Photography Advice

Demystifying Digital Camera Sensors: The Journey of Light to Pixel

Rethinking Photography in the Mirrorless Digital Age - Part 2

Digital camera sensors do not record light the same way that film does. Everyone acknowledges that; however, as an industry, we can’t get everyone on the same page about how to talk about how digital camera sensors work and how exposure is managed with them.

Much like politics, we may never get people to agree on one definition. Still, regardless of which side of the photography aisle you are on – there is only one truth because the truth is based on physics, electronics, and engineering.

If we find an easier way to discuss it, we will make it easier for more people to understand. Let’s set the record straight.

Shutter Speed + Aperture + ISO = Exposure

That equation is how exposure has been considered since the beginning of photography in the early 1800s. But that equation has been inaccurate for digital photographers for over 20 years. 

Many argue that ISO has never been a part of exposure, even for film. I definitely believe that ISO is not and never has been an “exposure setting,” but it certainly impacts your other exposure settings.

📸 Factoid
Exposure is the amount of light per unit area reaching a frame of photographic film or the surface of an electronic image sensor. An “exposure” is a single shutter cycle.

Every roll of film has a set ISO or sensitivity based on its chemical makeup and the chemical reactions that occur when the film is developed.

Yes, you have to “set” the ISO on your film camera, but that is just to inform the camera’s light meter what speed film you have loaded. It is not a setting that you choose on a frame-by-frame basis.

Fast forward to our digital cameras, and while we still have an ISO setting, the fact is that ISO is applied after the exposure has been made.

The last sentence may have caught you a little off-guard. Let’s review a few basics, and then I will explain in detail why ISO is not an exposure setting in a digital camera.


Shutter Speed

Shutter Speed is the camera setting that controls how long light is exposed to the sensor. Longer shutter speeds (seconds or even minutes) let in more light, useful for low-light situations like night photography. Conversely, shorter speeds (fractions of a second) let in less light, which is ideal for freezing fast-moving subjects or capturing bright scenes without overexposure.

Shutter speed is a “purpose-driven” setting because it significantly impacts how movement is depicted in your image. 

A diagram showing the different shutter speeds and how the render action in a photo.

If you photograph a moving object, you will use a faster speed (fractions of a second) to “freeze” the motion.

Shutter speed is also useful when hand-holding a camera to ensure that you will not introduce camera motion because of shaky hands due to heavy gear. This type of motion blur is often confused for an image that is soft (slightly out of focus – but not intentionally)

There are creative options where you would choose slower shutter speeds, such as intentional motion blur (ICM), which can convey a sense of movement or add a dreamy, ethereal quality to the image. 


Aperture

Aperture refers to the size of the opening within the lens through which light passes to reach the sensor. A wider aperture (lower f-number) allows more light, making it ideal for low-light situations. Conversely, a smaller aperture (higher f-number) lets in less light, which is useful for bright scenes or achieving more depth of field.

Aperture is an emotional setting that controls the overall feeling of your image. 

A diagram illustrating the various types of lenses used with digital camera sensors.

Depth of field is the range of focus in your image. The distance in front of your focal point to the distance beyond your focal point that appears to be in focus.

Wide apertures create a shallow depth of field, blurring the background behind your subject and making it stand out. 

Narrow apertures produce a deep depth of field, keeping both foreground and background sharp.

Smaller apertures with their extreme depth of field help ensure that everything in the scene stays in focus. Wider apertures can be used creatively to draw attention to your subject. Depending on your subject matter, both options dramatically impact the feeling or mood of the photo.


ISO

It has long been taught that for digital cameras, ISO refers to the sensor’s sensitivity to light. 🤦🏻‍♂️ 

As a holdover from the film days, we have been told that raising your camera’s ISO increases the camera’s sensitivity to light. Nothing could be further from the truth.

ISO is a setting that makes your photo lighter or darker in a digital camera.

A photo of a woman displaying various facial expressions captured by digital camera sensors.

The Fallacy of ISO and Digital Camera Sensors

A digital camera sensor is made up of pixels that contain photodiodes and capacitors.

Together, these two components form the core light-collecting and signal-generating unit of each pixel in a digital camera sensor. They work in tandem to convert light into an electrical signal. 

The sensor interprets ISO by amplifying the electrical signal generated by the photodiodes within the sensor. 

This amplification increases the perceived sensitivity to light, allowing you to capture images in lower lighting conditions. 

To understand this, we need to know that the sensors in our digital cameras are analog. (You read that correctly – analog, not digital

Each pixel on a digital camera sensor is like a tiny solar cell, converting light into an electrical signal proportional to the light intensity.

At the base ISO setting, the signal is amplified to a certain level. Increasing the ISO setting further amplifies the signal, essentially “boosting” its strength and making the image brighter.

While some amplification happens at the hardware level, most modern cameras also apply software-based “gain” adjustments after capturing the raw data. This helps mitigate noise but also introduces its own subtle processing artifacts.

