Ok, a long take, probably overkill but anyway: I have all kinds of adapters (maybe 50+), some expensive and many ultra cheap. Overall, in normal scenes, it's hard to get a sense of how the same scene would change, or if it would matter in practical terms, from a very expensive adapter versus a very cheap one. Often, it wouldn't matter.
But then you start to have all kinds of specific cases where it has a much larger impact. So understanding what are these cases will inform how much effort to put into adapters. I found some expensive adapters to not necessarily be much better. So it can't be generalized that more expensive ones are always going to result in a much better one. This is like wine, to make an analogy. You can spend a lot more money, and yes, it make be aged in expensive oak barrels, and have high quality source grapes, and so on, and not be interesting or any better, sometimes worst than a much affordable wine made by someone that really knows a lot about wine.
Here are some things I noticed:
- Mount fit (and for adapted, it always means two mounts that compound, or 4 if using a dual adapter like NEX-LM-M42). Some adapters fit less precisely. This makes the lens less stable. It makes precision focusing hit and miss, so if this is important, it needs to have a very good fit. When it doesn't, the lens will wobble when focusing, making it frustrating as well as self defeating.
- Light leaks. It's staggering that many lenses may have a design that makes so light from certain angle on the sides have an impact in the photo. This is self-evident when you have a lens with caps on, and you can see halos in scenes where there's strong ambient light. Of course, if there's a lot of light, the leaks get masked out. But if you are shooting relatively low light scenes and there's a strong illumination reaching the camera, this will ruin all the photos. One example is shooting from a location where direct sunlight hits the camera, but the scene itself is largely in the shadow (imagine shooting a mushroom scene in dense foliage, and the sun is hitting the camera). But it can also be shooting a far away night scape from within a well lit location.
- Internal reflections directions (forward and reverse). The lens design (or internal shape of the adapter) seems to matter more or less depending on how the actual lens works. In some cases, it's only important that no reflection happens from light incoming to the sensor. But in some lenses, it's very important that is also doesn't reflect light reflecting on within the sensor chamber (sensor plus other parts of the chamber). This is particularly important for lenses with very large image circles and more important when the camera flange is bigger, and the adapted mount is bigger. Often, one is adapting a lens that may have a much larger image circle, meaning that there's a lot of light that may reach the nearby areas reflections from the sensor become strong enough that it will affect the picture.
- Lock mechanism. In many cases, the lock mechanism, for some reason, isn't precise enough. As you focus turning the focusing ring in one or the other direction, there's some play in the adapter/lens locking that makes it so focusing very precisely a nuisance. It breaks the ability to associate an amount of rotation with an amount of focusing, as you need to compensate for the coupling having some rotational play.
- Flatness. Adapters can be less than perfectly perpendicular. It it wobbles, for some kind of photos, it can be a problem as the plane would shift a tiny bit, and if it's a very high aperture and to the extent the lens sits close to the sensor or is a wider field, it can be infuriating. I think precise macro work or when the precise alignment of all the plane matters, tiny problems with flatness are made extremely evident.
- Register. Almost all adapters are shorter than the precise required register. This is as different lenses in different eras, combined with lenses that may be tuned to infinity a little imprecisely, in addition to the effects of ambient temperature on this things, make it so a lens that is perfectly accurate will make several lens and temp and particular cameras not focus on infinity perfectly. This is convenient, but for wider lenses or those that depend on tight tolerances, it may greatly affect the photos. Complex lenses with floating elements and a tiny error in register in the adapter can have a very detrimental effect on photo quality. But for me, using old lenses and usually not very wide, it matter much less. But for some lenses and uses, it really becomes a huge issue. Some adapter can have the register adjusted very precisely, most cannot be adjusted at all. Most adapters do not say to what extend they are shorter, and the cheaper ones will have high variance, meaning one or another of the same maker will be different.
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Then there's the aspect of old lenses themselves. Many, many many very old lenses have wear in the internals, meaning the lens internal masking will be compromised. If find that this usually happens in specific places, as rings that reflect a lot of light. I think it gets compounded by how the actual optics work. In these cases, one may worry a lot about the adapter, but the huge effect is actually within the lens itself. The relation to the adapters is that one may worry too much about the adapter, when the cumulative effect from the lens may be so high that the any adapter improvement will result in little differences anyway. ie. it's like worrying about potential light rain and building a huge dome when one is surfing.
All in all, I think it's hard for someone that is not doing high precision photography $45 vs $28 will be much better. I think brand themselves are not very indicative as most brands are just like Vivitar, umbrella brands, but the adapters come out of very different producers, and two items from the same brand may come from different designs, providers.
I think it's thus very hit and miss, it's hard to make generalizations about brands, other that some much more expensive brands ($100+) are consistently better in all regards, but to what extent they will have a noticeable impact is largely dependent on the kind of photos and the particular type of lenses and shooting conditions.
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For old lenses (which are usually never too wide, and if wide, they are not very sharp) I found that the adapter matters a lot less overall, than being able to assess the wear of the internals of the lens and if one is inclined to improve things, send it to someone that can really do a very good service that includes internal masking of the lens) or doing it oneself. The tilting of locking matters more the faster the lens, little otherwise, the wobbling matters a lot more for wides when plane precision matters, the locking is important when one cares about precise photos (and the higher the sensor res) but even when not, it really can ruin the enjoyment of the manual focus experience), the precision needed skyrockets with dual adapters (intermediate adapters), and that I haven't found a vendor that removes the frustration with adapters in general, and even if one does research, it's camera and sample dependent.
