Full frame lenses on APS-C are a mixed bag. A 16mp sensor is going to outresolve all but the best lenses. On one hand, you are taking from the sweet spot which is where the lens resolves the most detail, but on the other hand you are not utilizing the full image circle of the lens and thus throwing away potential resolution. A MF lens would be even more challenged in terms of resolving power. The K-mount adapter for the Q is a great example of this. A 5x crop factor sounds really cool until your realize that only the best glass works well in terms of IQ. CA is a major problem with that setup too as you are magnifying CA tremendously. Ideally its best to use a lens that has an image circle that is the same as whatever format you are shooting. I hear some 645 lenses do OK on an APS-C sensor. They seem to be mostly ok on full frame, that's for sure, but then your crop factor is much less. 67 lenses were designed to cover an even larger circle, so I wouldn't expect amazing IQ. Some guy was using a 67 400mm lens with a k-5 and was getting decent results. Paul is totally right, MF lenses don't have to maintain the same resolution as 35mm lenses. Just like large format lenses have even worse resolving power over a given area. You can get a FF 135mm-200mm lens for probably cheaper. FWIW I have been told that the 67 lenses were in general better than the 645 lenses from back in the day. I don't know how true that is, but I would trust the source as being accurate and based upon experience.
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