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I would assume to play it safe using the hood from the 45mm to avoid possible vignetting. If the GB 71 or 72 would work we can save more money than having to buy yet one more expensive Contax hood! Let us know if it works.
Guy, I'm almost certain I've been using the 80mm GB-72 with the 55mm, Colin Elliot suggested this was the recommended hood when the lens was originally released. Absolutely no ill effects that I can see.
Incidentally, the 55mm is one of the unsung heroes of the Contax range, it's fully the equal of the Hasselblad 60mm which is a favourite of the Hassie cognoscenti.
You're right. I just looked at my 35mm lens hood and it is the GB101. Right there on the Contax web site it says that hood is for the 55mm. Obviously it is not. I wonder if anyone at Contax looks at their own site?
I checcked on the B&H site and they don't even have the hood for the 55mm listed.