An f-stop, f-number or aperture is a term that is very important in photography and video production. To be technically precise, the f-stop is the ratio of your lens’s focal length in comparison to the size of the iris or entrance pupil. The f-stop or f-number often relates to the ‘speed’ of a lens.
There a lot of mathematical explanations of f-stops to support it’s accuracy and relevancy to the application however in this short article we will be talking practically about the uses of an f-stop to a photographer, videographer or cinematographer.
The iris of the lens (seen below) consists of a varying number of plates that can collapse and contract (or open and close) according to how it is adjusted. The position that the iris is in, or the size of the ‘circle’ is what we assign an f-stop to. Each stop on the lens represents either an increase or decrease of the size of the iris. We call this f-stop increments.
The larger the size of the open iris, the lower the f-stop number thus the smaller the size of the open iris, the larger the f-stop. The fundamental use of the f-stop is for the camera operator to let either more or less light through the lens and onto the image sensor. This will result in an increase or decrease of brightest of the picture you’re capturing depending on which way you adjust your f-stop. It’s a vital tool for photographers videographers and cinematographers as one’s f-stop can mean the difference between an underexposed, correctly exposed or over exposed picture.
The image above is showing three separate images that have each had a different exposure to light. Although these images may have different light levels use to other settings in the such as ISO and shutter speed, this is still representing the fundamental effects that the iris has on your image.
Although you may be thinking that your f-stop is simple a tool to reduce or increase the amount of light that is going into the camera depending on your environment, the f-stop has other visual effects on your image. We call it depth of field.
Without getting too technical and off topic, the image on the left has whats called a shallow depth of field whereas the picture of the right has a very deep depth of field. Depth of field is essentially how much of the image on the z-axis is in focus. You’ve probably noticed the text on the pictures. From this data we obtain that the lens used for both of these photo was a 50mm. The reason for the difference in depth of field for these photos is the f-stop. The image on the left has a very wide iris and the image on the right has a very small iris. And look at the difference! This is a stylistic choice and you will see different extremes of bokeh or ‘blurriness’ from photo to photo. On most professional cameras these days, we have the ability to control our f-stop, and this is the reason why — you can literally change the entire look of your photo with your f-stop.
This should clear anything up for people looking at more information about f-stop.