Understanding Camera Exposure in 2 minutes.

Understanding camera exposure must be one of the confusing parts to photography. Once you get your head around this seemingly complex idea, you will become more creative in your photography and set the exposure with a particular creative feel in mind. For example, tweaking the time will have a different creative impact when the shutter speed is 1/1000 sec as compared to 30sec. Also having the aperture wide open (small f-stop number like f2.4) will have a significantly different result to the image than the same shot taken with the aperture very narrow (large f-stop number like f32).

I will explain exposure settings in a way that I have not read before and I believe that it makes a difficult concept easy to understand. I will take all of 2 minutes to read and hopefully understand, so get your stop watch ready. Here  goes:

Imagine you have an empty bucket and a hose is connected to a tap and placed into the bucket. You then fill up the bucket with water. Let us imagine that the amount of water in the bucket is the exposure.

Ok, so if we want to have the same amount of water (ie exposure)  in another bucket.  If we use a wider hose we can fill the bucket up in less time, but if we have a thinner hose then it will take longer to fill the bucket up with the same amount of water.

In terms of the camera,  the water, thickness of the hose and the time the water is running through the hose is equivalent to exposure, aperture (ie the width of the hole in the lens through which light travels) and the length of time the aperture is open to let light through.

So to get the same volume of light onto the sensor we can either use a small aperture and have light running through the lens of a long time or have the aperture wide open and have the light run through it in a relatively short period of time.

How are we going so far? all good? Ok, we are over the hard part. All we have to mention now is how much to tweak time when I use different aperture settings. To nail this down, let us return to the bucket and hose analogy.

If it takes 60 seconds to fill the bucket with a particular width hose, then how long will it take to fill if we use a hose with twice the diameter? Not half the time but actually 1/4 of the time (since the area of the hose is as circle and doubling the width quadruples the area), so it will take 15 seconds to fill.


Here are some aperture widths that double each time and so the area is quadrupled with each change in aperture setting.

small width  aperture                           large width aperture
f32           f16          f8            f4            f2

So how can we use this information when playing with the camera? Good question. If the camera is telling you that an exposure setting of 1 sec at f4 is needed that is the volume of light (the camera thinks) is needed. This volume (ie exposure) can be generated in many different ways.

1. keep f4 and 1 sec
2. Halve the aperture width and use four times the time; ie use f8 and 4 seconds
3. Double the aperture width and use a quarter of the time; ie f2 and 1/4 sec
4. Use f16 and 16 seconds
5. use f32 and use 64 seconds.
and so on. 

A natural question would be to ask: "why bother to change these settings if they all give the same exposure ?"

Using a small aperture (ie large f-stop like f16, f29, f32) allows more of the scene to be in focus, that is the depth of field is large; whilst at the same time the length of time the sensor is exposed to the light needs to be long (to ensure we have the correct exposure). Lengthening time makes: water turn to mist, blurs sparklers we wave around at night, creates star trails. make trains blur, creates the line of lights created by car headlights at night and in general introduces motion. A cool artistic effect! Like wise if I want to capture the motion of birds in flight, a child throwing a netball, a dog catching a ball in mid air, turn water in to droplets and so on, a very short time period is needed such as 1/100, 1/500. 1/1000 depending on what is needed, and a wide open aperture (eg f4, f3.2, f2) is used to achieve the correct exposure.

Hopefully I haven't lost you, now we are in the position to previsulalise an image and decide on a particular effect.

Effect 1: image sharp and background blurred. We need the aperture wide open (f4, f2.4 etc)
Effect 2: all of the image in focus. We need the aperture narrow (f29, f32 etc)
Effect 3: create motion in the scene. We need a long exposure time ie 1/4 sec, 1/2 sec, 1 sec, 10 sec etc
Effect 4: freeze everything in the scene. We need a very short exposure time 1/500, 1/1000, 1/2000 etc

You get the idea.
Here are three steps to help you with what we have covered so far.

1. Decide on the effect you want to create (use the above as guides).
2. If the effect needs you to manipulate the aperture, turn your camera setting to the aperture priority setting (AV) and set the aperture to the desired value and the time exposure will change automatically.
3. If the effect needs you to manipulate time, turn the camera setting to the time priority (TV) and set the time to the desired value and the aperture will be changed automatically to create what the camera thinks is the correct exposure needed to render a well exposed image.

In a matter of minutes we have gone from using a camera as a point and shoot device to us being a creative visionary using a camera to produce creative images we have preconceived. Now that is cool!

I suggest you go and have fun taking shots and explore each of the four effects mentioned above. Remember that I have given general guide lines to get you going on the correct  track, so experiment, be creative and learn how to get the most out of your camera!

have fun! :-)

I'd love to hear how you got on with your creative experiments.



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