Exposure (3): Aperture

[PhotoHues]

Thanks a lot for this valuable info.

[ccting]

1.8 is near 9 stops faster than 5.6
2.0 is about 8 stops faster than 5.6, 1 stop faster than 2.8
2.8 is about 3 stops faster than 5.6

am i correct?

[ccting]

50mm 1:1.8
This is a single focal length or prime lens of a maximum aperture of F1.8

Why there is a number before the ":"?

Up to date, i never see lens with number other than "1"...

[RichardTaylor]

Q.
Why there is a number before the ":"?
It indicates a ratio of the effective aperture to the focal length on a lens marking.

Look at the markings on your own lenses.


Q.
1.8 is near 9 stops faster than 5.6
2.0 is about 8 stops faster than 5.6, 1 stop faster than 2.8
2.8 is about 3 stops faster than 5.6

Incorrect
-----------------------------------------
It's this:
1.8 is near 3 stops faster than 5.6
2.0 is 3 stops faster than 5.6, 1 stop faster than 2.8
2.8 is 2 stops faster than 5.6

[ccting]

Thanks..

Ok.. i got the formula ;D

Question 1:

it is just a division? f2/f1?

For instance, how many stops faster = 1/(x/y) = y/x, where x is the referred f-number, and y is the base f-number.

5.6/1.8 = 3.11 = 3 stops (approx.)
5.6/2.0 = 2.8 = 3 stops (approx.)
2.8/2.0 = 1.4 = 1 stops (approx.)

Am i correct?

[RichardTaylor]

No.

It is lot more noticeable at the end of the aperture ranges like F1.4 to F2 and F16-F22.

I just remember the aperture "scale" as per the diagram in post #1 of this thread..

[Doug Sundseth]

F-stop is the ratio of aperture width to focal length. A 100mm lens at F/2 has a 50mm width aperture. A 100mm lens at F/4 has a 25mm aperture. Light is proportional to the area (not the width) of the aperture and area is proportional to width^2.

The result of this is that the "stops" in F-stops are powers of 2^1/2: 1 (=2^0), 1.4 (=2^0.5), 2 (=2^1), 2.8 (=2^1.5), 4, 5.6, 8, 11, 16 etc.

[ccting]

Originally Posted by Doug Sundseth

F-stop is the ratio of aperture width to focal length. A 100mm lens at F/2 has a 50mm width aperture. A 100mm lens at F/4 has a 25mm aperture. Light is proportional to the area (not the width) of the aperture and area is proportional to width^2.

The result of this is that the "stops" in F-stops are powers of 2^1/2: 1 (=2^0), 1.4 (=2^0.5), 2 (=2^1), 2.8 (=2^1.5), 4, 5.6, 8, 11, 16 etc.

Thanks Doug.. that is very clear explanation.. wow, that makes variable aperture easier to design.. ;D