Recognizing the three elements that affect exposure mentioned earlier (light duration, amount, & ISO sensitivity), you can probably see that they are directly related to each other. When one changes, the others must change to keep the same exposure value. A good way to understand the theory of exposure is to understand how each element is related to the other. Think of exposure in terms of filling a bucket with water. Let’s assume we have a bucket labeled 100 that takes 100 gallons of water to fill, another labeled 200, and another labeled 400. If the sun were a large water hose that was drenching our scene to the tune of 800 gallons a minute we would need some way to control the amount of water so that our bucket would not overflow. . The larger buckets represent a higher ISO. They are larger (more sensitive) and will capture more water and reach capacity (exposure) faster or with less light. The trade off is that they produce more noise in your image. One way would be to control the duration (time) that we allowed the hose to fill the bucket. At 800 gallons per minute we would fill the 100 gallon bucket in 1/8 of a minute. Since the shutter speed controls duration, that is one tool we could use to simply stop the flow after a specific time. However, the shutter speed we need may not be available or it could be that we prefer a higher or lower shutter speed. Another tool we have to use is the f/stop. The f/stop will control the amount of water that enters the bucket. We can use the f/stop to reduce the flow entering the bucket (not the flow onto the scene, just the flow into the bucket). So now if we used a shutter speed of 1/8 and if our f/stop were set a 1 (100%) allowing all the water to enter, we would fill the bucket in just 1/8 minute (given 800 gal/minute filling a 100 gallon bucket). But if we reduced the flow by one-half by changing the f/stop to f/2 (50%), at the end of the exposure (1/8 minute) the bucket would be only half-full. In order to fill the bucket at the new flow rate we would need to increase the duration of the flow to ¼ or twice what it was before. Change the size of the bucket and the shutter speed and f/stop will need to change accordingly. So, relative exposure typically refers to the relationship between the shutter speed and the f/stop. If one changes then the other must also to keep the exposure consistent. With film photography changing the ISO/ASA was not so simple (since you had to change the film) so this was not a typical component of relative exposure. However, with digital imaging all three need to be considered because it’s just as easy to change ISO as it is shutter speed and f/stop. Remember this about relative exposure: the shutter speed, the f/stop, or the ISO setting can be changed to adjust exposure. If the shutter speed is doubled (made faster) then the f/stop must be halved (opened up). If the f/stop is closed, the shutter speed must be slowed. Would that life were so simple! It turns out that there are additional considerations when you change any of the three exposure controls: Change the shutter speed and you will blur or freeze motion in the scene. Change the f/stop and you will increase or decrease the depth of field (the amount of blurred area) in front of and behind your plane of focus. Change your ISO and you will increase or decrease the amount of noise in your image. Several settings will give you a technically correct exposure; now you need to decide what you want your image to say. Read the article on Depth of Field to learn about this subject.