philippe.amard <philippe.amard@xxxxxx> wrote:
> Wiki: "the f-number N = f / D, with focal length f and aperture diameter
> D."
>
> The actual amount of light that goes through the lens remains the same, just
> the math tricks us into believing that we gain anything in term of f-stop
> ;-)
> I bet the light-meter won't be fooled by the advert.
The math is solid and the meter will definitely see the difference.
Suppose you put a 1.4x teleconverter on a 100/f2 lens. Effective
focal length increases but you lose a stop, so in effect you have
a 140/f2.8 lens.
The Speedbooster is just the inverse of that, a .71x (1/1.4) converter;
put it on a 135/f2.8 and you get a 100/f2 give or take a few mm.
Where there is a potential problem is in the quality of the conversion
lens; reviews I have seen say the Speedbooster is very good, so I'd
be inclined to grit my teeth and pay their $400 price rather than $99
for a Chinese copy on Ebay.
--
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