if the flange is too thin, you will focus "past" infinity when the lens is on the inf stop. if too thick, you will not get to inf focus.
how does your lens behave at inf?
I just checked both adapters with my Leitaxed Summicron-R 50 E55. On my D800E, infinity is spot-on with this lens (i.e. the infinity mark focuses to infinity). So much so, in fact, that I often don't even bother checking focus with this lens when shooting distant landscapes. I just set it to infinity.
I focused on a back-lit hill crest covered with trees about 4 km distant. With the Voigtländer adapter, the trees came in focus just about in the middle between the 10m and the infinity mark on the lens' focusing ring. With the Metabones, the same occurred about a third of the way between the 5 and 10m marks (i.e. much closer to the former).
also a 0.01mm thickness variation over about a 50mm flange diameter (arctan=5000) is about .01 degree; which seems like a pretty small tilt; were you using a tech camera, not discernible, i would think. one tenth degree tilt would be about arctan=500
Thanks - that's good to know. I will test on a suitable subject sometime soon to see if I can detect skew. As long as the blur due to skew isn't more severe than the accuracy of my focusing, I'm not bothered.
What I am concerned about, though, is the above finding with regard to infinity focus and how it relates to flange back distance (FBD). Am I wrong in assuming that the designers of a lens calculate its optimum performance within a relatively tight tolerance of the FBD of the camera system for which it is designed? If that distance is now as much reduced as found with the Metabones adapter, would we not expect sub-optimal performance of that lens? I realize that the FBD of the target camera system as well as the focal length and the general design paradigm of the lens likely influence the answer to the latter question. E.g. I would expect a symmetrical wide-angle M-mount lens suffering much more than a long SLR telephoto lens. Regardless, I'm curious if the observed reduction of the flange back distance by almost 0.5mm in the case of the Metabones adapter is something to be concerned about. Could anyone in the know shed some light on this?
I'd like to add that this is not splitting hairs to me, and I hope I'm not coming across as pedantic. As we have all seen, the sensor of the A7R is capable of capturing incredibly detailed images, given the right lens an proper technique. To me, pursuing the goal of maximizing IQ, particularly when using the camera for landscape photography, is worth while. In fact, it is the primary reason why I purchased the camera. Otherwise, I may as well have purchased the A7 or stayed with my NEX cameras. Knowing which factors need to be considered to maximize IQ is therefore important to me, and as we have seen from posts in this thread and others, the design and quality of adapters play a significant part in this.
