Since there is no size setting for M-mount, it is set for X-mount. Please note. This is a M-mount triplet Elmar 90mm F4. I purchased this item, which was rated as a good product by a famous camera store, in June of this year. I haven't used it much since I got it, and I've stored it in a dehumidified storage, so there are no problems with the aperture or helicoid. The inside of the lens is also in very clean condition.