Exposure Foundations

Wed Mar 04 2015


The foundation of any good exposure is understanding light, as an exposure is merely capturing light successfully. In the pre-digital days, photos were created by exposing negative or slide film to light for a certain period of time in order to record an image. Nowadays, it involves capturing that light digitally on a camera sensor.

At the core of that exposure is a combination of factors that allow the right amount of light into the camera for a specific period of time. We know these elements as aperture and shutter speed, which represent different “stops” of light. Understanding their relationship, and how they are impacted by the ISO setting of a camera, is essential to making the right decisions on how to set an exposure.

There are four elements of exposure. The first element is light sensitivity, or ISO. The second is the exposure mode you are shooting in (manual or aperture priority, for example). The third is aperture setting, or f-stop. The fourth is shutter speed.

Light Sensitivity

In the pre-digital days, photographers were capturing their images on film. When you purchased film, you always purchased film that had a set ASA, or film sensitivity. ASA 50, 64 and 100 film lay at the bottom of the light sensitivity spectrum. Thus, they were the film chosen for daylight landscape photography. Film ranged from those lower numbers to 400, 800 and 1600 ASA. There were higher numbers, but they were rarely used. The higher the number, the less light was necessary to capture an image. But also, the higher the ASA number, the higher visible grain you could see in the image.

Now, with digital cameras, you can find a range of ISO settings from 50 to 125,000. But ISO is more than just light sensitivity, it is a crucial element in determining what aperture and shutter speed to use in capturing an image. Selecting your ISO is key to setting the foundation for your exposure.

I always use standard starting points for ISO settings: 50 or 100 ISO for landscape photography, 400 ISO for wildlife photography, and 1600 ISO for aurora borealis photography. Choosing your light sensitivity setting drives the rest of the exposure process.

Exposure Mode

There are essentially four exposure mode settings for any DSLR (digital single lens reflex) camera: manual, aperture priority, shutter priority, and program. Manual exposure is when the photographer makes all of the exposure setting decisions: the ISO, the aperture, and the shutter speed. With aperture priority, the photographer sets the ISO and aperture setting, but the camera selects the shutter speed based on the meter reading in the camera. With shutter priority, the photographer selects ISO and shutter speed, but the camera sets the aperture. Finally, with program, the camera makes all decisions, leaving the photographer with no technical or creative decisions.

My preferred mode for daytime photography is aperture priority, as controlling the ISO and aperture setting are integral to my artistic control. For nighttime photography, I shoot exclusively in manual mode. The key thing to remember about any exposure mode is that the camera’s meter will want to translate the world into a neutral (18%) grey. That means, it will underexpose bright or white subjects and overexpose dark or black subjects. Have you ever taken a picture of a snowy scene and it comes back looking kind of muddy? That’s why. If shooting in any mode other than manual mode, you will need to useexposure compensation to get a correct exposure that provides detail in the blacks as well as the highlights.

Aperture and Shutter

Understanding the relationship between aperture setting and shutter speed is understanding stops of light. As a measurement unit of exposure, the f-stop corresponds primarily to the aperture setting on your lens: f/2.8, f/4.0, f/8.0 and so on. There is one f-stop between f/2.8 and f/4.o, one f-stop between f/4.0 and f/5.6 and all the way up the ring. These settings distinguish between an aperture that is really open, and thus bringing in more light when open (like f/2.8) or one that is really narrow, and thus bringing in little light (like f/16 or f/22). These settings also control depth of field, but that is a discussion for another time.

In contrast, shutter speed tells us how long the shutter on the camera body will remain open once the shutter has been triggered. The aperture on the lens controls how much light is coming in, but the shutter speed controls how long the film/sensor is exposed to light. Simply put, the wider the aperture, the shorter the shutter speed required for exposure; in contrast, a more narrow aperture will require a longer shutter speed for exposure. As the f-stop on the lens goes up, the amount of time for the shutter to be opened increases.

For example, to get the same exposure in mid-day light, a simple exposure would be ISO 100 with an aperture of f/16 and shutter speed of 1/125 seconds (also known as the Sunny F/16 Rule). Let’s say for artistic reasons, you need a slower shutter speed (say, to get some movement in water). In order to get movement, you will need to drop your shutter speed down to 1/30 of a second – that is a difference of two stops of light. In order to get that shutter speed, you would have to either increase your aperture to f/32, or, take other measures to reduce the amount of light coming in. A polarizing filter will take away about two stops of light. You could drop your ISO to 25 (no DSLR that I know of provides that). You could also add a full-frame 2-stop graduated neutral density filter.

My Common Settings

To illustrate how this all works in practice, I will share my primary settings for my most common lighting situations.

For daytime landscape photography, I typically shoot ISO 100 in aperture priority, with the aperture setting at f/16. Most often, shutter speed is either not a factor, or I want the slower shutter speed to get motion (like in moving water). But, if I am shooting a field of wildflowers and it is breezy, I may want to increase my ISO in order to provide for a faster shutter speed.

For wildlife photography, I start at ISO 400 in aperture priority, with the aperture set at f/5.6. I want a shallower depth of field to reduce background distraction and get a higher shutter speed, which is important when using a longer lens. And as a side note, the general rule of thumb for an ideal shutter speed to minimize camera movement is to have a shutter speed that equal to or greater than your focal length. So, for example, when shooting with a 500mm lens, you want a shutter speed of 1/500 or greater.

Finally, for nighttime photography, it all depends on the subject. If I am shooting the aurora borealis, I am going to start with ISO 1600 in manual mode, with the aperture set at f/2.8 and shutter speed at 8 seconds. Then I adjust to how bright or active the aurora is. For star trails photography, ISO 100 in manual mode on the bulb setting with the aperture set at f/2.8 and the shutter speed … well, the shutter is left open for 2-3 hours.

Understanding how all of this works takes a bit of time in the filed and a lot of tossed slides or deleted files. Experiment, try new things, and enjoy capturing the light! If you want to learn from me out in the field, keep an eye out for my listings of Alaska photo excursions. I always provide instruction on exposure as part of any photo tours or photo workshops.