Sony HDW-730 Operation Manual page 341

Fog-proof filter
Depending on the temperature and humidity, the protecting filter may
mist because of vapor or your breath. To ensure that the viewfinder is
always clear, replace the protecting filter with a fog-proof filter (Part No.
1-547-341-11, not supplied).
Fitting the fog-proof filter
Replace the protecting filter on the packing ring with the fog-proof filter.
Be sure to correctly assemble the fog-proof filter, the packing ring, and
the eyecup so that the reassembled eyepiece is waterproof.
Note
When cleaning the fog-proof filter, wipe it very gently with a soft cloth
to avoid impairing the anti-fogging coating.
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Chapter 6 Maintenance