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Old Apr 5, 2007, 12:14 AM   #11
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TDN wrote:
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actually, in theory the multiplying factor should be the square root of 2, which is 1.4142135623730950488016887242097...
And 1.4 alone isn't hard enpugh to calculate in your head without dozens of trailing numbers to make it near accurate? over several stops???


But you are correct is is Sq Rt of 2 based.


And anyone that has studied light is aware of the inverse square law (for distance from light source source) fall off.

It is NOT linear.
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Old Apr 6, 2007, 7:59 PM   #12
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Actually Canon had an f0.95 lens.

http://www.canon.com/camera-museum/c.../s_50_095.html

In my rangefinder days, I dreamed of having one of those monsters.






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Old Apr 7, 2007, 1:13 AM   #13
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Again at what price???? All I said is MOST wold never see a f/1.0 lens..... 1.2 and 1.4 being prohibitavely costly enough.

Picked up a pretty decent 50mm Pentax SMC-A f/2 for $20

Go below that f/ in the A range near $100, and AF well above double/that.... is that 1 or half stop REALLY worth the cost???? (assuming a 1.4 not just a half stop 1.7/8

Recent pime 50mm A f/2 Petax SMC for $20 shipped.... darn decent deal.

Have to have AF well $100 or more above that likely.








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Old Apr 7, 2007, 3:20 PM   #14
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TDN wrote:
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actually, in theory the multiplying factor should be the square root of 2, which is 1.4142135623730950488016887242097...

why? simple.

The amount of light is proportional to the area of the opening of the lens, right?

So an opening that's wice as wide, will let in twice the amount of light.

if we call the f-stop a (so we get f/a for the diameter of the opening), the area is equal to

(f/a)²
xPi

If we want to let half the light in, we need to divide that area by 2., so we get

1/2x(f/a)²xPi

if we bring the 2 inside the brackets we get:

(f/(sqrt(2)xa))²
xPi

if we state that sqrt(2)xa = a' , we see that the area of the new opening (with half the amount of light coming trough and as an f-stop a') is:


(f/a')²xPi

and that a' = sqrt(2)xa


so to let half the amount of light in, we need to multiply the f-stop with sqrt(2)


so if we do this for every stop, when we do it twice to go 2 stops down, we need to multiply twice with sqrt(2), which is of course 2




I guess it isnt exact on the aperture ring, because I think they'd rather write "4" isntead of "3,9597979746446661366447284277872"
Tom, would you fly to Florida and tutor my daughter in algebra? :lol:
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Old Apr 8, 2007, 4:52 PM   #15
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Not a pro and will never be one...Can't ever see the need for less than F/1.4. Don't think I'll ever try to get a 50mm AF at 1.4. Rather get an a to replace my M. Just cheaper and will fill the bill for my amateurish ways.

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Send money and I'll get better lenses!
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