The B&H Guide to Exposure: Understanding ISO
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, which stands for International Standards Organization, 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 color negative film, black-and-white negative film, and color 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. (Thank the folk at the ISO for saving us from the Inverse Square Law!) 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.
Now let us talk about the "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 favor.
|Two images of the New York City skyline with the same exposure value. The first image is at ISO 200. The second image is at ISO 6400.|
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, no one has really proven to me that sensor noise is an artistic improvement in a digital image!
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. The camera's native ISO is not usually published by the manufacturer in the specifications for a given camera, but a quick Internet search will show you what your particular camera's native ISO is. Amplified ISO is an ISO that requires an increase in voltage to the sensor to achieve. This is when noise starts to creep into your images: the higher the ISO, the greater the noise. And, finally, simulated ISO is when the camera uses a software algorithm to simulate even higher ISOs. Regardless of whether the increased ISO is amplified or simulated, you will see an increase in noise over the native ISO setting. And, regardless of the type of ISO, the numbers still adhere to the same linear relationship with EVs.
It's all EVs!
So, let us bring this exposure journey full circle, shall we?
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 that we can utilize 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 getting a roll of film of a different ISO).
We also discussed how each of these three adjustments can be quantified into EVs and that by doubling or halving the amount of light (aperture and shutter speed) and by doubling or halving the sensitivity of the photosensitive surface (ISO) we can adjust EVs up or down.
In order to maintain the exposure you are searching for, be it a proper exposure or intentional over- or under-exposing of an image, when you allow more photons into the camera, or increase the sensitivity of the film or sensor, you will need to compensate in the opposite direction with the other camera controls.
So, let's make this all practical by looking at a few photographic scenarios where you would adjust one of these three elements.
A wide aperture blurs the New York City skyline in this portrait.
1) Outdoor portrait. I am outdoors in sunlight and I want to take a photo of a friend. I saw some photos the other day that featured a lovely blurred background behind the model and I want to emulate this characteristic of those photos. In order to do this, I know from reading B&H Explora that I need to open my aperture as wide as I can to shorten the depth of field. When I turned the camera on in Auto mode, the camera said it was going to take a photo at f/8 and 1/500th of a second. I want to shoot at f/2.8 to get my blurred background. I am outside in sunlight, so my camera should be set at its native ISO and there should be no reason to adjust that. I have a few options now:
- I can switch my camera to Aperture Priority mode and let the computer determine the correct shutter speed for a proper exposure.
- I can switch to manual mode and, in my head, figure out that when I went from f/8 to f/2.8 I made a +3 EV shift to my exposure as I have opened up the aperture by 3 stops (f/8 to f/5.6 to f/4 to f/2.8) and thereby allowed more light into the camera. To maintain the balanced exposure using shutter speed, I need to reduce the amount of time that the shutter is open by 3 stops in the other direction. Therefore, I will change my shutter speed from 1/500th to 1/4000th of a second (1/500th to 1/1000th to 1/2000th to 1/4000th).
Mount Shuksan in North Cascades National Park, taken from a US Navy Sikorsky UH-3H Sea King helicopter. A narrow aperture allows greater depth of field to keep the helicopter tail and mountain relatively sharp.
2) Landscape Image. I am taking a picture of a city skyline or beautiful snow-capped mountain range and my friends want to be in the image in the foreground and are forgoing the opportunity for yet another selfie (thank goodness!). I have my SLR camera and we can get a better shot than with our phones. I want to keep my friends sharply in focus but also show the cityscape/mountain in focus, as well. The camera is suggesting a photo at f/4 and 1/1000th of a second at the native ISO. I want to increase the depth of field to keep my foreground friends in focus while making sure the background does not become a bunch of unrecognizable blurry shapes—f/16 might work. My options:
- I can use Aperture Priority mode, change the setting to f/16 and let the camera determine the best shutter speed.
- I can, using Manual mode, shift to f/16. This will reduce the size of the aperture opening by 4 stops that I must now compensate for with shutter speed. Why can't I compensate with ISO? Well, in theory, you can, but if your camera is already on the native ISO setting, you probably will not be able to desensitize the sensor or film by the needed 4 stops in this scenario. So, you will have to compensate with a slower shutter speed. Four stops from 1/1000th is 1/60th of a second (1/1000th to 1/500th to 1/250th to 1/125th to 1/60th).
High shutter speed freezes the action on a football pitch.
3) Kids' Soccer Game. You need action shots of your kid's heroics on the local youth soccer pitch and want to try out your new telephoto lens. The key to freezing action is a fast shutter speed. Because the light on the field might be very dynamic, this is a good opportunity to use either Shutter Priority mode or Aperture Priority mode and let the camera do the EV math for you at high speed. This may be a scenario where an ISO adjustment will come into the fray.
- If you use Shutter Priority mode, you can set you shutter speed to something very quick, say 1/1000th of a second. You will see the camera compensate by opening up the aperture near or all the way to its maximum width to let in as much light as possible for the quick shutter opening.
- If you use Aperture Priority mode, you can open up your aperture as much as possible. The camera will compensate by giving you the fastest shutter speed available for that amount of light.
- So, hold the phone. It is a cloudy day and you noticed that in Shutter Priority mode your images (if you are shooting digital and can instantly review the photos) are looking dark even when the computer is giving you your maximum aperture. In Aperture Priority mode, your images are looking blurry because the shutter speeds are not sufficient to freeze the action. What do you do? This is where you can increase the EV value for the given aperture or shutter speed by making the sensor more sensitive to the photons. So, dial up the ISO number by one or two or three or more EVs and see if that helps brighten your Shutter Priority photos or sharpen your Aperture Priority pictures. Be careful, turning the ISO up too high will result in unattractive noise!
A low-light, handheld image from a New York street. After opening my aperture to f/1.8, I had to increase my ISO to 800 to get the shutter speed sufficiently quick to avoid camera blur in the low light.
4) Handheld Low-Light Photo. One of the things that many photographers dream of is the ability to capture a photograph in a dark space, such as a bar or restaurant, without using a flash for extra light or a cumbersome tripod. In the past, you needed a very fast (high ISO) film and a lens that allowed a very large aperture. The lens portion of the equation remains true in the digital age, but not quite as much as the low-light performance of high-ISO digital images is getting better and better. So, here is how I approach the image:
- Depending on how dark the environment is, how large I can make my aperture, and how long I have to set up the image, I will either start at the native ISO or bump it up one or two EVs. I will then open my aperture to its maximum diameter and see what kind of shutter speeds the camera is giving me in Aperture Priority mode. If they are still too slow for me to eliminate the blur caused by camera shake, I will turn the ISO up even more.
You may have noticed that in all of the above examples I was adjusting the camera in full-stop or EV increments. Many cameras let you select and change exposure settings (ISO, Aperture, Shutter Speed, and Exposure Compensation or Shift) in 1/2 stop or even 1/3 stop increments. If your math skills are on par with mine, I recommend setting your camera to work in full or 1/2-stop increments to simplify the calculations. I'm sure there are those who will argue against that, as you might lose some of the precision-exposure capabilities offered by 1/3 step adjustments, but I accept tradeoff in the name of easier math.
I hope you enjoyed this series on exposure and how to control exposure in a camera. One key to getting the photographs you want is knowing how the different variables not only affect each other when exposing an image, but also knowing the different side effects of adjusting each variable (depth of field, motion blur, noise, etc). As I wrote at the beginning, photography is art. You should always feel free to experiment with changing your camera settings in order to achieve the objectives of your artistic vision by altering exposure, depth of field, motion blur, etc. Good luck, have fun, take great photos, and make art!