What does ISO mean in Photography?
One of the questions I get asked a lot by beginners is, what does ISO mean in photography, and how does it affect my images? In this article I will try and explain how to use and better understand ISO on your digital camera.
ISO – a Definition
ISO stands for the International Organisation for Standardization — an organisation that sets international standards for all different kinds of measurements. But, when in reference to your camera, the ISO is your camera’s sensitivity to light. ISO is displayed in a number like this: 100, 200, or 400.
Are you Still Using the Command Dial Auto Modes?
Do you use your camera in manual mode or do you still use the fully automatic and semi-automatic modes such as AV and TV on your DSLR?
Before I launch into ISO, you should really understand the relationship between these three elements?
Aperture
This basically helps you to control the depth of field or plane of focus.
Shutter speed
This has a massive effect on the way moving subjects are captured by your camera; to low and subjects will blur.
ISO
When using film, the film you place in the camera will come with its own film speed. When you load the film into the camera, you should select the corresponding ISO setting. So when the film you are using is 100 ISO, you should also change the ISO setting on your camera to 100.
What does ISO mean in Photography?
Manipulating ISO in a digital camera
Without a doubt digital photography has forever changed the relationship between the camera and ISO.
Unlike film where the camera’s and film where ISO ratings and the chosen film have to match, using a digital camera means that you can select a different ISO (film speed) for each individual shot.
Being able to change the film speed on your camera lets you match the sensitivity of your film or sensor to the amount of readily available light. If you think about it, ISO is an amazing thing. It enables you to take photos in any situation, from bright sunlight to candle light.
For this handheld image of the inside of the Wigwam in Liverpool I used ISO 1250 f4 1/125 @17mm. There is a lot of noise, although not as much as I would have expected.

ISO 1250 f4 1/125 @17mm
It’s thanks to ISO that your digital camera is so versatile.
So let’s take a closer look at what does ISO stand for in photography, and what the choice of ISO setting could possibly mean for your photos.
What is ISO?
ISO is a measure of the sensitivity of your camera’s sensor to the available light. The lowest ISO setting of most digital cameras varies from 50, through to 200. My Canon 7D Mkii offers me 100 ISO as its lowest setting up to some ridiculous number like 64,000! I would love to be able to access ISO 50, but 100 works fine. If I need to operate in very low light I can whip out the tripod and alter other settings.
Once you start accessing higher ISO settings, and I mean anywhere over ISO 800, the sensor is more sensitive to light. However, you must understand that at these higher settings the sensor will be ‘hunting’ for light and noise will start to be introduced into your images. This is evident in my image of a Pine Marten which was taken at very high ISO, (6400.) Lightroom actually did a reasonable job of removing some of the noise, but I would rather it wasn’t there in the first place.

