A reminder for some memebers as to what the F stop is all about.

Started Jan 26, 2014 | Discussions thread
boggis the cat Veteran Member • Posts: 6,329
Keep it clear and simple

mapgraphs wrote:

I'll try and make it more understandable,

The f/ number is the Focal Length (FL) divided by the Diameter (D) of the entrance pupil.

so the equation is f/ = FL/D

You're defining f as focal length one moment and then defining f as aperture the next. It just doesn't work that way. So no they aren't the same.


Bob is saying that relative aperture (note relative) is defined as f/D, where f is the focal length and D is the entrance pupil diameter.

The trick is to make it clear whether you are talking about relative aperture (e.g. 'f/2') or physical aperture (e.g. '25 mm').

To get f/2 at f = 50 mm requires an aperture of 25 mm: 50/2 = 25. So, as an example:

  • FourThirds: the ZD 50 mm f/2 lens has a maximum aperture of 50/2 = 25 mm
  • 135: a 100 mm f/2 lens requires a maximum aperture of 100/2 = 50 mm.

Both the above lenses would have the same EFL of 100 mm (EFL means 'Equivalent Focal Length' in 135 terms).  The 135 lens requires twice the physical aperture to achieve f/2 because the lens must cover a much larger sensor.

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