1/3 increment fstops

I've been trying to work this out for several days and lord knows I'm no wizard

at math but I just can't seem to figure it out. I have Ansel Adams 3 book series

but he just teased me. I'll explain how in a moment.

 

My question is this. If a change of 1 full fstop doubles or halves the light

reaching the film, what does 1.4 and 1/3 stops do in that regard. How much less

light reaches the lens if I close down 1/3 of a stop?

 

Ansel Adams says see book two because it is equivelent to a change in asa number

of one asa number. But in book two he merely says ever 3 asa numbers = 1 stop.

 

I know I get hung up on the numbers, maybe because I'm not good at math, but I

ran it past a friend of mine who is very good at math and he couldn't figure it

out either. Can someone please tell me by what percentage the light increases or

decreases at a change of 1/3 fstop and 1/4 fstop?

 

Thanks

Tim

Tim,

 

Have a think about this:

 

...the formula for any ISO value is the base value multiplied by 2 to the power of the stop increment. So 125 opened up 1/3 of a stop is 160: (125 x 2^0.33)<div></div>

Numbers in the full stop step sequence are related by a factor of 2^(1/2) = (1.414), adjacent numbers in the 1/2 stop sequence are related by a factor of 2^(1/4) =(1.189) and in the 1/3 stop sequence they are related by a factor of 2^(1/6) (1.122).

 

The amount of light passing at he various aperture is given by 2^(fractional stop), so for 1/3 stops it's 2^(1/3), which is the cube root of 2 which is 1.26, so opening up by 1/3 stop allows 26% more light to pass.

 

For a 1/4 stop it's 2^(1/4) which is 1.189, so opening up by a quarter stop allows 18.9% more light to pass.

 

Bob Atkins - Technical Editor, Photo.net

The old 1960's and 1970's SPIE engineering books have the asa specs as being grouped to the nearest 1/3 stop. Thus the recommended film box numbers are marked in a sequence of 100, 125, 160, 200 . . . the cube root of 3 is about 1.26. For practical reasons the numbers are rounded.

And third stop increments sorta match .1 logarithmic increments used in density and signal/noise ratios.

A 1/3 stop increase is about 26 percent more light. A 1/4 stop increase is about 19 percent.

 

What I've never understood is why, since film speed increments are 1/3 stop, lenses are adjustable in 1/2 stops.

"What I've never understood is why, since film speed increments are 1/3 stop, lenses are adjustable in 1/2 stops."

 

And who promised you that photography was going to be simple ? It is the same reason for which Leonardo used to write using a mirror image and a code: only people smart enough to figure this out should be allowed to practise the art...:D