F Number again

Had a trawl thro old threads and was sure this had been discussed but can't find anything.

How do f number compare when it comes to the amout of light they let in.

ie: does an f/1.4 allow twice a much light as a f/1.8?

I was sure there was a site that gave this info

Take any f-stop number and multiply or divide by 1.414 (the square root of 2) to get the next full stop up or down. A full stop doubles or halves the amount of light allowed through the lens.

A higher numbered f-stop lets in less light than a lower-numbered f-stop.

"Standard" full f-stop numbering is: f/1.0 (seldom seen), f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8, f/11, f/16, f/22, f/32, etc. Most lenses will not go smaller than f/32.

Thanks

So f/1.4 will let double the amount of light thro than a f/2. F/1.4 and f/2 being full stops?

So f/1.8 is a half stop?

Thats right.

You've got an awful lot of expensive kit considering you don't know the basics about aperture. Maybe it doesn't matter so long as the photos are OK, but it does seem pretty fundemental.......

petrolhead wrote in post #2105575
So f/1.4 will let double the amount of light thro than a f/2. F/1.4 and f/2 being full stops?

So far, so good.

A half stop would be a factor of the square root of the square root of 2, or 1.1892071...

So a half stop wider than f/2 would be f/1.6817928...

robert_a_woods wrote in post #2105597
Thats right.

You've got an awful lot of expensive kit considering you don't know the basics about aperture. Maybe it doesn't matter so long as the photos are OK, but it does seem pretty fundemental.......

I know quite a bit but had forgotten that bit

SkipD wrote in post #2105462
Take any f-stop number and multiply or divide by 1.414 (the square root of 2) to get the next full stop up or down. A full stop doubles or halves the amount of light allowed through the lens.

A higher numbered f-stop lets in less light than a lower-numbered f-stop.

"Standard" full f-stop numbering is: f/1.0 (seldom seen), f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8, f/11, f/16, f/22, f/32, etc. Most lenses will not go smaller than f/32.

I'm glad I'm not the only one trying to understand this. I have been trying to understand this for three years now. And this "A full stop doubles or halves the amount of light allowed through the lens." is the part I keep falling over. Is there a way that you can tell if you are "doubles or halves" just by looking at the f-stop numbering? I have these memorized " f/1.0 (seldom seen), f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8, f/11, f/16, f/22, f/32".

This short read helped me when I was learning. READ

cthomas wrote in post #2105739
this "A full stop doubles or halves the amount of light allowed through the lens." is the part I keep falling over.

Two things to remember that might help.

First, the f/ number is the denominator of a fraction. It represents the diameter of the aperture as a fraction of the focal length. So an f/8 aperture has half the diameter of f/4, just as 1/8 is half the value of 1/4.

Now if you make the diameter of a circle twice as large, it will have four times the area and let in four times as much light (remember that "Pi * radius squared" thing from geometry class?).

Ergo, when you increase the diameter by a factor of the square root of 2 (approximately 1.4), you double the amount of light it lets through.

If you start with the number 1 and repeatedly multiply it by the square root of 2, you will get the progression of numbers that you have memorized.

robert_a_woods wrote in post #2105597
Thats right.

You've got an awful lot of expensive kit considering you don't know the basics about aperture. Maybe it doesn't matter so long as the photos are OK, but it does seem pretty fundemental.......

it's never stopped a blonde driving a Ferrari

fivegallon wrote in post #2105890
it's never stopped a blonde driving a Ferrari

LOL

fivegallon wrote in post #2105890
it's never stopped a blonde driving a Ferrari

and you know she couldn't do the math .

(i better be careful.....my wife, a blonde, is busy in her office studying for a test today for her second masters degree )

ed rader