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

Started Jan 26, 2014 | Discussions thread
Great Bustard Forum Pro • Posts: 43,009

boggis the cat wrote:

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').

Not "physical aperture" but "virtual aperture" (entrance pupil). The virtual aperture is the image of the physical aperture through the FE (front element).

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.

This isn't so.  A FF 100mm lens at f/4 (or a 100 / 4 lens) still has an image circle that covers the entire sensor.  In fact, a 100mm lens at f/4 on FF would project the exact same amount of light on the whole of the sensor as a 50mm lens at f/2 on 4/3 for a given scene luminance and shutter speed.

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