The camera’s processor then converts the amplified signal from analog to digital (A/D). The digital data is further processed by the camera’s firmware, correcting for color variations, white balance, and other factors.

Interpretation of ISO

The work done by the camera’s processor and firmware interprets what we historically knew as ISO or sensitivity. Please note the word “interprets”. The ISO or sensitivity is not changed; it is interpreted.

Higher ISO settings amplify the signal more, making the sensor appear more sensitive to light. This allows you to capture images in darker environments without needing longer exposure times or wider apertures.

Unfortunately, amplifying the signal also amplifies existing noise within the sensor itself. This manifests as unwanted grain or speckles in your images, particularly at high ISO settings.

Different sensor types vary in their inherent sensitivity and noise characteristics. Full-frame sensors tend to be better at low light performance than smaller sensors, allowing them to offer higher usable ISO ranges. This happens because full-frame sensors have more real estate to use bigger photodiodes.

Photographers who adopted digital cameras in the early 2000s probably still suffer from a form of digital noise PTSD because raising your ISO two to three stops above the base would render an image almost unusable.

Fast forward to 2023, and the camera sensors, firmware, and software are much better at interpreting the higher ISO sensitivities. We also have incredibly effective noise reduction software, which is being improved dramatically by AI.

How Did We Wind Up With Such a Misleading Setting?

In 1975, Steven Sasson, an engineer at Kodak, developed a portable, battery-operated, self-contained digital camera. It weighed 8 pounds (3.6 kg) and used a Fairchild CCD image sensor having only 100 × 100 pixels (0.01 megapixels). The black and white images were digitally recorded onto a cassette tape, a process that took 23 seconds per image.

In 1991, Kodak created the first-ever digital SLR. The Kodak Digital Camera System (DCS) was a modified Nikon F3 whose film chamber and winder were modified to make room for digital sensors. The camera had a built-in 1.3-megapixel Kodak CCD to capture images.

While all this development was occurring, engineers, likely with some input from marketing people, realized that to get photographers to adopt this new revolutionary technology, they would need to make the transition as smooth and straightforward as possible.

I realize it is hard to imagine photographers complaining or whining about anything (Think AI, Competition, Customers in general… 🫣 Wait, that is a whole different article).

The simple solution for Kodak and Nikon was to keep the same settings that photographers were comfortable with. For other manufacturers, it was a matter of why rock the boat; it just made sense to follow along.

ISO, even though it is a gain setting, was still referred to as adjusting the sensitivity of the camera’s sensor, and the big lie began. 

Ironically, even in 2024, Nikon’s website on one page refers to ISO as signal gain, and on another, states, “increase in ISO will make the camera’s sensor more sensitive to the lack of light.” Yes, even their marketing and education people don’t have a solid handle on it.

📸 Did you know?
Before the 1980s, photographers used ASA or DIN, not ISO, depending on where they were located in the world. ASA (American Standards Association) and DIN (Deutsche Industrie-Norm) were competing film speed rating systems. As an international organization, ISO created a unified standard incorporating both, which eliminated confusion and simplified communication across borders. The ISO system’s greater precision could also account for digital gain, which made it more adaptable to the evolving world of photography.

The Final Frame

The easiest way to understand ISO is this:  

ISO functions by amplifying the signal from the image sensor, thereby adjusting the brightness of the image.

or…

Raising the ISO makes the image brighter, and lowering the ISO makes it darker.

We are more than two decades into the digital era and already in the second generation of digital cameras (mirrorless), and the way we talk about exposure is still the same way we talked about it in the last century when everyone shot film.

It is time to catch up to make the most effective use of the gear we spend thousands and thousands of dollars on. It is time to take advantage of the fact that our cameras can do much more for us than film cameras.

It is time to realize that there are more accessible and better ways to teach this technology, beginning with the updated definition of ISO I just shared with you.

Please read on to Part III to learn a new photography term you didn’t know you needed. In Part IV, I will share a quote that will forever change how you approach setting your exposure.

I hope you found this information useful. Now go pick up that camera and shoot something! Because – “Your BEST shot is your NEXT shot!” — Joe Edelman

Have more questions about becoming a successful photographer? Would you like to continue the conversation? Join my TOGKnowledge Photographic Community, where you will find photographers from over 30 countries passionate about learning and sharing their photography as they develop their craft.


The Exposure Series

Part I: What the Exposure Triangle and Brooklyn Bridge Have in Common 

Part II: Demystifying Digital Camera Sensors: The Journey of Light to Pixel

Part III: The Must-Try ISO Tolerance Test: Max Out Your Mirrorless

Part IV: Use Purpose and Feeling for Correct Exposure In Every Frame


Joe Edelman

Joe Edelman is an award winning Photographer, Author, and "No Bull" Photo Educator.  Follow this link to learn more about Joe or view his portfolio. Please be sure to connect on the social media platforms below.
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