As we noted in our previous segments, Understanding Aperture and Understanding Shutter Speed, photography is the capture of light on a photosensitive surface, be it film or a digital sensor. In order to expose a photograph, we have to control the number of photons that connect with the image surface, as well as control the sensitivity of that surface to the light.
We compared aperture to the eye's iris that opens and constricts the diameter of its opening to limit the amount of light allowed into the eye. Shutter speed is akin to the duration of time your eye is open to admit the light. ISO is similar to the sensitivity of the rods and cones at the rear of the human eye.
In Part Three of this three-part series, we will discuss ISO and then talk about the symbiotic relationship between ISO, shutter speed, and aperture.
ISO
ISO, which stands for International Organisation of Standards, is a measure of the sensitivity of film or a digital sensor to light.
If you have ever photographed with film, you are familiar with the different numbers associated with a package of film that you used to buy at the store. Kodak Gold 200, Fuji Velvia 50, Agfa APX 400, etc. The numeral in the film's nomenclature referred to its speed, sometimes given as an ASA number, and the higher the number, the more sensitive the film was to light. This sensitivity was a function of the size of the photosensitive grains on the piece of film, as well as the characteristics of the different chemicals used to create the film.
There are different international standards applied to colour negative film, black-and-white negative film, and colour slide film, but the numbering system remains constant.
In order to adjust ISO, film shooters have only the option of changing to a different speed film. Once loaded, the ISO is a constant for a given roll of film.
When digital photography entered the fray, the ISO developed speed standards for digital sensors. Thankfully, they assigned the same number system to the digital sensors as they did to the film standards. If you never shot film, you probably are not concerned that the ISO is spoken in the same language, but for those transitioning from film to digital, it is a welcome convenience.
One huge game-changer of digital photography is that, when it comes to ISO, you can change the sensor's sensitivity while you are shooting. ISO is now an electronic function of the camera and not a fixed value as it was with a particular roll of film. So, with the turn of a dial or flick of a switch, you can change your camera sensor's ISO for each image, if you want.
The technical ins-and-outs of how ISO is calculated are fairly complicated and not especially useful to most photographers, but what is important is how the ISO numbers relate to exposure and exposure values. Just like with aperture and shutter speed, we want ISO to work in EVs, because we are still controlling exposure and we want all three variables to speak the same language.
ISO, like shutter speed, is linear. A film with an ISO of 200 is half as sensitive to light as the same film with an ISO rating of 400. Double the ISO, double the sensitivity. Half the ISO, half the sensitivity. Easy peasy!
So, assuming all else is constant, if you change your camera's ISO from 400 to 200, or change to a roll of 200 speed film from a roll of 400 speed film, you will introduce a -1 EV shift, as you have just made the sensor, or film, half as sensitive to the given light. ISO 800 changed to ISO 1600; double the sensitivity; +1 EV.
Side effects of ISO
With film, the higher ISO films had larger grains that were impacted by greater numbers of photons. The larger the grains, the "grainier" the image would be. Depending on the type of photo and film, the grain of the film could be a real benefit to the texture and feel of an image. It was something that you had to deal with when shooting film and you could definitely make the grain work in your artistic favour.
With digital sensors, when we adjust ISO, we are not changing the size of the pixel, but we are increasing its sensitivity electronically by, in layman's terms, increasing the voltage to the sensor. The unfortunate side effect of this is called noise. The higher the ISO, the more digital noise is introduced into your image. Camera manufacturers, while seemingly always adding megapixels to their new cameras, are constantly trying to reduce the amount of noise at a given ISO. While grain could improve a photograph's feel and texture, noise rarely looks artistic in digital images.
In the digital realm, there are three types of ISO: native, amplified, and simulated. Native ISO is the ISO setting that does not require the camera to increase the voltage to the sensor. Amplified ISO requires an increase in voltage, which introduces noise. Simulated ISO uses software algorithms to mimic higher sensitivity, often with even more noise. Regardless, the EV relationship stays linear.
It’s all EVs!
So, let us bring this exposure journey full circle. In the three segments we discussed how, in order to control the amount of light striking a piece of film or digital sensor, we have two physical controls inside the camera — aperture and shutter speed. We can further control the sensitivity of the sensor by adjusting the ISO on a digital camera (or by using a roll of film of a different ISO).
Each of these three adjustments can be quantified into EVs. By doubling or halving light (aperture and shutter speed) or doubling or halving sensitivity (ISO) we can adjust EVs up or down.
To maintain the exposure you want — whether “properly” exposed, intentionally over, or under-exposed — any increase in photons through aperture/shutter or increase in ISO sensitivity must be compensated in the opposite direction with the other controls.
Practical scenarios
1) Outdoor Portrait: Open the aperture to f/2.8 for background blur. Compensate with a faster shutter (e.g. 1/4000s instead of 1/500s). ISO stays at native.
2) Landscape: Stop down to f/16 for front-to-back sharpness. Compensate with a slower shutter (e.g. 1/60s instead of 1/1000s). ISO stays at native.
3) Kids’ Soccer Game: Use 1/1000s shutter to freeze motion. If light is low, raise ISO to keep exposure balanced.
4) Handheld Low-Light: Open aperture to f/1.8. Raise ISO (e.g. ISO 800 or higher) until shutter speed is fast enough to avoid blur.
Conclusion
I hope you enjoyed this series on exposure and how to control it in-camera. The key to getting the photographs you want is understanding how aperture, shutter speed, and ISO interact — not only in balancing exposure, but also in their side effects: depth of field, motion blur, and noise. Photography is art, so experiment freely with your settings to achieve your vision. Good luck, have fun, take great photos, and make art!
