Oly 45 1.8 with Pana GTC1 converter... works !!!

Started May 17, 2012 | Discussions thread
azazel1024 Senior Member • Posts: 1,139
Re: Oly 45 1.8 with Pana GTC1 converter... works !!!

MAubrey wrote:

azazel1024 wrote:

A 2x teleconverter results in a two stop loss of aperature. So the 45mm f/1.8 is actually a 90mm f/3.6 with a 2x teleconverter on there (I am not speaking of depth of field, I mean out right the aperature if f/3.6 with a 2x teleconverter on there).

That's only true when they're attached to the back of the lens. teleconverters attached to the front of a lens are an entirely different issue. Back-attaching teleconverters cause light loss because they move the lens groups further away from the sensor, but that doesn't happen in the case of ones that attach to the front.

Not true in the least. The focal length with a 2x teleconverter, no matter WHERE it is placed in the optical assembly, doubles focal length. The physical Iris of the lens stays the exact same size. The 45mm lens has a 25mm Iris (45/25 = f/1.8), doubling the focal length to 90mm no matter how you do it results in a 90mm focal length, but you still have the same 25mm Iris as the Iris of the lens in this case is where the aperature blades are located (controlling the iris size). If the Iris placement where to somehow shift (which can't happen without changing the internal optics of the existing lens), the current physical obstruction of the aperature blades even when wide open would result is severe vignetting and the apertature blades would no long be able to control the light levels and DOF of the lens. The result 90mm/25mm = f/3.6.

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