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

Started Jan 26, 2014 | Discussions thread
Re: A reminder for some memebers as to what the F stop is all about.
1

bobn2 wrote:

veroman wrote:

Sergey_Green wrote:

There is no f2, there is f/2 ...

Wow. Big difference. Surprised nobody caught this before. What a terrible "error."

It's a silly little thing, but quite important. The 'f2' notation doesn't tell you what's going one. For instance, if you say

'The aperture of the lens if f2' it sounds like the aperture is 2 and it's measured in 'effs'. That is quite wrong, the aperture isn't two.

If you say 'the aperture of the lens is f/2' what you are saying is 'the aperture of the lens is f divided by two', so then you ask, what is 'f' - of course it's the focal length, so 'the aperture' is 'the focal length divided by two' and is measured in units of distance, usually millimetres.

So, if someone tells you that the aperture of a 50mm f/2 lens and a 25mm f/2 lens is the same, they are wrong. in the case of the 50mm lens it is 50/2 mm, = 25mm and in the case of the 25mm lens it is 25/2 mm = 12.5mm. If both lenses are collecting light from the same angle of view (for instance because the 50mm lens is working with a sensor of twice the linear dimensions of the 25mm lens, then it is collecting from the same solid angle over four times the area, so it collects four times the amount of light.

Yes it is true that 4 times the amount of light will enter through a 50mm lens at f/2 than a 25 mm lens. But since area of the sensor of the of the camera with a 50mm lens is 4 times larger than the camera with the 25 mm lens, the amount of light falling on the sensor per unit area is the same for both cameras. In other words the light intensity at the sensor is the same for both cameras even though the camera with the 50 mm lens is working with 4 times the amount of light as the camera with the 25 mm lens. I think the problem arises from confusion about the difference between light intensity and total light

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