Question about the Rangefinding Mechanism

[Robert J]

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And BTW, the prism system in the M camera isn't actually a pentaprism. A pentaprism is used to invert an image in an SLR. But I understand there are also SLRs that use mirrors to do the same thing for cost reasons.

 

One of the most famous slr cameras to use a porroflex mirror design to replace the traditional pentaprism were the Olympus pen line of half frame cameras.

[giordano]

One of the most famous slr cameras to use a porroflex mirror design to replace the traditional pentaprism were the Olympus pen line of half frame cameras.

 

There's a cutaway diagram here: Olympus Pen F und Halbformate. The finder defies simple classification because it's neither a porroflex mirror design nor a porroprism design: instead, the "Porro" element (at least in the Pen FT) is provided by a prism followed by a fixed mirror.