Photography Resource: Understanding Exposure, Part 4: ISO
ISO, which stands for International Organisation of Standards, is a measure of the sensitivity of film or a digital sensor to light. Formerly, ASA which stood for American Standards Association until it was changed to the more universal ISO, as above. The scales are an arbitrary rating of film speed that is the sensitivity of the film to light. If everything else is kept constant, the required exposure time is inversely proportional to the ratings guide using handheld light meters and now of course integrated into all digital cameras.
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, Fujifilm 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 virtually 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 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 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 process of increasing ISO, performed internally with the camera’s software and sensor, is very similar to increasing the gain on a microphone. Discussion about camera ISO can get incredibly complicated and nuanced but, for most photographers, thinking of ISO as the sensitivity of the sensor is the best way to avoid headaches.
The unfortunate side effect of increased ISO is called digital 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 film grain could improve a photograph’s feel and texture, no one has really proven to 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 may show you what your particular camera’s native ISO is not always the lowest ISO available on the camera. 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 (or lower) 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! (Evaluative Increments)!
So, let us bring this exposure journey full circle, shall we?
In the three segments, we discuss 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 utilise 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).
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. Most aspects of photography exposure are the other way round or back to front, for example, a small aperture is referred to as a large f number (f/16) and a large aperture is a small f number (f/2.8). Here are some common scenarios set out below.
1) Outdoor Portrait
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 you want to emulate this characteristic of those photos. In order to do this, you would need to open the aperture as wide as you can to shorten the depth of field. When turning 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. If you want to shoot at f/2.8 to get my blurred background.
- Switching the camera to Aperture Priority mode and let the camera determine the correct shutter speed for a proper exposure.
- Switch to manual mode and, in your head, figure out going from f/8 to f/2.8 I made a +3 EV shift to my exposure which is opening up the aperture by 3 stops (f/8 to f/5.6 to f/4 to f/2.8) and thereby allowing more light into the camera. To maintain the balanced exposure using shutter speed you need to reduce the amount of time that the shutter is open by 3 stops in the other direction. Therefore change the shutter speed from 1/500th to 1/4000th of a second (1/500th to 1/1000th to 1/2000th to 1/4000th).
2) Landscape Image
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 unrecognisable blurry shapes f/16 might work. My options:
- Use Aperture Priority mode, change the setting to f/16 and let the camera determine the best shutter speed.
- 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 desensitise 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).
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 fast shutter speed 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.
- 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 camera 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 (grain).
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.
Hoping you have 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 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!
Originally published at https://www.bhphotovideo.com.