Digital Photography: ISO Explained

It seems everyone has a digital camera these days. Unfortunately for most people it's a cellphone. Believe it or not people still use "real" cameras, and you may be one of them. There are a lots of reasons why you want to take pictures with one, but the number one reason is that you can get out of auto and take control of the settings for each picture.

This all begins with the exposure triangle (shutter speed, aperture, and ISO). From previous articles I've written you know that shutter speed controls the length of time the image sensor is exposed to light, and aperture (f/number) controls the amount of light that enters the camera. Aperture also controls your depth of field while ISO is the sensitivity of the image sensor to light. A change in one requires a change in at least one other to maintain proper exposure.

On its face ISO is easy to understand - The higher the number the more sensitive your camera is to light. But it’s not that simple, and increasing your camera's ISO comes at a cost. What is ISO and why do you need to know about it?  Let’s find out.

What Is ISO?

International Standards Organization (ISO) is the standard measure of light sensitivity of your camera’s imaging sensor whether that’s film or a digital sensor.  It is a standard designation that all camera manufacturers agree to.  Film may also have a sensitivity rating called ASA, but for the scope of this article I will only talk about ISO.

ISO is also known as “film speed”.  When we look back to antiquity known as “The Film Days” you may recall having the choice of 100, 200, 400, 800, etc. ISO ratings when purchasing a roll of film.  Today, digital cameras use a similar designation, and can go as high 25,000 ISO or more.

A film's ISO rating is a reference to the light sensitivity of the film emulsion. It’s speed represents the amount of time the film needs to be exposed to light to get a proper image, which is controlled by the length of time the shutter is left open. Aperture plays a role here too with the amount of light the iris lets in.  

The higher the ISO number the more sensitive the film is to light. That is, 800 ISO is more sensitive than 400 ISO by one full stop of light.

Film sensitivity is a simpler design when compared with a digital sensor’s light sensitivity.  Higher speed films were more sensitive to light because the silver crystals in the film emulsion were larger allowing them to capture light faster when compared to smaller crystals. The tradeoff however, was that the larger crystals were visible and made the image look grainy. 

Digital image sensors have a similar problem. As the pixels become more sensitive to light they too become "grainy". Here is where we make the comparison between the crystals in the film emulsion and the digital imaging pixels. Larger emulsion crystals increases the grain. Whereas, a stronger electrical signal increases the electrical noise (grain).  

The Digital Photo Sensor

A digital photo sensor is made up of millions of smaller sensors called pixels. Each one of these pixels independently gather light in the red, blue, and green spectrum. When a pixel is active and highly sensitive (6400 ISO) with no light coming into the camera (your lens cap is on), there will be a small amount of color sprinkled throughout the background of the black image. This is electrical noise (grain) from the increased electrical signal that is stamped into your image when snapping the photograph.

Below are two images with the lens cap on. To the right is 100 ISO and to the left is 25,000 ISO.  Click the image to enlarge.  You may need to dim the lights in your room, but the electrical noise is noticeable. 

The below diagram represents the image quality of one properly exposed pixel at 200 ISO on a digital sensor. It demonstrates a representative amount of red, blue, and green light (shown as red).  The electrical noise is represented as blue. 

Notice how much of the pixel is in the "red" with a small amount of electrical noise in "blue". This would create a tack sharp image.

Now let's consider what would happen if you took a picture in low light with the camera set to 200 ISO. When you increase the ISO from 200 to 3200 the pixel increases the light sensitivity by four times, but it also increases the amount of digital noise by that same amount. It does this by increasing the electrical signal to boost the sensor's light sensitivity.

The increased ‘noise’ (electrical signal represented in blue) is the digital equivalent of the larger crystals in the film emulsion. This is your grain.

Benefits Of Digital Camera ISO

One benefit of using a digital camera when compared to film is that the digital camera can adjust ISO on the fly to meet the lighting demands of your current environment.  This occurs because the output of the photosensor is able to be electronically amplified on demand.  Whereas a 36 exposure roll of 800 ISO film had you locked in at 800 ISO for the entire 36 exposures.  

Amplifying the sensitivity of the image sensor does not change it's native light sensitivity. When you change the ISO from 100 to 1600 what you are actually doing is electronically amplifying the output signal making it more sensitive to a given amount of light. 

Here’s the key to this whole thing. When the photosensor ISO increases for low light the electrical signal increases and you lose image quality. Is this a bad thing?  Not necessarily. Grain can add an artistic effect, especially with black and white photos.

Notice how Uncle Fuggles is tack sharp at 400 ISO and how the image degrades at 25,000 ISO. Both photos were taken at 50mm with my Nikon D7100 and Sigma 17-50mm f/2.8 lens.

400 ISO                                                                                    25000 ISO

Why Do You Need To Increase The ISO?

The primary reason you need to up the ISO is to increase the photo sensor's sensitivity to light to keep the shutter speed fast enough to avoid motion blur when hand holding or during fast action. If your subject isn't moving you can turn down the ISO and mount your camera on a tripod to shoot a longer exposure.

One thing to note is that you want to lower the ISO when shooting flash photography, but flash will often alter the artistic impression of the image you are trying to create.  

When Do You Adjust The ISO?

Well, it depends. It depends on what you are trying to do with the image you are trying to create. Grab your camera and work through the examples below to help you better understand what the settings do.

Turn up the ISO when…

1. You are in low light situations without a flash.

Example: 1/100 shutter speed; f/2.8 aperture; 6400 ISO

2. You need a fast shutter to freeze fast action.  This can also occur in low light.

Example: 1/500 shutter; f/5.6 aperture; 3200 ISO

3. You want to shoot a landscape while hand holding the camera with the majority of the image in focus without any bokeh.

Example: 1/125 shutter speed; f/22 aperture; 1600 ISO

4. You want to create an artistic black and white photo

Example: 1/500 shutter speed; f/1.8 aperture; 6400 ISO

5. You are using a variable aperture lens.  At 55mm your aperture is f4.5 whereas at 200mm you are shooting at f5.6.  You need to adjust the ISO for the number of stops of light you are reducing your aperture by as you zoom. There is also the rule of thumb that as you zoom your shutter speed should not drop below the length of your lens in millimeters.

Example 1: 1/125 shutter speed; f/4 aperture at 55mm; 1600 ISO

Example 2: 1/250 shutter speed; f/5.6 aperture at 200mm; 6400 ISO

Turn down the ISO when…

1. You are trying to recreate an image just as you see it without any grain whatsoever.

Example: 1/125 shutter speed; f/4 aperture; 100 ISO (bright light or with flash)

2. You are outside on a bright day

Example: 1/250 shutter speed; f/8 aperture; 100 ISO

3. You use a flash (direct or indirect).

Example: 1/200 shutter speed; f/5.6 aperture; 200 ISO

4. You change from a ‘slow’ lens to a ‘fast’ lens.  That is, when you use a larger aperture lens to let in more light.

Example: 1/125 shutter speed; f/1.4 aperture; ISO 100

Late model cameras can handle ISO’s as high as 3200 ISO with little grain. Each sensor generation is better than the last, and many of the new models can handle low light well, especially when you use quality glass.

One last note regarding noise, and photography with high ISO.  It can be argued that 'some' noise is not only an acceptable part of imaging, but can lend character that can add to an image's artistic qualities rather than detract.  

Consider old World War II era photographs.  If they were tack sharp like today's digital images would they possess as much nostalgia if they didn't have that grainy texture?  That sepia tone?  That point is of course debatable.  What you must consider when capturing an image is whether you are creating a photo documentation, or are you looking at photography as an art form?  The choice is yours and ISO will help get you there.