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I have been using the 'wild country' chemical hand warmers with film lenses.
i stock up on them from rays outdoors (they only sell them in winter) they cost 1.50 each and are good for 8 hours.
i found that it is best to open them approx 15minutes before use and keep them in a pocket. then set f stop and focus, attach with rubber band to the topside of the lens, check focus then dangle a sock or something to insulate from cold air. (they dont work great on cold metal or plastic surfaces with out insulation)
The heat does not seem to affect the image or the focus during exposure.
i've attached an image, temp was 1 degree that night.
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