Exposure Made Simple

Exposure made simple

Modern digital cameras have a built-in exposure meter and are, for the most part, extremely accurate. Most have some sort of multi-segment or matrix metering that evaluates the scene and comes up with the best exposure for the scene. While they are really good these days, sometimes, you want a little more control in picking your exposure. That’s when you can switch to manual mode.

Somewhere, each camera is different, as you are looking through the lens and changing settings, (generally at the bottom middle of your viewfinder) there is an exposure indicator that tells you whether or not you are right in your settings for the scene that is being seen through the lens. The indicator needle at Zero means you have the correct exposure, if it is in the minus you are underexposed and if it is in the plus you are overexposed for the scene. Each manufacturer will have a different version of this, consult your user manual for your specific make and model.

In order to set the proper exposure (without flash) with the camera you need to be concerned with three basic controls:

CCD sensitivity level– Measured in ISO (International Standards Organization) or ASA (American Standards Association). For example, 100, 200, 400….
Shutter Speed- Measured in seconds and fractions of seconds. For example 30 seconds, 1/500th second.
Aperture- Measured by predetermined hole sizes in the lens. Common aperture designations have an f/ then number…ie…f/8, f/11.

CCD sensitivity level is what we set in today’s modern digital cameras. In film cameras, it was the film speed indicator. Common ASA’s today include 100, 200, 400. 800 and 1600 ASA/ISO. Most digital SLRs have ratings in that range, point and shoots may only go up to 400. These are the sensitivity ratings for the recording surface.

Basically, the higher the number, the more sensitive to light. Most situations occurring in everyday life will have the photographer setting a 100 ASA/ISO to 400 ASA/ISO rating on their camera, with bright sunlit days lending themselves to 100 ASA/ISO or 200 ASA/ISO and cloudy days better suited to using 400 ASA/ISO. In modern digital cameras, you can change this anytime.

Higher ASA/ISO ratings like 1600 will be when you are in a low light situation, like an arena or school theatre and you can’t use flash. The problem, the higher the ASA/ISO rating, the more noise or grain that shows up in the image. So, while ideally, we would like a smooth picture, circumstances may have us have to suffer from some noise to get the photo we like.

Now, the tough part, the numbers listed above are common ASA/ISO ratings that have certain properties that need to be noted. 200 ASA/ISO is twice as sensitive to light as 100 ASA/ISO, 400 ASA/ISO twice as sensitive to light as 200 ASA/ISO, 800 ASA/ISO twice as sensitive to light as 400 ASA/ISO and 1600 ASA/ISO is twice as sensitive to light as 800 ASA/ISO. The difference between these is measured in stops, the difference between each, 1 stop. More on this later.

The next two controls govern light entering into the camera body and striking the recording surface.

Shutter speed and Aperture

First, we will look at the control governing how long the recording surface is exposed, the shutter speed. Quite frankly, a shutter is just that, a shutter or a curtain, that opens and closes in front of the recording surface. It is measured in seconds and fractions of seconds.

Shutter speeds on most of today’s modern cameras can be set between 30 seconds for long exposures, the curtain blades remaining open for that amount of time, to 1/8000th sec., even faster, opening and closing the curtain blades very briefly exposing the recording surface to light entering the camera.

Cameras even have BULB settings which can have the shutter open for really long exposures, even hours, which people have used for photographing night skies and star trails.

One thing to note, shutter speeds also have relations to be aware of, though with today’s modern cameras it is hard to track. Basically, a shutter speed of 1/125 sec. lets in twice as much light as a shutter speed of 1/250 sec., while a shutter speed of 1 sec. lets in half the amount of light as a shutter speed of 2 sec. This difference is measured in stops and if it lets in half as much light or allows twice as much, the measurement is 1 stop difference.

The standard shutter speed scale from 30 sec goes: 30 sec., 15 sec., 8 sec., 4 sec., 2 sec.,1 sec. (denoted by “), 1/2 sec., 1/4 sec, 1/8sec, 1/15 sec., 1/30 sec., 1/60 sec., 1/125 sec. 1/250 sec., 1/500 sec., 1/1000 sec., 1/2000 sec., 1/4000 sec., 1/8000 sec.

These are the scale of shutter speeds with 1 full stop between them. Today’s modern cameras have more accurate exposure control that has shutter speeds adjustable in third stops so you may see shutter speeds like 3 sec. or 1/400 sec.

The light enters into the camera through the lens which has in it several blades, referred to as diaphragm or lens aperture. These blades open and close making different sizes of holes for the light to pass through.

The holes are standard sizes, measured in stops. Starting with the biggest hole and listing to the smallest, common apertures are f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 and f/32 depending on the lens you have. There are other aperture measurements out there, these are the most common. The lower the number, f/1.4, the bigger the hole in the lens, the higher the number, f/22, the smaller the hole. So, basically, a big hole lets in a lot of light, a small hole lets in less light.

These numbers listed above also have a relationship between them, they are measured in stops as well, and f/2 lets in half the amount of light as f/1.4 while f/8 lets in twice the amount of light as f/11. The difference between each, 1 stop. Today’s modern cameras also allow the apertures to be set in third stops as well, form more accurate exposure control, so you may see apertures such as f/17 or f/10.

I know, I know, the title of the article was exposure made simple. How can it be simple with fractions and stops and f/?????? These are all the important technical terms and factors you need to be familiar with for your basic camera operations.

Your camera has a built-in method of metering to get the correct exposure, you just need to tell it the sensitivity of the recording surface, how much light to let in, and how much time to let the light in to make it all work.

Why is all this important if you can just leave your camera on program? Well, you may find situations where the program mode isn’t always the best solution.

While the camera evaluates the scene for proper exposure, it doesn’t look at the subject, the action, and the effect the photographer is trying to accomplish. For this, you will need manual exposure or, try shutter-priority or aperture priority modes.


Choosing your shutter speed
Choosing your aperture
Setting your ISO sensitivity
Putting it all together, coming up with a proper exposure