive as 100.

Lowest and highest ISO settings

Most cameras have the lowest ISO setting of 50, or 100. On the other end of the scale it can be a lot more diverse from one camera model to the next, and that has something to do with the side effects high ISO settings have. The side effect (spoiler alert: image noise) can make it impractical to use with very high settings, as the image quality is not usable anymore. Expensive cameras with a larger sensor might provide high ISO settings without degrading the image quality too much. Common highest ISO settings are 25600, but there are cameras which can be set to e.g. 3280000. Not sure how usable that is, my camera is not very good at high ISO settings in a low light setting, so I rarely use ISO 3200 or higher as that gets too noisy for my taste. But you have to test that with your camera. It also depends on how good you are with post-processing (getting rid of noise), and how tolerant you are about seeing noise in an image. Some people don’t bother, some are very sensitive (pun intended) to the higher sensitivity settings.

Native ISO setting

Each camera has a native ISO setting. Most often that is the lowest ISO setting, but not always. The native ISO setting is the setting where the voltage to the sensor is not increased to boost or reduce the sensitivity level, and that setting is the one with the least amount of image noise produced. Most cameras have a native ISO 100 setting.

How do I use ISO in my photography?

That is probably the only important thing to know. Knowing how the technology works might tickle your brain, but if you can’t translate that knowledge into your photography, then it does not help you much.

If we leave out all the technical details, we are interested in ISO settings because it offers another way to increase or decrease the brightness of the exposure. Besides the aperture (controls the brightness by increasing or decreasing the size of the hole in your lens) and the shutter speed (controls the brightness by setting how long the light from that hole can expose the sensor), the ISO setting gives us a third method to control the exposure. The benefit of having a third method is that you have another tool to avoid the pitfalls or side effects of the other two methods. An example:

Your image might be correctly exposed with an aperture of f/2.8 and 1/15s shutter speed, ISO at 100 . You decide that 1/15s is too slow for your subject, maybe it is something that moves and you want it frozen in your frame. So you want a faster shutter speed. What do you do? Well, you can let more light in by opening up the aperture, but wait, f/2.8 is already the maximum your lens can do! ISO to the rescue, you just crank up the ISO setting, and your shutter speed drops to a faster setting (if you are in aperture priority mode, having set f/2.8). Basically, you have now three tools to juggle in the air to keep your exposure correct, but each of these three balls has their own side effect. Changing Aperture changes the depth of field. Changing shutter speed changes the capture of movement. And changing ISO? Changes the amount of digital noise in your image. Juggling these three parameters of exposure is discussed in the exposure triangle. There will be a separate article to put a spotlight on that, as it might take a little longer to give it justice. For now it is enough to know that with the ISO setting you have a third tool to increase the brightness of your resulting image, but at the cost of having noisier images. There is always a trade-off in photography. If you are a person who finds it difficult to decide between options, then photography is the ideal training ground.

Where do I find the ISO settings?

Each camera manufacturer has its own way to place the controls for ISO. Some have a dedicated knob for that, but as real estate on the tiny camera bodies is precious, most cameras will have that as a setting you control by pressing a button, and then turning a general multi-function knob to increase or decrease the setting. Some have touch screens to set it, some have programmable function buttons which can be set to control ISO. This is just something you have to learn how it works with your own camera. Sorry, can’t help with that.

What is auto-ISO?

Auto-ISO is something your camera might have, or not. It means that it allows the camera to choose an ISO setting it believes is best for a specific scenario. They might hide it in some kind of program mode, where the camera chooses some settings for you. It might also be something you can select actively, e.g. you can tell the camera to have the freedom to select between ISO 100–800 as it finds fit, but not above that (to protect you from getting too noisy images). I tend to not use auto ISO, but try to keep ISO as low as possible, and juggle only between aperture and shutter speed. That is enough complexity for my simple brain. I most often increase ISO to get faster shutter speeds when I take images of something fast-moving (like birds), and the light is not enough to get the fast shutter speed I need. I get noisier images, and as a punishment I have to deal with that in post-processing. That leads to the next point.

Find out how far you can go with high-ISO settings on your camera

Before you select a higher ISO setting to increase the brightness, it would be good to know if the result is something you would consider as acceptable. Every camera is different with how well they can handle noise with higher ISO settings, so just test it. Put your camera on a tripod (or plant it on something stable to remove camera movement). Set your camera to aperture priority setting and set it to f/8. Does not matter too much, just leave it constant. The camera will then select a suitable shutter speed, but as the camera is on a tripod, you get sharp images regardless of shutter speed, and the depth of field is constant. Now start with the lowest ISO setting and take an image. Increase the ISO setting and take images until you reached the highest setting. Make sure you get a properly exposed image each time. If you started in a too bright environment, your camera might not select a short enough shutter speed to compensate, in that case just go somewhere dimly lit and repeat.

Back at your computer, load your raw files (I forgot to mention, set your camera to capture raw files) into your favorite image processor, and inspect the images at 100% magnification, to make sure that one pixel on the screen relates to one pixel of your sensor. When do you see noticeable image noise? ISO 400? 800? 1600? Even better? Just make a mental or written note about that. Now use your image processor to reduce the noise to something you find acceptable. Note down the ISO setting where you can still use post-processing noise reduction to get an acceptable result. These two ISO settings are now your guideline in what maximum settings you should normally not exceed. Let’s have a look at an example. These two are at the opposite sides of the range I can shoot with my camera, ISO 100 and ISO 25600: