The final word? or a total miss?

Started Aug 20, 2012 | Discussions thread
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Roar Arne Velle
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The final word? or a total miss?
Aug 20, 2012

I always like to find explanations and how to make it right, or better. So also dealing with the phase AF issue on some of the new top Nikon cameras. I have followed numerous threads about the issue reading what different people have experienced. This is what I think is right. Please correct me if you think otherwise! (sorry for my english, - it is not my native language.)

First some assumptions: Why lens fine tune at all? Well, I think it is about communication between camera and lens and small lens variations and small miscalibration to the sentral AF point. I will point at the AF motors and the moving sensors inside the lens. The data sometimes needs correction values to find the correct position for the lens fast or at all.

Second: The AF point and the final calibration of each of them: Small errors can be corrected and allow the mechanical assembly prosess to be little off and still produce good information. That fact allow for cheaper production and superb end result.

All parts of the AF system works correct if assembled correct. The evidence is all the cameras working correctly. But some does not and some cannot be corrected easily. Why: I think it all comes from bad mechanical assembly.

All parts dealing with picture making and AF must be adjusted after the lens mount. The image sensor must be placed in the correct distance from it and perfect parallell. (The contrast AF will still work good without that). The two mirrors and the AF sensor must together make up for the same distance and parallellity. For the matt-screen to function as a focus mechanism, - it must too.

When these positions are two wrong, the AF sensor receiving light from different directions, from different parts of the lens and the second mirror, sees a too misaligned image point (area) to produce reliable data. In theory the AF point shall receive light from different directions, but originally from the same image point. Misaligned It will not. Small failures can be allowed and corrected by telling the AF point a little different place of correct focus. (AF pont calibration we call this). To function realy fast, all must be as correct as possibly not needing any calibration.

I think a few iterations (loops) are allowed also for phase detection. The telling about focus hunting, indicates that. Loops makes AF more accurate, but slows it down a bit. More prosessing power makes it possible to do more iterations in newer systems without slowing it down so much. I think that is an important reason for more accurate AF in newer cameras. For contrast detection, loops are a necessity until good enough results are obtained.

A system with too much misaligned mirrors can't be corrected without adjusting the mirrors. If the image sensor or the AF module is misaligned, it is all much worse. One needs to disassembly much of the camera. That is very laborous and therefore expensive.

What about worse on the left side? That involves more the right side of the mirrors (seen from the back of the camera). The mirrors have adjusting points only at their left side. The right side is not supported and can be more way off, I think. For small failures this can be calibrated, but not for major issues. In the future it is possibly to put adjusting points on both sides of the mirror.

If the lens need a lot of lens fine tuning to get sharp focus, it indicates a bad lens or a camera badly adjusted, I think.

To calibrate the AF point one need a fast and perfect lens with flat focus plane or alternatively a lens where the field curvature (often like rings in a water after throwing a stone in it) is known and can be corrected for. It is a factory task.

Lens fine tune can be done for many apertures (to deal with focus breathing), many different image distances and for different focal distances (for zooms). I hope this will come in later program updates. This prosess can be made automaticaly by let a program alternate between contrast detection and phase detection and can be done for each lens. I have written about that before.

Nikon has had water flood, earthquakes and crashed factories. New factories are built and new workers needs training. I think that is the main reason for the bad assembly and needs some time to be sorted out. I hope that will happen soon, - the D800 and D4 deserves that. I also hope that Nikon and other will work on reengineer the sentral box where all the impotant parts are fastened, to a higher degree of precision. That will in time make camera producing cheaper and still accurate.

 Roar Arne Velle's gear list:Roar Arne Velle's gear list
Panasonic Lumix DMC-LX5 Nikon D7100 Nikon AF-S DX Nikkor 35mm f/1.8G Nikon AF-S DX Nikkor 18-140mm f/3.5-5.6G ED VR +9 more
Nikon D4 Nikon D800
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