Definition of a "stop"

[Destin]

Alright guys, your all gonna think I'm a moron for asking this (or at least I feel like one).

What is the exact definition of a stop? I was always taught that each stop let in double the amount of light. Which numbers are full stops? Wouldn't it be logical to double the number? ex.) f1.4- f2.8 should be one stop? Then why does everyone say that 1.4 is a stop faster than 1.8? Which is right? I really should know this, guess I just never asked or needed to know...

 

[reznap]

Alright guys, your all gonna think I'm a moron for asking this (or at least I feel like one).

What is the exact definition of a stop? I was always taught that each stop let in double the amount of light. Which numbers are full stops? Wouldn't it be logical to double the number? ex.) f1.4- f2.8 should be one stop? Then why does everyone say that 1.4 is a stop faster than 1.8? Which is right? I really should know this, guess I just never asked or needed to know...

Actually.. f/2 (not f/1.8) is a stop less than f/1.4.
And that full stop less (f/2) has half of the light gathering area than f/1.4.

If you want to look at a few decent scales, wikipedia has a decent write-up.

F-number - Wikipedia, the free encyclopedia

Stops double ISO numbers, so 50, 100, 200, 400 and so on..
Shutter speed doubles/halves respectively too.

 

[sanpan]

From what I understand, each full stop allows double or half the light depending whether you are stopping up or down. Instinctively one feels that the f numbers should always be double as well.
This is true in case of shutter speeds, AFAIK. 1/50 allows twice as much light as 1/100.
If my memory serves me right, if you numerically halve the f-number, the amount of light passing through increases 4 times. So f/11 allows 4 times more light than f/22.
Hope this helps.

 

[reznap]

If my memory serves me right, if you numerically halve the f-number, the amount of light passing through increases 4 times. So f/11 allows 4 times more light than f/22.
Hope this helps.

I've never heard that before but I did the math and it seems to work out that way. Nice and easy to remember. :thumbup:

 

[Destin]

Alright guys, thanks!! That seems to make sense! I've been meaning to ask that for weeks now lol

 

[OrionsByte]

For ISO and shutter speed, doubling and halving the number is equal to one stop, because it doubled or halves the exposure. The reason aperture is different is because it refers to an AREA.

So here's a quick mental exercise: imaging you have a square tile that measures 1-foot by 1-foot, and you want to double the area that is covered by tiles. If you simply double the width of the tile surface by adding a tile below and to one side, then to make a square you'd have to add another tile, right? That's four tiles now. So by doubling the WIDTH you quadrupled the AREA.

Without going in to all the math, the correct number to use to double the area is the square root of 2, which is roughly 1.4. 1.4, 2, 2.8, 4, 5.6, 8, 11, 16... That's where those come from.

Since the square root of two times itself IS two (by definition), doubling the aperture value is equivalent to two stops (Av * 1.4 * 1.4 = Av * 2), or four times the light (or area) as pointed put before.

Hope that helps.

 

[Boomn4x4]

Alright guys, your all gonna think I'm a moron for asking this (or at least I feel like one).

What is the exact definition of a stop? I was always taught that each stop let in double the amount of light. Which numbers are full stops? Wouldn't it be logical to double the number? ex.) f1.4- f2.8 should be one stop? Then why does everyone say that 1.4 is a stop faster than 1.8? Which is right? I really should know this, guess I just never asked or needed to know...

You are correct in that one stop is half (or double) the amount of light. Where you are missing the logic is in understanding what those numbers mean. You are alos missing a critical part of your "number".

The correct typing is f/1.4 or f/2.8. You are missing the "slash"... or more correctly the "divided by" sign.

The "f" is an abbreviation for "focal length"... So the "number" f/2 is literally "Focal length divided by 2" So if your aperture is f/2 and you have a 50mm lens, the diameter of the lens aperture is 25mm (50/2).

So now using math, pi * radius squared, we can find the area. 3.14 * 12.5 ^2.... or 3.14 * 156.25 = 490 square milimeters.

As mentioned 1.4 is one "stop down" so we would expect that to let in twice as much as 2... so 980 mm

Doing the math

50mm / 1.4 = 35.71
3.14 * 17.8^2
3.14 * 317.8 = 998 mm

Now, I rounded some number so they don't come out exact, but as you can see f/1.4 has double the area as f/2 thereby letting in double the light

This is why telephoto "fast" lenses are so expensive. When you have a 200mm lens with a f/2.0 aperture.... you end up having an opening of 100mm in diameter, 10 cm, or about the size of a grape fruit.... that is a lot of glass to engineer.

 

[table1349]

A Tedious Explanation of the f/stop

 

[KmH]

OMG, I love math.

 

[Destin]

Alright guys, your all gonna think I'm a moron for asking this (or at least I feel like one).

What is the exact definition of a stop? I was always taught that each stop let in double the amount of light. Which numbers are full stops? Wouldn't it be logical to double the number? ex.) f1.4- f2.8 should be one stop? Then why does everyone say that 1.4 is a stop faster than 1.8? Which is right? I really should know this, guess I just never asked or needed to know...

You are correct in that one stop is half (or double) the amount of light. Where you are missing the logic is in understanding what those numbers mean. You are alos missing a critical part of your "number".

The correct typing is f/1.4 or f/2.8. You are missing the "slash"... or more correctly the "divided by" sign.

The "f" is an abbreviation for "focal length"... So the "number" f/2 is literally "Focal length divided by 2" So if your aperture is f/2 and you have a 50mm lens, the diameter of the lens aperture is 25mm (50/2).

So now using math, pi * radius squared, we can find the area. 3.14 * 12.5 ^2.... or 3.14 * 156.25 = 490 square milimeters.

As mentioned 1.4 is one "stop down" so we would expect that to let in twice as much as 2... so 980 mm

Doing the math

50mm / 1.4 = 35.71
3.14 * 17.8^2
3.14 * 317.8 = 998 mm

Now, I rounded some number so they don't come out exact, but as you can see f/1.4 has double the area as f/2 thereby letting in double the light

This is why telephoto "fast" lenses are so expensive. When you have a 200mm lens with a f/2.0 aperture.... you end up having an opening of 100mm in diameter, 10 cm, or about the size of a grape fruit.... that is a lot of glass to engineer.

HOLY CRAP!!!! BOOMN4x4 can actually give helpful information?!?!?!?!?! :mrgreen: My jaw just hit the floor

Haha just kidding, Thanks man, your explination clarified alot for me man. Thank you!

 

[Boomn4x4]

HOLY CRAP!!!! BOOMN4x4 can actually give helpful information?!?!?!?!?! :mrgreen: My jaw just hit the floor

Haha just kidding, Thanks man, your explination clarified alot for me man. Thank you!

I don't give bad information, i just have an unpopular opinion that there are better subjects to choose to photograph than your own children when trying to learn photography.