ISO 6400 f.5.6 1/800 sec
Note: this is a very basic explanation.
The idea for having standards is that different camera and lens combinations all produce the same results at the same aperture, shutter speed, and ISO settings.
In other words, the Pine Marten photo taken at ISO 6400, f/5.6 and 1/800 sec, should look the same as an image taken at the same settings on your camera, whatever that may be.
Having a universal standard is important when photographers use light meters. For example, if a studio photographer sets up the lights and uses a flash meter to work out that the required exposure is, say, f/11 at ISO 100, then it’s important to know that these settings work for any camera.
In practice, there are minor variations between different cameras but for the most part are nothing to worry about.
Practical Use of ISO
What does ISO mean in Photography, and how does it relate to the exposure triangle. The diagram below looks at the relationship between shutter speed, aperture and ISO in you camera. If you are anything like me, just getting out and playing with the settings helps me to understand the principles way faster than any diagram.
ISO works with shutter speed and aperture in order to give you the correct exposure for the available light level. I wrote about aperture in a previous article.
The advantage of a digital camera is that you can use it in just about any lighting situation without the need for flash or tripod, (within reason.) When you are struggling for light, you have the option of raising the ISO, in addition to using a larger aperture or a longer shutter speed, to help you obtain a good exposure.
In this image of a juvenile Kingfisher the light was fairly poor under the trees. Despite using the widest aperture available on my lens, (f5.6), I was forced to up my ISO to 400 which gave me a reasonable speed for a mostly static subject (1/250.) This helped me to avoid image blur.
However, as I mentioned previously, you need to be aware that raising the ISO can increase the amount of noise in your photos, especially in the darkest tones. This is not so much of a problem as modern sensors are very capable at high ISO settings. It is one of the reasons I chose the Canon 7D Mkii.
Avoiding Auto ISO
If you use Program, Aperture Priority, Shutter Priority, or Manual exposure modes on your camera you can set the ISO yourself rather than let the camera decide. In turn, it makes you think about the relationship between ISO and the other settings and the impact on your final image quality. However, you really need to understand what does ISO mean for your photography in the field.
This is more relevant in low light. For example, let’s say you find yourself in a situation where you are hand-holding the camera (therefore you cannot use a slower shutter speed) and need to either raise the ISO or open the aperture to obtain the correct exposure.
If you open the aperture, you’ll have less depth of field. If you raise the ISO, you’ll have more noise. You have to make a choice. What’s more important, noise or depth of field? You have control, not the camera.
Full-Frame vs Crop Sensor
It used to be that digital cameras with full-frame sensors created images with less noise at any given ISO setting than crop sensor cameras (that is, the sensors in APS-C and Micro Four-thirds cameras.)
However, as ISO performance has increased, the performance gap between full-frame and crop sensor cameras has decreased. Image quality (noise) is no longer the major consideration it once was. I can quite happily shoot at 1200 ISO without noise becoming an issue.
ISO and Image Quality
The best quality images (i.e. those with the least noise and the most information) are always taken at the lowest possible ISO setting. In bright sunlight there is no issue using ISO 100 and being handheld, but it becomes increasingly more difficult as the light conditions degrade. I shoot weddings professionally, and some church interiors were very dark and gloomy. A lot of these institutions didn’t allow the use of flash so there was always a balance to be found between light and quality of image.
In those situations I sometimes am able to use a use 100 ISO together with a tripod, but only if I can to pause the bride and groom mid procession; this isn’t always appreciated or possible.
Using a faster lens is an option. Having access to f1.4 or f2.8 makes a difference, but sometimes upping the ISO is the only solution.
Gloucester Cathedral looks well lit in the image below, but in reality it was very gloomy and forced me to up my ISO so I could use f.8 for the image.

ISO 500 f.8 1/30sec
Using a tripod allows you to use low ISO and longer shutter speeds without having to worry about camera shake. For that reason, low ISOs are ideal for landscape, architectural and product photography, where it’s normal for photographers to use tripods.
Do not be afraid to Use the Higher ISO Numbers
There is no reason to be afraid of using a high ISO on your camera. If you are nervous, then do some testing with your camera set at the higher ISO settings (800, 1600, 3200, 6400, 12800, etc.)
Make sure you shoot in RAW format as this captures maximum information. Import your images into Lightroom and see how much noise you actually get in your images and how far you can push your camera before noise becomes insurmountable. It is amazing how much noise can be reduced post shoot.
Subjects such as natural light portraits, or photos taken indoors, often require high ISO settings, especially if they are done when the ambient light levels are low, such as dusk. The high ISO settings of modern cameras are a great benefit in low light conditions because they let you experiment with taking photos hand-held that years ago you could only have attempted with a tripod (and slow shutter speeds) or by using a flash to light the scene.
I used ISO 100 1/250 f5.6 @100mm for this photo at the ruins of Haughmond Abbey. It was a bright sunny day so there were no problems using ISO 100.
However, this image of the dam at Lake Vyrnwy was taken in the shade of the dam so the light wasn’t as good. I was forced to up my ISO to 400 1/125 f.11 @35mm.
In contrast this image taken a few minutes later had far better light so I was able to adjust my settings accordingly. ISO 100 1/125 f.13 @50mm.
A Conclusion
As camera technology advances it means that many photographers can get great results from their cameras at ISO settings up to 6400 and beyond. It’s definitely changed the way I work creatively in low light conditions. However, it is very important to remember that using a higher ISO setting will sacrifice image quality. I wrote about aperture priority in a previous article which you can find here.
If you are unsure or want to find out how to use your camera in something other than full auto then my ‘Get Off Auto‘ workshop might be just the ticket for you.
As a teacher I love questions? Please leave yours, together with any comments below!
Trackbacks/Pingbacks