F or sometimes referred to as f/ number, it simply mean how much shutter opens up while taking a photo. Smaller the number, wider the shutter/aperture opens. f/1.4 means shutter very wide open. Don't get confused by F 1.4 or f/1.4. They all refer to as aperture 1.4. The smaller the number, the wider the aperture opens. On a 50mm lens, f/1.4 means that shutter is around 36mm wide (diameter) open!!! On the same lens, f/8 means the shutter will open around 6mm and f/22 means shutter will be open around 2.3mm!
How does f number matter for photos?
Even though 22 number for f is 15 times bigger than f/1.4 as a measure of diameter, in terms of area or light allowed to pass through in x amount of time, f/1.4 will take in whooping roughly 225 times more light!!!
In other words to take in same amount of light as f/1.4 in 10 milliseconds, f/22 will need 2250 milliseconds!!!
This article can be very technical but I am going to keep it useful from practical applicability.
The aperture or the opening of the lens or the f number also determines following:
- The higher the f number the smaller the lens opens. The smaller the f number, the lens opens wider.
- To take in same amount of light (or to get same exposure), smaller f number will need the shutter to stay open for smaller time period compared to a bigger f number.
- Importantly, f number also determines Depth of Field. Depth of Field is nothing but how much distance is in focus in your camera. With a smaller f number, very narrow band of area will be in focus. Say for f/1.4, if you are shooting an object 2 feet away, only an inch can be in focus. Say like the between 23.5 to 24.5 inches away from the lens!! Now for the same object, if you change f to f/22, a good 5-6 inches can be in focus! As an example, from 18 inches to 24 inches can be in focus.
- If you taking photos handheld and if you need to have more depth of field, you will need smaller aperture (higher f number). This will require you or the camera to use higher ISO and this can bring in some noise in the image.
When I changed to f/5.6, the second flower also came in some focus and some top leaves too. Now see the third image when I took a photo with f/16. Both flowers, full plant and some of the wall is also in focus. In first photo, you can't even see that there is a wall behind the plant but in the third photo, you can clearly see it. Also with respect to point number #4 above, the first photo has ISO of 100. The image has no noise. f/5.6 required ISO to be at 1250 and f/16 forced the ISO to 5000!! If you see carefully, in the last photo, you can see some noise or graininess.