MorrisMini@xxxxxxx wrote:
>
> Hi all,
>
> I'm looking forward with great antipication to Gary Reese's lens tests with
> converters. I started thinking about stacking the two converters (2X and
> 1.4X) and came up with a quandry. *Duh* As a mathematician, I make a good
> plumber.....
>
> How is the focal length calculated? Do you add the converters and then
> multiply times the lens length, or multiply one converter times the lens
> length and that times the other converter? For a 300mm, for example, is it:
>
> 300mm X 1.4 = 420mm
> 420mm X 2 = 840mm
>
> -or-
>
> 300mm X (1.4 + 2)= 1020mm?????
>
> Weird, I can't think "outside the box" on this.
>
> -Tim Clark
>
Tim;
The first method is correct. Since the converters are focal length
multipliers, that's what you do. To write it like your second equation,
it would be:
300 X (1.4 X 2) = 840
I hope this helps.
George
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