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I've searched online a lot, and found plenty of sites that quote both numbers.

There appears to be agreement that the C-mount flange focal distance (FFD) is 17.526 mm (exactly 0.69 inches).

The CS-mount is either 5.0 mm less or 5.026 mm less.

Does anybody have a definitive answer?

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    \$\begingroup\$ why do you care \$\endgroup\$
    – aaaaa says reinstate Monica
    Commented Jul 28, 2018 at 22:05
  • \$\begingroup\$ Why do I care? To know where to put the lens so it'll focus to infinity. (Whyever else?) \$\endgroup\$
    – nerdfever.com
    Commented Jul 30, 2018 at 3:51

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CS mount is 5mm less than C mount. However, there is no such thing as "exactly", and a 0.026mm difference isn't significant as long as lens elements can move far enough to focus.

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