F-numbers, that is.

A post for beginners, today. About the “F”-number and why it is important. Very important.

The most important number is the minimum f-number a lens can be set to. In other words, the maximum aperture, or lens opening that this lens can go to.

F-numbers? Yes, you know. These numbers: f/16,  f/11,  f/8,  f/5.6,  f/4,  f/2.8,  f/2.0,  f/1.4, etc.  And the lower the F-number, the larger the opening.  (The diameter of the opening, by the way, is the lens’s focal length, f, divided by this number. So a 100mm lens set to f/4 would have an aperture diameter of 100/4 = 25 mm).

So how low can we go?

  • On your consumer lens, that minimum f-number is 3.5 when you zoom out, or 5.6 when you zoom in. So on your lens it says “1:3.5-5.6”. Look at the top or at the very front of the lens.
  • On my photojournalist zoom lenses it says “1:2.8”, meaning that the lens can open to f/2.8 whether I zoom in or out.
  • On my fixed, or prime, lenses, this number says “1:1.4”: i.e. I can open to f/1.4.

This is important for two reasons.

First: lower F-numbers means more selective depth of field, i.e. the ability to blur the background. See these pictures, of a student the other day:

f5.6- 1/60th second

f2.8 - 1/250th second

f2.8 - 1/250th second

f1.4 - 1/800th second

You can see clearly that if you want those blurry backgrounds, you need the low f-numbers. No substitute will do. That’s why we pay for “fast lenses” (this just means “lenses with a low “F”-number”).

Second, the larger the opening (the smaller the f-number), the more light comes in. And hence, the faster the shutter speed can be. Look again at the above images. The “consumer lens” needs 1/60th second – barely fast enough for a sharp image. The “pro zoom” needs just 1/250th second. Nice. And  the fast prime lens needs only 1/800th second: a stunningly fast shutter speed that will freeze any motion.

The above images, and those numbers, show very clearly why I would rather shoot a party with a prime f/1.4 or f/2.0 lens than with a zoom lens. If there is one investment you might want to consider making it is that fast lens.


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