再试……
这次使用的器材是P家的
F50/2.8macro
FA28-70/4
Z-1p
Z-20

对焦距离1.5m

当用50的时候，两台机的重复对焦远比上次用28-70/4要好，对50次只有3次到4次在左右1mm内。
当用28-70/4时，用70对（忘了用28！），有5次和7次在左右1mm内，却没有发现在这个范围以外的。

这里我想到一个问题，会不会对焦快的机，它的对焦精度相对低一点呢？同一只镜头，Z-1p比Z-20快得多，但同时对焦重复性也差了，请各位DX分析一下。

我是28-70/4配MZ3，在较暗的情况下一样误差较大，但在主体明亮时要好不少。不知实验时的光线情况如何?
总的来说28-70/4的AF机构有些问题，试过FA80-320、80-200/4.7和FA100/2.8M表现都好得多。

看来我要换只裂像对焦屏至得，不过除此之外有什么办法让其准确地对焦呢？

AF精度也和镜头有关。
这有篇文章或许有用：

====================================================
   From: Ed Mathews <edwardamathews@hotmail.com>
     To: pentax-discuss@pdml.net <pentax-discuss@pdml.net>
Subject: Re: My autofocus tests on the MZ-5n
   Date: 01-1-31 8:53:00
====================================================


Rob,
     I m not sure what the lenses have in common, but here s what I found,
going from best to worst (I ran this test multiple times, using Delta 100,
focusing to infinity on fine detail of ships at the Baltimore Inner Harbor.
I focused using AF at wide open and two stops down and every marked focal
length for zooms, and then focused manually at the same apertures and focal
length.  I examined the negs with a 25X Loupe on my lightbox):

  FA 43mm F1.9: Outstanding AF accuracy.  Such that even wide open it s
sharper than most other lenses just because the AF is so accurate.
  F35-80mm F4-5.6:  A cheap lens with average optics, that still performs
better than most, again simply because the AF was dead on at every focal
length.
  F80-200mm F4.7-5.6:  Very accurate AF, and real decent optics for such a
cheap lens.
  Tamron FA 28-105mm F4-5.6:  Again, very accurate, but average optics and
high linear distortion, and prone to flare.  Nevertheless, performs well in
most situations due to the accurate AF.
  FA 77mm F1.8:  Accurate most of the time, but it often would lock on
slightly out of focus, but then would fine tune itself and achieve accurate
focus with another lift and touch of the shutter button.  A bit of a pain in
the ass to focus that way, but as long as I know it.... and the optics are
very good.... and the lens has almost no distortion..... and it feels so
good...... and it cost so much......
  Sigma FA50mm F2.8 EX Macro:  Not too bad, but inconsistent.  Sometimes it
would lock on and be in focus and other times it would lock on and not be in
focus.  But another touch of the shutter button did nothing with this lens
once it thought it was focused.  Great optics, though.  Alas, I sold this to
KEH right after the test because I never used it.
  Tamron FA 28-200 F3.8-5.6:  The newer version, but not the latest with the
zoom lock.  Most of the time, it would focus slightly ahead of the subject,
regardless of focal length, but the error was greater at the wide end.  Even
at two stops down, sharpness never equaled my manual focus wide open.  I
sold this too.
  FA 28-70mm F4:  It s really too bad about this lens.  Focused short of
infinity every single time at every focal length.  At two stops down, AF
performance was always so bad that it was far, far behind my manual focus
wide open.  However, the optics (as shown by my manual focus) are very good,
so I m keeping the lens and hoping it works better on the MZ-S.

That s all, folks.

Thanks,
Ed

昨晚看了一下，反光镜的背面（AF小窗）有点发梅，于是擦了几下，再试对焦，试验条件如下：
用FA28-70/4.0在一个开了20W光管的9平方米的白墙房间内，对1m外的黑色电线AF，合焦后再让其转到无限远或近拍的距离上。

结果：
50次中有13次是不能重复到同一个位置，是左或右约1mm（对焦距离的表示环），还有几次是有接近2mm的差别。

请问DX们，这样的机还正常吗？

看看反光板下面的聚焦透镜上有没有灰尘?

我的Z1P没有这个问题。
镜头的对焦系统是否有磨损？这也是一个可能的因素。

也许会有影响。我测试70-200/2。8的时候，P卡口的Tokina一直得分不高。

最近试了一只M口的，硬是要的。