Digital photography tutorial - Understanding exposure

Understanding exposure, even if you use a digital camera with automatic exposure modes, can help you have more control over your photographs and to be more creative.

Shutter speed/aperture/ISO table - find out what camera settings to use outdoors.

Table for indoor shots - for photos taken without flash

So let's start by considering the three variables that work together to achieve a well exposed photograph. They are the ISO setting, camera shutter speed and aperture. As many different combinations of these three can be used it can be quite confusing for digital photography beginners.

ISO ratings

ISO originally referred to the speed of the film you placed inside your camera. Each ISO was suitable for particular lighting conditions; you would use a fast 400-800 ISO film if you were shooting where there was little light, and a slower 100 film on your bright, sunny summer holiday.

Nowadays the term ISO relates to the sensitivity of the sensor in your digital camera and you can change it in between shots, no more waiting to finish the film in the camera!

So how do these numbers help us in understanding exposure? The higher the number or ISO setting, the greater the sensor's sensitivity to light. At an ISO of 800 or more, the camera's aperture only needs to be open for a short time in order to create an image, whereas using lower numbers means that the sensor needs more light to reach it in order to record a properly exposed shot.

For normal, everyday photography ISO ratings in the region of 100-200 are the most commonly used, and won't usually need changing during a photographic session.

Camera shutter speeds

The shutter speed, on the other hand, may change with every shot.

There are two reasons for changing the shutter speed; to let more or less light onto the sensor, or to stop or blur action. The first determines the correct exposure, whereas the second option allows more creativity. A well exposed photograph of something moving, that is blurred because the shutter speed chosen was too slow, won't make an attractive photo. Likewise, a shot of someone on a horse won't make it into the photo album if, although it caught the moment that the horse cleared the jump, you can only just pick out the animal because the whole picture is too dark.

Camera aperture settings

Thinking of the camera aperture as a hole that can change in size assists us in understanding exposure. When it is tiny, the shutter needs to stay open longer in order for enough light to reach the sensor to ensure correct exposure. When it is expanded then the shutter needs to only open for a fraction of a second.

The aperture is measured by f-stops. Starting at the smallest number, often f2 or f2.8, we can say that each time we change the setting it halves the size of the aperture, ending up at f11 or even f16 at which point only a tiny pinprick of light will enter the camera lens.

The size of the aperture will have an effect on how much of the photograph is in focus. Click here to learn about how shutter speed and aperture can be used in creative ways to put the emphasis on part of a photograph and throw the rest out of focus.

How to put them all together

The following tables will help you in understanding exposure. Try them out by setting your camera to manual and studying the results you get.

Outdoor settings

ISO ratings	Sun on snow or sand	Bright sunshine	Hazy sunshine	Cloudy but bright	Overcast
100/125	1/125 sec @ f/11	1/125 sec @ f/8	1/125 sec @ f/5.6	1/125 sec @ f/4	1/60 @ f/4
200	1/250 sec @ f/16	1/250 sec @ f/11	1/125 sec @ f/11	1/125 sec @ f/8	1/125 @ f/5.6
400	1/500 sec @ f/16	1/500 sec @ f/11	1/250 sec @ f/11	1/250 sec @ f/8	1/250 @ f/5.6

Indoors, without flash

Subject	100	200	400	800
Well lit room	1/15 sec @ f2	1/30 sec @ f2	1/30 @ f/2.8	1/30 sec @ f/4
Dimly lit room	1/4 sec @ f2.8	1/15 sec @ f2	1/30 @ f/2	1/30 sec @ f/2.8
Candle light (close up)	1/4 sec @ f2	1/8 sec @ f2	1/15 @ f/2	1/30 sec @ f/2
Bright fluorescent light	1/30 sec @ f2.8	1/30 sec @ f4	1/60 @ f/4	1/60 sec @ f/5.6