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

Started Jan 26, 2014 | Discussions thread
Almost.
2

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.

Sorry.

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).

Yes.

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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