An uncoated Sonnar must be very old indeed! Probably it is just (lightly) single coated. The advantage of a multicoating in a Tele lens like this is not very great. I would go for the best condition.
Concerning Ed Lau's recommendation to fit a filter in front of the lens: once the light has passed through the filter, it will behave exactly the same as when there wsa no filter, except thar now light reflected by the lens will be reflected back into it by the filter. That is not an improvement.
zeiss Jena coated vs zeiss Jena uncoated
in Medium Format
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An uncoated Sonnar must be very old indeed! Probably it is just (lightly) single coated. The advantage of a multicoating in a Tele lens like this is not very great. I would go for the best condition.
Concerning Ed Lau's recommendation to fit a filter in front of the lens: once the light has passed through the filter, it will behave exactly the same as when there wsa no filter, except thar now light reflected by the lens will be reflected back into it by the filter. That is not an improvement.