Tuesday, July 16, 2013

What is this f word (I mean f number) in photography?

F is popular not only in Hollywood movies as some word but f number is even more popular among photographers!

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:
  1. The higher the f number the smaller the lens opens. The smaller the f number, the lens opens wider.
  2. 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.
  3. 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.
  4. 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.
Here are 3 photos I took of a flower in my backyard. I used Canon EOS-M and a 50mm Super Takumar F1.4 lens. As you can see, at f/1.8, only the flower in the center is in focus! Nothing else! Background is blurred and it gives a nice 3D kind of view!
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.

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