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Auxiliary Wide Lens Focal Length & Aperture Conversion


Devon
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I just got me an old set of Yashica auxiliary lenses from the antique store. See the image below for the included manual.

Instruction Sheet for Yashica Auxiliary Lens set (Electro 35)

Doesn't an auxiliary lens keep the same aperture in relation to it's COMBINED (taking lens + auxiliary) focal length? For the wide auxiliary lens, why does it specify "f4" for the maximum aperture? (I think the multiplication factor of the wide auxiliary lens is really .84x aside from the manual stating "0.8x".) These lenses were usually sold for the Yashica Electro 35, which came with a fixed lens: 45mm F1.7.

For example:

Electro 35 Lens with no adapter:
Focal length: 45mm
Entrance pupil diameter (aperture): 26.47mm (f1.7)

Add the wide auxiliary lens:
Focal length: 45 x 0.84 = 37.8mm
Entrance pupil diameter (aperture): 26.47 x 0.84 = 22.23mm
Divide focal length by entrance pupil: 37.8 / 22.23 = 1.7 (f.17)

Electro 35 Lens WITH wide auxiliary adapter = 37.8mm f1.7

Can somebody help me understand where "f4" comes from? Is my math/thinking wrong?

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