Noob question. how does aperture relate to f-stop?

[ulrichsd]

I've read multiple sites say the difference between the f/1.8 and f/1.4 is 2/3rds of a stop different... but the aperture difference of 1.8-1.4 = 0.4, not 2/3rds (0.667).

So I guess my question is why is 1.8 to 1.4 a 2/3rds stop difference, not 2/5ths?

Thanks for answering,
Scott

 

[Compaq]

Well, there's a formula to calculate this. The square root of 2 is important.

One stop reduction (halving the light that hits the sensor) is a....well... just see here:

f/1.4 = f/(sqrt2)^1
f/2 = f/(sqrt2)^2
f/2.8 = f/(sqrt2)^3

 

[clanthar]

A stop is twice as much or half as much light (factor of two).

f / 1.8
focal length divided by 1.8 = diameter of aperture opening.

If you double or halve the diameter of the aperture the area of the aperture will change by a factor of 4 (two stops).

Joe

 

[KmH]

I've read multiple sites say the difference between the f/1.8 and f/1.4 is 2/3rds of a stop different... but the aperture difference of 1.8-1.4 = 0.4, not 2/3rds (0.667).

So I guess my question is why is 1.8 to 1.4 a 2/3rds stop difference, not 2/5ths?

Thanks for answering,
Scott

Because a stop is a halving or a doubeling of light, and the lens aperture AREA, not the lens aperture diameter, is what is being doubled or halved, is why the square root of 2 -1.4142 is used and not 2 itself.

F-number - Wikipedia, the free encyclopedia

 

[ulrichsd]

Thanks everyone for the responses, makes much more sense now.

f/1.4 to f/2 is one stop reduction, same as f/4 to f/5.6 is one stop reduction.

 

[KmH]

Yep. The full stop steps are f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16/ f/22, f/32.

You can also make full stop steps strating from a f1normally 1/3 stop value, like f/6.3. a full stop reduction from f/6.3 is to f/9.

Note though that they make many lenses that have a max aperture of f/2.8, not f/2. Why do you think they do that?

To keep the same exposure, but slightly alter the DOF in the scene because you reduced the aperture 1 stop, increase only the ISO 1 stop , or only the shutter speed 1 stop, or any conbination of the 2 that adds up to a full stop. Like a 1/3 stop change of the ISO, and a 2/3 stop change of the shutter speed.

The same applies for any of the 3 exposure triad settings.