[obican]

Your knowledge? Boy, you know nothing. You have been saying otherwise all the time but everytime someone asks you anything you try to divert. 

 

To put everything together really short:

 



 

 

This is a DSLR cross section. There are two mirrors, one of them diverts the light to the focusing screen, other to the AF sensor. When an image is being made, both get out of the way and light reaches the image sensor.

 

In order for AF sensor to function correctly, it has to be very carefully calibrated so that the distance the light takes to reach the AF sensor should be exactly the same as it takes after the mirror flips up, reaching the image sensor. If there is a miscalibration within the lens, lens mount, main mirror, secondary mirror, image sensor or AF sensor, you dial in a microadjustment so that the camera compensates for the amount. You can set this value per lens or per camera body on most cameras.

 

The same thing goes for the focusing screen too. If there is a miscalibration anywhere within the system, the focused area on the viewfinder screen will not be the same as the image sensor.

 

Viewfinder screen is (in theory, most of the time it's not really) carefully shimmed to ensure it is calibrated. However, if there is a problem with the lens or something had occured with the mirror later or if the whole system is not up to specs to begin with (quite common actually), you can't rely on the viewfinder to ensure good focus. You can't compensate for this electronically neither.

 

Got it now?

 

It has nothing to do with the PDAF algorythms, it has nothing to do with lens taking a long step. It is just a mismatch of distances because belive it or not, your camera is made quite cheaply.

 

Prove me wrong.