Front/Back or random Focusing

[ckanthon]

Hei,

On a Chinese Website there is an article (on 2005 May) talking about this topic (FF/BF).
Reference blogs (in chinese)
h**p://dcdv.zol.com.cn/2005/0508/168329.shtml
h**p://blog.pixnet.net/mydcmag/post/2891359

However, it was written in simplified Chinese. I translated it onto Traditional Chinese in office 2003 and extract the main points as follows:

The Arthur himself owned a 2XD. He asked the technician in Singapore while sending his Camera (2XD + EF 7X-2X0/F2.XL USM) for a repair:

The meanings are summaried as follows:

a) Source of Focusing errors: It can be caused by body, by lens or by body + lens

b) Sort of Focusing errors: always front focus (FF), always back focus (BF) or FF-BF randomly.

c) Severity of FF/BF: Full Frame system is not so noticable. APS-C format is much noticable because the focal lenght has a mutiplier (for example x1.5) Therefore the focusing error also multiply by a factor (x1.5).

d) Cause of Focusing errors:
- Mainly the position error of the mirror splitting the light to the focusing sensor (TTL). The error can be "+" or "-" (i.e. systematic error) or randomly "+/-" (i.e random error)
- Error can also be introduced by the focusing motor (too faster?)
- It is easier to happen for System with mixed lens for SLR and for DSLR at the same time.

(that guy did not mention any other factors such as sensitvity of focusing sensor, different lighting sources& conditions etc.)

How to implement corrections:
- Better alignment of the optical mirrors (also the mirrors repeat it actions in consistent manner [here meant the shutter action])
- Better motor drive (for example update fiemware can achieve this)
- Allow the adjustment of the CCD/CMOS focal plane position to fit different situations
- tighten the error tolerance in production or QC control.

Translation not 100% correct or if you know this article already. sorry.