[Davor]

Big Thanks to everyone here, you are great people. Specially thanks to member "672", your answer was most helpful, when my camera returns I will try all of this and post news here...

[donesteban]

The fact the sensor is build out of two sensors is true. It is the same for all sensors bigger than 24x36mm - and if the sensor is bigger than 36x48mm, it is built of more than 2 sensors. The reason is: The steppers used in the production do not offer more than 24x36mm. As many things can go wrong during builing one sensor out of two, prices for sensors bigger than 24x36mm are much higher than 24x36mm.
The question is now: Where does the company set the limit of "something went wrong, let's throw this sensor away". Brighten a RAW picture by 4 stops in post processing is certainly extreme and leading to noise, banding and many other bad things with most of the digital cameras, expect a few cameras, like the Nikon D600, D800. A sensor showing bad behavior in this situation is not broken and will most likely not be replaced. On the other hand, making a picture brighter by 1 or 1.5 stops is very common, if this is sufficient to cause a problem, they will have to fix it.

[noser]

Ahhhh... Is this why the 645D only goes to ISO 1600? Because anything higher is off-chip signal amplification in software that would surely show the split?

That kind of info would actually be good to know; any future 645D may have a similar limit, unless there is some sort of factory calibration to bring the two halves into perfect agreement - and unlikely, too.

[Davor]

Here is an update about my problem with 645d: after exactly 3 months that it's been on repair service, 645d finaly came back to me. But, unfortunately, I did not make any tests about the line streak or split sensor issue, because after only a few shoots I decided to send my camera to repair service again because I found a new problem which didn't exist earlier (before the repair). I don't know exactly how to describe this problem, but now I cannot view images in the viewfinder normaly. The image is blurred and there is no way to correct it and get a clear view with the diopter correction slider. Autofocus works well I think (but I can not be 100% sure) and images taken on that way are ok (not blury) but when the image is in focus - in the viewfinder I see it blury. If I try to focus manualy, I can get the image in viewfinder much clearer (not 100% clear) but images taken on that way have the focus moved a lot. The problem is much more visible with a 35mm lens - when I turn the focusing ring to infinity, the image in viewfinder is very blury. With a 200mm lens the image in viewfinder is much less blury but still is blury.
The seller asked me if I have problems with my eyes??? But 3 months back - the viewfinder on this camera worked fine. I also have a new K-5 II and K-r and on those cameras I see the image in viewfinder completely clear and right.
I can get the green icons in the viewfinder of 645d completly clear but not the image coming from the lens.
The repair service staff tells my seller and he tells me that after all repairement the camera was checked and marked as good but I found that it isn't.
What will happend next I don't know :-(

[Giklab]

They probably messed up the focusing screen calibration when rebuilding the camera. Somebody who knows about these things should jump in here.

[scratchpaddy]

I think Giklab's got it right. I'm not sure if I qualify as someone who "knows these things," but I have replaced the focusing screens on my K-5 and K-r, and your symptoms sound familiar to me. I'd say your shim got jiggled or lost.

The fact that you can get the viewfinder icons clear means there's nothing wrong with eyepiece/diopter. The focusing screen acts as a replica of the sensor for you to look at, so it and the image sensor have to be the exact same distance from the lens to work properly. One of them is out of whack. Could be that the sensor is further forward or back, but I don't know if that's even possible. Actually, no, that would mess up a whole bunch of other things. It has to be the focusing screen, which is only an optical aid to you, and has no effect on the pictures.