Tips on Exposure

Posted by Alan Tunnicliffe

A photographs exposure determines how light or dark an image will appear when it has been captured by your camera. This is determined by just three camera settings: aperture, ISO and shutter speed (the “exposure triangle”). Mastering their use is an essential part of understanding how to achieve the result you want.

As a very basic way of understanding how to obtain the correct exposure, think of it like collecting rain in a bucket. While the rate of rainfall is uncontrollable, three factors remain under your control: the width of the bucket, the duration you leave it in the rain and the quantity of rain you want to collect…. You just need to ensure you do not collect too little (“underexposed”), but that you also do not collect too much (“overexposed”). The key is that there are many different combinations of width, time and quantity that will achieve this. As an example, for the same quantity of water, you can get away with less time in the rain if you pick a bucket that is really wide. In photography, the exposure settings of aperture, shutter speed and ISO speed are equivalent to the width, time and quantity discussed above, just as the rate of rainfall was beyond your control, so too is natural light for a photograph.

Each setting controls exposure differently:

• Aperture: controls the area over which light can enter your camera
• Shutter speed: controls the duration of the exposure
• ISO speed: controls the sensitivity of your cameras’ sensor to a given amount of light

You can therefore use many combinations of the above three settings to achieve the same exposure. The key is to know which trade-offs to make, since each setting also influences other image properties. Eg., aperture affects depth of field, shutter speed affects motion blur and ISO speed affects image noise.

The next few sections will describe each setting.

A camera shutter determines when the camera sensor will be open or closed to incoming light from the camera lens. The shutter speed specifically refers to how long this light is permitted to enter the camera.

By the Numbers. Shutter speeds influence on exposure is perhaps the simplest of the three camera settings:  it correlates exactly 1:1 with the amount of light entering the camera. E.g., when the exposure time doubles the amount of light entering the camera doubles. It is also the setting that has the widest range of possibilities:

Shutter Speed	Typical Examples
1 – 30+ seconds	Specialty night and low-light photos on a tripod
2 – 1/2 second	To add a silky look to flowing water Landscape photos on a tripod for enhanced depth of field
1/2 to 1/30 second	To add motion blur to the background of a moving subject Carefully taken hand-held photos with stabilization
1/50 – 1/100 second	Typical hand-held photos without substantial zoom
1/250 – 1/500 second	To freeze everyday sports/action subject movement Hand-held photos with substantial zoom (telephoto lens)
1/1000 – 1/4000 second	To freeze extremely fast, up-close subject motion
(95%+ of your photographs will be taken within this range)

 

 

 

 

Shutter speed is a powerful tool for freezing or exaggerating the appearance of motion. With waterfalls and other creative shots, motion blur is sometimes desirable, but for most other shots this is not wanted. All most people care about with shutter speed is whether it results in a sharp photo — either by freezing movement or because the shot can be taken hand-held without camera shake.

So, how do you know which shutter speed will provide a sharp hand-held shot? As a rule of thumb with digital cameras, the best way to find out is to just experiment and look at the results on your cameras rear LCD screen (at full zoom). If a properly focused photo comes out blurred, then you’ll usually need to either, increase the shutter speed, keep your hands steadier or use a camera tripod.

Aperture Settings

A cameras aperture setting controls the area over which light can pass through your camera lens. It is referred to in terms of an f-stop value; this is the most confusing with beginners, because the area of the opening increases as the f-stop decreases. In photography slang, when someone says they are “stopping down” or “opening up” their lens, they are referring to increasing and decreasing the f-stop value, respectively.

Depth of Field Chart

Every time the f-stop value halves, the light-collecting area quadruples. (Inverse Square Law)

 

The above aperture and shutter speed combinations all result in the same exposure.

The above f-stop numbers are all standard options in any camera. The range of values may also vary from camera to camera (or lens to lens).

A camera aperture setting is what determines a photos depth of field (the range of distance over which objects appear in sharp focus). Lower f-stop values (eg. f1.4 to f4) correlate with a shallower depth of field: Higher f-stop values (f11 to f22) with greater depth of field.

ISO Speed

The ISO speed determines how sensitive the camera is to incoming light. Similar to shutter speed, it also equates 1:1 with how much the exposure increases or decreases. Unlike aperture and shutter speed, a lower ISO speed is almost always desirable, since higher ISO speeds increase image noise. As a result, ISO speed is usually only increased from its minimum value if the desired aperture and shutter speed are not otherwise obtainable. Common ISO speeds include 100, 200, 400 and 800, although many cameras also permit lower or higher values.

With compact cameras, an ISO speed in the range of 50-200 generally produces acceptably low image noise, whereas with digital SLR cameras, a range of 50-800 (or higher) is often acceptable.

Camera Exposure Modes

Most digital cameras have one of the following standardised exposure modes: Auto ( ), Program (P), Aperture Priority (Av), Shutter Priority (Tv), Manual (M) and Bulb (B) mode. Av, Tv, and M are often called “creative modes” or “auto exposure (AE) modes.”

Each of these modes influences how aperture, ISO and shutter speed are chosen for a given exposure. Some modes attempt to pick all three values, whereas others let you specify one setting and the camera picks the other two (if possible). The following charts describe how each mode relates to exposure

Exposure Mode	How It Works
Auto ()	Camera automatically selects all exposure settings.
Program (P)	Camera automatically selects aperture & shutter speed. With some cameras, P can also act as a hybrid of the Av & Tv modes.
Aperture Priority (Av or A)	You specify the aperture & ISO; the camera’s metering determines the corresponding shutter speed.
Shutter Priority (Tv or S)	You specify the shutter speed & ISO; the camera’s metering determines the corresponding aperture.
Manual (M)	You specify the aperture, ISO and shutter speed — regardless of whether these values lead to a correct exposure.
Bulb (B)	Useful for exposures longer than 30 seconds. You specify the aperture and ISO; the shutter speed is determined by a remote release switch, or by the duration until you press the shutter button a second time.

 

The camera may also have several pre-set modes; the most common include landscape, portrait, sports and night mode. The symbols used for each mode vary slightly from camera to camera, but will likely appear similar to those below: