When I first go on-set, I find the composition that I want. Then next step for me is determining which F-stop I want to shoot at before I even set up my lights. When you first start out in photography, one of the hardest things to grasp is what camera combinations should you use to get the correct exposure. There are three factors that determine what the exposure of your image will be: ISO, shutter speed, and aperture (F-stop). There are a lot of posts out there that tries to explain this but it gets a little confusing with all the technical mumbo jumbo that’s not needed to understand how it works. This post will explain it in layman’s terms to help you get the right exposure every time.
F-stop is the ratio between the diameter of the aperture in the lens and the focal length of the lens, but we won’t get into the mathematics. The F-stop which is also known as the aperture, regulates how much light is allowed through the lens by varying degrees. The wider the opening the more light it lets in and the reverse is true. It functions similar to an iris, at night your pupils dilate to let in more light, during the day when it’s bright the pupils gets smaller because not as much light is needed.
When I’m on-set, the reason I determine the F-stop first before anything else is because F-stop also allows you direct your viewers eyes. If you shoot wide open, meaning that you allow the maximum amount of light through your lens, your depth of field will be smaller. This allows you to focus on your subject and blur out details of the surrounding background. (The smaller the F-stop number, the larger the opening, the more shallow depth of field.) So if you were to shoot a portrait, and you set your camera to shoot at F1.2, you can literally just get the focus on the tip of the nose and blur everything else out. The higher the F-stop number, the more detail your images will show. So if you were to shoot a fashion spread and wanted every detail of the environment to be in focus, you would set your F-stop to F8-F11.
After you figure out what F-stop you want to shoot at, you will then adjust your shutter speed to get the correct exposure. Shutter speed is simple and straight forward. The only time that you would want to figure out what shutter speed to shoot at before figuring the F-stop is when you’re wanting to add motion blur or freeze motion.
Most of the time your ISO will be set to 100, or 200 for Nikon users. But when you can’t achieve the right exposure with the combination of F-stop and shutter speed, you bump up the ISO to allow more light into the camera. The higher you go in ISO the more grain your image will have, although high end cameras now can handle ISOs up to 1600 without any noise.
Using these three together you can come up with many different combinations for the same exposure, but how you choose to get there is all about your creativity. I hope this helps.