The upside to A-mount popularity going down

Started 3 months ago | Discussions thread
ProfHankD
OP ProfHankD Veteran Member • Posts: 7,860
Some details about focus micro adjustment
1

SQLGuy wrote:

ProfHankD wrote:

I've certainly heard of people having that issue. However, some Sony E bodies didn't provide micro adjustment, so I think Sony might have been a bit more careful about alignment back when they were current. Either that or I just got lucky with the three I have?

E? Or A? I think you meant A.

That's true for A bodies too, but I do mean E using A-mount lenses via an LA-EA2.

BTW, the micro adjustment doesn't really fix things -- different lenses might need different micro adjustments if it's off. I suppose I should also note that most of my A mount lenses are slow enough that DoF could hide a small error.

That's why the adjustment is per-lens rather than one setting for the body. It's a problem, though, with some 3rd party lenses that use the same ID for different models.

Ideally, MFA would also allow different adjustments at different focal lengths of a zoom. Sigma allows that, and, I think, also different adjustments at different distances, for some of their EF lenses through their dock.

Among my A mount cameras, the A900 has it, but earlier and/or lower models, like the A390 and KM7D, do not.

Film cameras could have benefitted from MFA, too, but I don't believe it was available in any of them. I guess I could adjust my Sigma lenses with the dock for use on my EOS A2e, but what a pain that would be - shoot some frames, develop them, adjust, shoot some frames, development them...

On the other other hand, this could be handy for my EOS 5D classic or EOS 1D classic, since those also don't have MFA, and, IIUC, the Sigma dock adjustments have no effect when I use those lenses on the MC-11.

It shouldn't be per lens, but is because it isn't implemented correctly. The "correct" way to do micro adjust would be using something like a screwdriver or insertion of shims -- it's logically the exact same problem as making the OVF screen align with the film plane (or sensor) well enough to judge focus. I vaguely recall it not being too terrible to mechanically adjust the PDAF sensor alignment on some DSLRs, but you had to go in from the bottom, and it wasn't a user-friendly thing to do.

It's honestly shocking that the PDAF alignment in SLRs was ever good enough. Typically, the PDAF sensor was fed by having a portion of the main mirror semi-transparent, and having a second (hinged) mirror on the back of the main mirror reflect the image down to a series of optics that brought the image to the PDAF sensor angled below the mirror box. The SLT design makes precise alignment of the PDAF sensor an order of magnitude easier and easily much more accurate: it basically sits where the OVF focus screen would be, with light split to it by the non-moving pellicle mirror (what SLT really should have been called).

In other words, Sony's placement of the PDAF sensor in SLTs, even the LA-EA2/4 adapters, is much easier to make accurately aligned than PDAF sensors in SLRs.

So, why did Sony start adding micro adjust to their bodies? I can think of three good reasons:

  1. Film SLRs never really had things very well aligned because the film plane wasn't very planar nor even very consistent from frame to frame, while sensors are flat at the molecular level. More fundamentally, emulsion thickness was greater than the thickness of light-sensitive pixels. This is a large part of why sensors can effectively resolve so much finer detail than film. In other words, higher resolution digital sensors made alignment tolerances much tighter than they had been for film... and tighter still as pixel pitch got finer.
  2. People didn't understand that software-implemented "micro adjustment" wasn't needed if you were simply more careful about mechanical alignment so other companies having this "feature" nudged Sony toward adding it. My impression is that, for example, Canon really needed it. That's not a criticism of Canon, it's just that Sony didn't make film cameras, so they were not reusing an old subsystem design (which is a lot of how Canon keeps their prices so low while maintaining a good profit margin). In fact, the reason micro adjustment is lens dependent, even copy dependent (especially in some non-screw-drive lenses), is that it is applied as a focus-distance-dependent offset in the lens focus control, and generally is a poor approximation to moving the PDAF sensor a fixed amount to bring it into the sensor plane -- especially for floating-element lenses or zooms.
  3. Although it's as easy to align the PDAF sensor in an LA-EA2/4 as it is in a native A-mount SLT body, the adapter itself adds some imprecision in alignment by adding a pair of mating surfaces. Thus, there's actually a slightly greater need for adjustment on E bodies than in A bodies. In practice, I haven't found this to be significant -- Sony's adapters fit their bodies remarkably well -- but YMMV, and people who accidentally broke/deformed the plastic "shock absorbers" in the original E mounts probably had the adapters sit a little differently.

In any case, using main-sensor focus mechanisms (CD, PD, dual pixels, depth-from-defocus, etc.) removes all imprecision in alignment. Once AF performance using the main sensor became comparable to a separate PDAF sensor, PDAF sensors really needed to go -- and then you also get to lose the SLT mirror... and the box that housed it. And, while you're removing the mirror box, why not upgrade the lens communication protocol?

All of that is a big part of why evolution of E-mount was always destined to obsolete A-mount. Just be thankful that Sony is honoring the Minolta heritage so strongly that they've actually invested any effort at all in continuing to make even most Minolta AF lenses that don't implement Sony's full A-mount electronics still able to function better than they did on the cameras they were designed for. Note I didn't say function as well as the latest lenses, because that is probably a technical impossibility.

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