The exposure of an image is controlled by three parameters.
- The size of the lens opening.
- The length of time the shutter is open.
- The sensitivity the image sensor is set to, known as ISO
In the first two installments we saw that light is first captured by the lens, controlled by the lens opening or aperture, then the shutter controls how long the light falls on the image sensor.
So, after this intro to exposure we'll start at the lens with aperture and work back to the image sensor. As a further note: The image sensor could be film or a digital device. We'll be speaking about digital only here.
I get asked this question a lot by photographers just beginning to experiment with adjusting their exposure settings. My answer is always the same. There is no 1 'best' set of exposure settings for everything. It turns out there are 6 or 7 correct combinations of exposure settings for most scenes. This is where a lot of new photographers get that, Huh! look on their face. Don't worry, it's easy because it's all about factors of 2.
You've probably heard the term before. It is frequently used without the f/, and just called a stop. The term stop is most often used when discussing aperture settings. It can also be used to discuss shutter speeds and ISO settings as you'll soon see.
A stop is either times 2, or divide by 2. A lens opening that is twice (times 2) the size of the previous lens opening is a full stop more of aperture. A lens opening that is 1/2 (divide by 2) the previous lens opening is a full stop less aperture. See, I told you it was easy.
It works the same way for shutter speed and ISO also. Twice (times 2) as fast a shutter speed is a full stop, 1/2 (divide by 2) as fast is also a full stop. Most digital cameras today allow you to adjust the three exposure settings in 1/3 stop increments so you can have exquisite control of your exposures.
We'll take a look at the standard shutter speeds that are each 1 stop from the next. They are:
- 1 second
- 1/2 s (1 divided by 2)
- 1/4 s (1/4 divided by 2)
- 1/8 s (1/8 divided by 2)
- 1/15 s
- 1/30 s
- 1/60 s
- 1/125 s
- 1/250 s
- 1/500 s
- 1/1000 s
- 1/2000 s
and so forth. Top of the line DSLR cameras have fast shutter speeds of 1/8000 s. Don't forget that most DSLRs are going to display 1/3 stop increments so there will be 2 shutter speeds between each of these. It gives you really fine control of your exposures.
Your DSLR has a built-in light meter that will help you select exposure settings so your images are neither to dark nor to light. Just remember, it's just a dumb 'ol machine that can only do things by rote. It can be fooled by things that are really bright, really dark, or scenes that have both. We'll cover those issues in a couple of installments.
So that's the intro to Exposure and next we'll get into some nitty gritty about aperture.
See you then,
All images courtesy of Wikimedia Commons.