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Dust on the mirror turned out to be a cracked prism on a Canon FTb
I was working on a Canon FTb that came in from a guy in Portland, the viewfinder showed this weird dark crescent shape that I figured was just some stubborn dust on the mirror. Spent a good 40 minutes blowing air and using a brush, even tried a swab on the mirror surface before I noticed the crescent moved when I shifted the camera angle. That's when it hit me, the light path goes through the prism before the mirror, so what I was seeing had to be inside the prism itself. Cracked the prism housing open and sure enough, there was a hairline fracture running along the top edge, probably from a drop that the owner didn't even remember. I ended up swapping in a used prism from a parts body, cost the guy about $120 total instead of a full CLA. Has anyone else chased a viewfinder ghost for way too long before realizing it wasn't the mirror or the screen?
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umab868d ago
$120 for a used prism swap sounds like a steal compared to what a full teardown would've cost. Gotta love when the "dust" turns out to be a fracture that moved every time you tilted the camera, like it was messing with you on purpose. I once spent half an hour scrubbing a "smudge" off a Pentax screen that turned out to be a scratch catching the light at one angle. Lesson learned: if the spot changes when you rotate the body, stop cleaning and start looking somewhere else.
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