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

Started Jan 26, 2014 | Discussions thread
Messier Object
Messier Object Veteran Member • Posts: 8,422
Re: A reminder for some memebers as to what the F stop is all about.
7

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.

Bob, the words have no "sound" until read out loud, in which case it sounds like "eff too". Who would read it out loud as "eff divided by too". Maybe in a formal setting a speaker giving a presentation would read it out as "an f ratio of two", or maybe even "a focal ratio of two". But most would read it out loud as "eff too"

As for "f2" sounding like the measure is in "effs", the only time we see the unit coming before the quantity is with currency $2

So really you should have written "if you write", not "if you say", and "reads like", not "sounds like".
Yes it's a silly little thing, but quite important, especially when being critical of others.

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.

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Bob

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