This page owes its use of the acronym 'L.A.S.T.' to the article here, which is such a good one that this page has been limited mainly to a more in-depth discussion of what exposure means in terms of the finished photographic product as well as reinforcement of the concept of exposure through analogy. That article is called Four Factors of Exposure. The four factors are Light, Aperture, Sensitivity, and Time. On this page and elsewhere, you will also see those same factors referred to, respectively, as scene brightness, F-stop, ISO, and shutter speed. At the end of this page, there is a link to the glossary, where you will find lengthier discussions of the exposure factors individually.
The Four Factors
Scene brightness is a composite measure of how much light is either radiated and/or reflected by the scene toward the camera. Examples of radiated light include red hot charcoal, a table lamp, or the sun or moon, if any of these are actually in the scene, meaning they will be seen in the photo. On the other hand suppose that there are no radiated light sources in the scene. As an example, consider a sunny day at a sporting event. You will not likely be pointing your camera at the sun, but rather at the athletes. The light entering your camera originated at the sun but was reflected off of objects and people in the scene. This is what is meant by reflected light. Someone catching a ball under direct sunlight has a higher scene brightness than two persons chatting in a dimly lit room. It is interesting to note that of all the four factors, scene brightness is the most difficult to control with the camera. The photographer can introduce additional continuous lighting such as lamps (or remove them), or the camera flash can bombard the scene with a very brief and very powerful burst of light. However, if the scene is too bright the camera has no way of removing light from the scene. With the remaining three factors, however, the camera can block light (F-stop), become less sensitive to light (ISO), or simply use less light (shutter speed).
F-stop (aperture) is a number that represents the size of an opening inside the camera lens. This size can be changed and is controlled either by the camera or by the photographer. The larger this opening is, the more light will flow through the lens in a given amount of time. Now, you're really going to like this part: the larger the F-stop number, the smaller the opening. See? I said you'd like it.
The ISO (sensitivity) is a number that expresses how rapidly your film or sensor reacts to light. The higher the number, the faster it reacts.
Shutter speed (time) gets a big paragraph to make room for a small rant. The bottom line of this rant is that SHUTTER SPEED IS NOT A SPEED. Shutter speed is a time, specifically the length of time the shutter is open and allowing light to strike the film or sensor. A speed is a distance divided by a time. When you select a shutter speed on your camera, however, you won't be selecting '55 miles per hour' or '2 inches per second'. You will in fact be selecting something more like 60 (1/60th of a second) or 500 (1/500th of a second). These are, of course, times (or durations). Nevertheless, the term shutter speed will continue to be used here because later when you are reading your camera manual or another article on the Internet, 'shutter speed' is what you will see. It is worth noting that the numbers you see on your camera when selecting shutter speed are actually reciprocals of the time unless indicated by a double apostrophe ( " ) after the number, in which case they are being expressed in actual seconds.
It is also worth noting that often rather than selecting a shutter speed on your camera, you will more likely be observing what shutter speed your camera has selected for you. Once you understand what effect different shutter speeds will have on your photographs, you can decide whether or not you like the value your camera proposes to use in a given situation. If not, you can change it. This also applies to the F-stop. And, if you are using a digital camera, it also applies to ISO. There is a caveat on which we will elaborate below. In short, if you change the value of one exposure factor, one or more of the other factors must be changed automatically by the camera (or manually by the photographer) if the resultant exposure is to remain unchanged.
What is Exposure?
Something has been said now about each of the four major factors. However, little has been said about how the four factors influence exposure, and nothing has been said about how all four unite to form an overall exposure. In fact, what is exposure? Let's just say that exposure can be thought of as how light or dark a photo is with respect to how light or dark it should be. Thus, an overexposed photo is lighter than it should be, and an underexposed photo is darker than it should be.
Another definition of exposure which encompasses a bit more subtlety would be to say that exposure is a measure of how well the lightest and darkest elements of a scene fall within the dynamic range of the medium on which it is to be displayed. Suppose that a scene consists of 10 objects, each of a somewhat different brightness. The brightest object is completely white, and the darkest object is completely black. Suppose that a photo has been taken of that scene and that the photo is being viewed on a computer monitor. If the 2 brightest objects in the photo are completely white on the screen, the photo is overexposed. If the 3 brightest objects in the scene are all completely white on the screen, the photo is badly overexposed. Conversely, if the 2 darkest objects in the scene are both completely black on the screen, the photo is underexposed. If the 3 darkest objects in the scene are all completely black on the screen, the photo is badly underexposed. The above is an oversimplification because some scenes have such a high contrast, or some media have such low dynamic range that it is impossible to avoid 'washing out' the highlights (displaying objects that are almost white as completely white) and/or 'blocking up' the shadows (displaying objects that are almost black as completely black).
Exposure by Analogy
Not even the briefest of brief glossaries of photographic terms would be complete without a separate article on each of the four major factors of exposure. Return to the table of contents to find them. Exposure, if incorrect, can obviously be altered by changing any one (or more) of the four factors. Which of the factors to change in any given situation is a decision best left until after one has studied the factors individually. The point of this article, alluded to earlier only briefly, is to assist the reader in conceptualizing the effect of the four factors on exposure through the use of three sets of analogies. Within each analogy, each of the four factors has its own analogue. The word analogue must not be confused with the word analog. Example sentence 1: This article applies to both analog and digital photography. Example sentence 2: In the analogy between photographic exposure and distance travelled by an automobile, the analogue of F-stop is gear ratio. (See the table below.)
Table of conceptual analogues for the four factors of photographic exposure
Note that the above analogies do not stand up to mathematical scrutiny. In other words, the mathematical formulae that relate the factors in one column are not necessarily the same as those relating the factors in another column. For this reason, they are referred to as conceptual analogues. This brings us to an important point.
Imagine we are told that a house has a water pressure of 60 psi. The kitchen faucet has a pipe diameter of 1.5 cm. We are given a bottle with a mouth with a diameter 0.5 cm and and told that it will take 15 seconds to fill the bottle. Then we are asked how long it would take to fill the bottle if the kitchen pipe were 3 cm instead of 1.5 cm. Obviously, this is not a calculation we would want to do in our heads between photographs. Enter the stop.
In photography, the word stop is used in many situations. If we change the shutter speed, we change it by a certain number of stops. If we change the ISO, we change it by a certain number of stops. If we change the F-stop, we change it by a certain number of stops. What's great about this is that if the shutter speed needs to be changed by 1 stop, this can be perfectly compensated for by changing the F-stop by 1 stop. All you need to know is which direction to change it, and that is where conceptualizing the factors comes in handy. The number scales for each factor are graduated in stops (or sometimes fractions of stops). These number scales are not terribly difficult to remember, as you will see when you read about them in their respective articles—so read on.