A reminder about F-stops

A bit of revision, since I was asked this three times in the last few days. “Why are F-stops important?” Yes – repeat answer, but important enough to repeat, I think. So here goes.

F-stops (the clicks on the aperture adjustment ring on old cameras, hence “stops”) are important. You know: f/5.6, f/8, f/11, that sort of thing. Or on a good lens, maybe f/2.8, or for a prime (non-adjustable) lens, even lower. The lower a lens can go, the better.


  • The lower the f-stop (it is “f divided by that number”), the larger the diameter of the lens opening while the picture is being taken.
  • In other words, “lower f-number” equals “larger lens opening”.
  • The main f-numbers (16, 11, 8, 5.6, 4, 2.8, 2, 1,4…) mean twice as much light each time you go to a smaller number.
  • (That is why the ratio between these numbers is the square root of 2: after all, since the area of a circle equals pi x r squared, to halve the light that comes in you must reduce the diameter of a circle by the square root of 2)

This means two things:

  • That lower f-number means that more light gets in. Hence, you get the ability to take pictures in lower-light conditions at the same ISO. See yesterday’s post for why that is important.
  • The low f-number also means blurrier backgrounds. The wider the lens, the more paths an out-of-focus part of the picture can follow to reach the sensor; hence, the blurrier that out-of-focus element becomes.

Blurry backgrounds and low-light pictures are good things. So look at the front or top of your lens. If the lens says “1:3.5-5.6″ it is a kit lens that can open to f/3.5 at wide angle and f/5.6 when zoomed in. A pro zoom lens will just say 1:2.8”.

And the new 50mm prime lens I am buying, used, tomorrow says 1:1.2, meaning f/1.2 – meaning it is faster (more light gets in) than the 1.4 lens I now have.

Alas, more light means more glass, and hence more dollars. But you will see why this is worth it.

In general, get the fastest lens you can get, if you can afford it.

One thought on “A reminder about F-stops

  1. Except that faster lenses are often softer… Canon’s f/1.2L 50 is notably softer than the f/1.4 at say f/2. The f/1.0L, (which is just an AWESOME piece of glass) is incredibly soft even stopped down. Notably several of Canon’s f/4 zooms are sharper than the f/2.8s too. Nitpicking maybe, I’d still always chose the f/2.8s, but in the case of the REALLY wide lenses like the f/1.2L, I guess it depends on what kind of picture someone is looking for! Love your blog! 🙂

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