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View Poll Results: f/2.8 + 1.4 TC =
3.5 1512.50%
4.0 9075.00%
4.5 97.50%
What are you talking about? 65.00%
Voters: 120. You may not vote on this poll

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11-21-2007, 04:44 PM   #1
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f/2.8 + 1.4x TC =

Survey says?

11-21-2007, 05:08 PM   #2
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It cuts out 1 stop of light. But you will still be able to set your lens to f2.8, but just not as much light will get through and the camera's auto exposure settings will adjust for the lesser light.
11-21-2007, 06:03 PM   #3
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I'm pretty sure that it effectively "eats" 1.4 stops of light. However, it is usually rounded to 1 stop for ease of calculation. The correct answer would be 4.5, IMHO.
11-21-2007, 06:24 PM   #4
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Mathematically it's actually 4.595 (approx).

11-21-2007, 07:08 PM   #5
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F-stops in 1stop increacements :1.0, 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11.0, 16.0, 22.0, 32.0, 45.0, 64.0 ......

are full stops. So correct answer would be F4 as a 1.4xTC cuts 1 stop of light.

Thanks

"standard f-stop scale that corresponds to the sequence of the powers of "

Last edited by Mikhail; 11-21-2007 at 07:15 PM.
11-21-2007, 07:12 PM   #6
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I'm pretty sure 1.4× is used precisely because it approximates √2, which is the ratio between successive full F stops (rounded by convention). Which would make it f/4. A 2× teleconverter is conveniently back to integers since of course √2×√2=2, so that costs two stops.
11-21-2007, 07:18 PM   #7
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QuoteOriginally posted by jfdavis58 Quote
Mathematically it's actually 4.595 (approx).
What math are you using? 2.8 x 1.4 = 3.92 (~4) By that math a 2x TC would drop me back to f/5.6.
11-21-2007, 07:18 PM   #8
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QuoteOriginally posted by mattdm Quote
I'm pretty sure 1.4× is used precisely because it approximates √2, which is the ratio between successive full F stops (rounded by convention). Which would make it f/4. A 2× teleconverter is conveniently back to integers since of course √2×√2=2, so that costs two stops.
What he said!

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11-21-2007, 07:21 PM   #9
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Well its actually √2 x F2.8, and √2*2 x F2.8 = F5.6 is the correct way to do it, but they made the numbers work out nice and easily because not everyone is good at math!

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11-21-2007, 07:26 PM   #10
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QuoteOriginally posted by jfdavis58 Quote
Mathematically it's actually 4.595 (approx).
Okay, I give up. How are you calculating that value?
11-21-2007, 07:43 PM   #11
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a: aperture
D: Effective diameter of lens opening
f: focal length

a = f / D - that's the definition of aperture

You have in your example a = 2.8:

2.8 = f / D -- we will use this ratio below.

Now you add the 1.4x converter, so the new focal length f' = 1.4 x f

The new aperture

a' = f' / D = (1.4 x f) / D = 1.4 x (f / D) = 1.4 x 2.8 = 3.92 ~= 4

Last edited by Ole; 12-02-2007 at 09:37 AM.
11-21-2007, 07:44 PM   #12
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I be the one not good at math.
11-21-2007, 09:53 PM   #13
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I'm getting a headache:-).
11-21-2007, 09:56 PM   #14
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OK, I am going to go with 4.
11-22-2007, 07:39 AM   #15
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Nice thread, cleared that up for me !
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