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Re: [OM] Standard lens?

Subject: Re: [OM] Standard lens?
From: Moose <olymoose@xxxxxxxxxxxxx>
Date: Wed, 02 Apr 2003 01:32:50 -0800
I guess my math doesn't agree with yours. I come up with 1/3 stops of
1.000
1.095
1.225
1.414
1.549
1.732
2.000

I didn't use meters or tables. After all, they have things like a progression of shutter speeds of ...1/30, 1/60, 1/125, 1/250, which don't follow the correct math sequence. I used simple math to convert changes in area of the front element to diameter and then to focal ratio (f-stop), i.e., a decrease of 1 stop is a decrease of the area of the FE by 1/2, a decrease of 1/2 stop is a decrease of the area of the FE by 1/4, etc. I know that this doesn't mean anything in practice, but it does in theory.

As 1.8 is closer to and slightly larger than 1.789, I conclude that it 1/4 stop faster than 2.0, not 1/3. Lama, that's the reason I used 1/4 stops in the first place, all the apertures I was illustrating fell on 1/2 or 1/4 stops increments except f1.2, which is partway between the 2 sets of increments.

Moose

John A. Lind wrote:

That's definitely on the right track, but not quite completely correct. Nearly all interim f-stops are on 1/3 stop increments, not 1/4 stop increments. f/1.2 is one of the exceptions . . . 1/2-stop faster than f/1.4.

Some common lens speeds at other than full stops:
  f/1.2:  1/2-stop faster than f/1.4
  f/1.7:  1/2-stop faster than f/2
  f/1.8:  1/3-stop faster than f/2
  f/3.5:  1/3-stop faster than f/4
  f/4.5:  1/3-stop slower than f/4
  f/6.3:  1/3-stop slower than f/5.6

These all taken from my Weston meters (1/3-stop: Master V; 1/2-stop: Ranger 9) and confirmed by my math regarding 1/3-stop and 1/2-stop increments. In general, f-stops and film speeds run in 1/3-stop increments.

-- John

At 16:43 3/31/03, Moose wrote:

The simple math is that 1/4 stop increments (to more decimal places than have any possible practical meaning) from f1 to f2 are
1.000
1.069
1.155
1.265
1.414 (sqrt of 2)
1.512
1.633
1.789
2.000

As you can see, 1.8 is 1/4 stop faster than 2.0 and 1.4 is 1 stop faster than 2.0. 1.2 doesn't fall directly on either 1/4 stop or 1/3 stop, but is roughly 1/2 stop faster than 1.4.

Moose

Andrew Dacey wrote:

I could be wrong here, but I believe that 1.0 -> 1.2 -> 1.4 -> 1.8 -> 2.0 are all half-stop increments. So the 1.4 is 1/2 stop faster than the 1.8 and the 1.2 is a further 1/2 stop faster still.




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