When I took my first photography class in high school the teacher, Greg Wahl-Stephens, a man who I owe so much, taught us how to use an all manual Pentax K1000.
He explained exposure in fairly simple terms, which I have heard recently described as the exposure triangle. Like all triangles, there are three sides, but unfortunately, two of those sides have numbers that don’t scale linearly.
F-stops, often referred to as aperture, is the diameter of the opening of the back of your lens that lets the light pass through. These numbers aren’t measurements and their scale basically makes no sense. A smaller number is a big opening that lets in more light, and a larger number is a small opening that lets in less light. Full stops in order from eﬀectively brightest to darkest where each number left to right halves the volume of light passing through are 1.4, 2, 2.8 4, 5.6, 8, 11,16, 22. Your camera will likely will let you adjust these numbers in 1/3 or 1/2 stop increments and some lenses can have f-stops that are smaller or larger than these numbers. The larger the opening the smaller the number, the more your background will be blurred, which is referred to as a bokeh. The smaller the opening the larger the opening, the more will be in focus. This relationship is referred to as depth-of-field. A lens may be at its sharpest at f5.6-8 (you’ll need to find test data for your lens for the answer) and you may want a lot of depth-of-field to record texture and other details.
The shutter speed is the fraction of a second that your sensor is capturing the image. These numbers are actually logical. 1/125 is twice the amount of time as 1/250. While your camera is capable of shutter speeds between these numbers, full shutter speed stops are for example 1/4000, 1/2000, 1/1000, 1/500, 1/250, 1/125, 1/60. Many cameras are capable of shutter speeds from 1/8000 to multiple seconds. Most cameras interact with flashes efficiently at their maximum sync speed, which is often 1/200. Keep in mind, the slower your shutter speed the blurrier your image may be. And another rule of thumb is if you want to avoid your image being blurry from your hands shaking, your shutter speed shouldn’t be lower than 1/ the mm of your lens.
The sensitivity of your sensor to light. The lower the ISO typically, the higher the quality of the image. Most cameras begin at 100 ISO and double in sensitivity every time you double the number. So, these numbers look like this—100, 200, 400, 800, 1600, etc. Your camera likely allows you to change the ISO in 1/3 stop increments, too.
In order to record an image at, let’s say 100 ISO, you’ll need the same volume of light to hit the sensor regardless of the brightness or darkness of the scene. That volume is based on the f- stop and the shutter speed. Think of it as if we need to fill a bucket (the sensor) with water. The f-stop is the diameter of the hose and the shutter speed is how long you need to have the faucet on to fill the bucket.
In the real world, if you want to take a photo outside on a sunny day, your exposure will likely be 1/100 f16 ISO 100. This is an example of the sunny 16 rule, which states that in full sun, the exposure will be 1/ISO f16. Just so we are dealing in “whole” numbers, let’s say the exposure is 1/125 f16 ISO 100.
1/125 of a second is too slow to freeze even the motion of a posing model (for the most part), so we’ll want to use a higher shutter speed, and this is where Mr. Stephens comes back in. He had a slider with all of the shutter speeds on the moving part and all of the f-stops on the fixed side. If you lined up one exposure combination, you had every corresponding combo before your eyes.
Exposures for ISO 100 on a Sunny Day
Let’s say we are outside on a glorious sunny day with the chart above and for whatever reason, we need to shoot at 1/8000 f2.8. The only way we can do that is to increase our ISO to 200 as seen below.
Exposures for ISO 200 on a Sunny Day
Whenever you have a correct exposure and you wish to change one of the three numbers, like the you have to change one of the other two in order to maintain the same volume of light as recorded and the triangle remains in balance. In this example we increased the ISO so we had to increase the shutter speed to remain in balance.