How would you define a "stop"

[tirediron]

I believe, as I understand it.

A stop is a unit of measurement used in exposure, in terms of ISO full stops are measured by doubling or halving ISO value.

"A stop is the unit of measurement used in the three elements of the exposure triangle. A difference of one full stop represents a doubling or halving of the value of that unit relative to the next full stop above or below it"

That works perfectly! Thanks all!

 

[tirediron]

ISO is easy to explain.. wait til they ask you why f/2.8 is not a full stop from f/1.4

"That's covered in the hand-out you received at the beginning of the evening".

 

[phiya]

ISO is easy to explain.. wait til they ask you why f/2.8 is not a full stop from f/1.4

"That's covered in the hand-out you received at the beginning of the evening".

speaking of which, I've never really thought about this, but now that I do - it would be a function of pi am I correct? We're talking about the area of the circle (aperture), doubling that area doubles the light, and it doesn't happen at full aperture values? Or am I completely off?

 

[Big Mike]

The F-number is a ratio between the focal length and the diameter of the entrance pupil.
And since the amount of light entering the pupil, has to do with the area (which is circular) then yes, Pi in a factor in the equation.

Maybe a more simple way to thing of numbers, is that you get from one stop to the next by multiplying or dividing by the square root of 2. (1.41423562)

 

[Studio7Four]

You're just about right, it is related to the area of the opening, which is Pi times radius(squared). Pi is a constant, so it's actually related to the radius. To double the area you multiply the radius by the square root of two...which happens to be (approximately) 1.4. That's why (with some rounding), your scale is 1.4, 2, 2.8, 4, 5.6...

 

[Sonoma]

Try this...........

Definition of stop - iDigitalPhoto Dictionary

Maybe you can get what you need from this; condense it.

Good luck
Gary