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I wonder if there is also a difference between heavier, longer SLR lenses (where the weight may cause a cheap adapter to warp out of alignment) and lighter rangefinder lenses. I definitely feel like SLR adapters are prone to a little more play.
No adapter would warp but the camera bodies do - and it's measurable too. Even quite visible in many combinations! You can actually see the front face of the camera bow out and twist. On M4/3, the two sony Nex's I looked at, three Canon and one Nikon DSLR (all mid-range or cheaper DSLR models) it's pretty bad. And if the lens is heavier than the camera and it's mounted via a lens collar then the camera actually has enough weight on it's own (500 to 700 grams) to also warp it's face. But the adaptors themselves you would need a truck to warp them.
I have used a micrometer on the thicknesses of my Chinese cheap-0 $20 adapters and 14 of 16 were right on (within the tolerances of the micrometer itself). One was just not seated right (fixed), and the other is a tilt-adapter which doesn't zero properly.
it takes a very shoddy machining job to make an adapter that is uneven thickness. i feel sure 95% (or more) of the $20 adapters you buy will show no measurable deviation in thickness with a standard micrometer. the much more difficult part is machining the mount and insuring that it holds both the camera and the lens perfectly flush against the adapter surfaces. this is the source of most of the issues and even if the adapter is made by the OEM vender to the same tolerances as their camera and lens mounts (say a canon EF mount to...Show more →
Yeah, that's a point. I had some that were a little loose till I figured out there's a tension adjustment - thanks to the folks here! (-_-)/ Now the only one I have that doesn't hold everything super tight and flush is that Tilt-adapter and really, that's because it uses a hinging pin design designed by someone's pet chimpanzee - I'm sure of it!
I would have to guess based on my own experiences, that the inner surface and the area inside the camera around the sensor is more critical to IQ than anything else. In my case I've flocked the inside area of my adapters which helped with a few lenses actually. This is probably what Rusty experienced as that surface shape and texture (coating) can be pretty critical for internal reflections and stuff - which can cause soft corners, and low contrast edges, reduced detectable resolution (due to lower contrast), and so on - especially with WA/UWA lenses I would think.
For m4/3 and APS-C users (which I guess is the majority of folks adapting) I'm going to say that besides the inner surface of the ring there's no difference between a cheap and expensive adapter when it comes to accuracy. Certainly for those formats centering and 4 or 5 microns of tilt isn't going to make any noticeable difference in 99.999999% of shots - again, the camera bodies themselves warp more than that. The worst one I measured had a 1.93mm outward warp at the top of the mount and a 1.12mm inward warp at the bottom of the ring. This was a Cannon 300/4L lens attached to the GH1. GH2 seems a little better tho I haven't tried to measure it yet. etc. etc. The others were less but I noticed with careful study and measurements that at about 450g (either from the camera or the lens) the face of Nex, GH1, and one of the Canon bodies distort slightly - enough to measure.
With that kind of distortion happening already, what are we talking about with adapters? Meh...