Putting It All Together - Exposure

Background
I've already explained the basics of ISO speed, apertures and shutter speeds. Now it's time to put them all together and see how they relate to the overall exposure of an image. The EV or Exposure Value for any given scene and lighting is always a constant.

Theory
The correct exposure for an image is governed by a complicated formula. However, all you really need to know is what effect altering any of the three options has on your pictures. So, for a particular scene and lighting conditions, your camera might suggest this exposure:

ISO 200, 1/250th @ f/5.6

If we were to change the ISO setting to 400, the film or sensor is now twice as "quick" at gathering the same amount of light, so the exposure time can be halved:

ISO 400, 1/500th @ f/5.6

Or, the same amount of light will be let in using 1/250th of a second as before, but decreasing the size of the aperture (increasing the f-number):

ISO 400, 1/250th @ f/8

Assuming we leave the ISO setting as it was originally, the following are also equivalent exposures to the first:

ISO 200, 1/125th @ f/8

ISO 200, 1/500th @ f/4

To see more equivalent values, you might like to look at this interactive EV calculator.

What does this mean for your pictures? Well basically, the combination of shutter speed, ISO and aperture is always a trade-off, depending on your priorities - or what you can change. Sometimes you may have low light and hit the widest aperture of your lens, which will give you slower shutter speeds. If you still want pictures which don't suffer from camera shake, you will need to increase the ISO setting.

Other times, you might have plenty of light (on a sunny day), so use a slow ISO setting, and choose the aperture/shutter speed to either freeze movement (fast shutter speed) or maximise depth of field (small aperture).

Examples
Here are some shots I took of the moon, in October 2004. With this sort of subject, you often don't know what will be the "correct" exposure, so by keeping two of the three parameters the same, and just changing the third, you can bracket the exposure to see which is best.

In this case, I used ISO 200 (to preserve fine detail) and the lens was set at it's widest aperture, f/5.6. The only option left was changing the shutter speed accordingly. Apart from the blue hue, see what effect this has on exposure:

1/250th

This version looks a little bit too bright, it's overexposed.

[1/250th @ f/5.6 with ISO200 - overexposed. Loss of detail in the highlight areas, bottom right]

1/400th

This version looks about right, it's correctly exposed.

[1/400th @ f/5.6 with ISO200 - nicely exposed. Enough detail in the highlights and shadows]

1/640th

This version looks a little bit too dark, it's underexposed.

[1/640th @ f/5.6 with ISO200 - underexposed. Loss of detail in the shadow areas, centre]

As good practice, why not take a few pictures of the same subject, but using a combination of different shutter speeds, apertures and ISO settings. Change one at a time and take a series of shots, then see what effect this has on your subject, and the way the camera records it.