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Photography Q&A
DSLR focusing and AF fine tuning on old AF-D lens question?
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<blockquote data-quote="dieselnutjob" data-source="post: 762080" data-attributes="member: 47318"><p>There is something that I don't understand.</p><p>On a DSLR a small proportion of light (if I understand correctly) goes through the mirror and hits a second mirror that points the image down to an AF sensor.</p><p>This AF sensor detects whether light is in phase in specific focus points, or not, and if not how much it is out and which way.</p><p>In the context of an AF-D lens the camera would pic a spot (or maybe just the one that the user selected) and wind the focus motor screw either anti-clockwise or clockwise until best phase is acheived.</p><p>Is this correct?</p><p></p><p>So I rebuild a 180mm f2.8 AF-N lens myself last year. There was a shaft with a cog on each end that goes from the focus screw on the mount to the focus ring. A previous owner had (I guess) forced the focus ring and busted the shaft. So I repaired it. It works now.</p><p></p><p>I have been wondering whether this lens was spot on or not as focus seemed a little soft, and then I read Ken Rockwell's review of this lens saying that focus was off so he sent it back, so I had a go at playing with the AF fine adjust (on a D750). The D750 has a +/-20 fine AF adjustment, and I found that focusing about 2.5m away that it needed all +20 points to get it right. By default it was about maybe 1/2 or 1 centimetre out.</p><p></p><p>I repeated on my 24-120 f4 and that is about right at about +2. So I would say that there's probably nothing wrong with the camera.</p><p></p><p>I don't understand why this should be. Surely if the focus is out wouldn't the sensor detect this and turn the screw? Surely either the image focuses on the sensor, or it doesn't? There is no electronic link between the position of the focus ring and the camera. All it can know is whether it's too far forward or two far back and turn that screw?</p><p></p><p>What does this AF fine tune actually do on an AF-D lens?</p><p></p><p>What am I not understanding about this? Is there anything I can adjust in this 180mm to make it calibrate better?</p></blockquote><p></p>
[QUOTE="dieselnutjob, post: 762080, member: 47318"] There is something that I don't understand. On a DSLR a small proportion of light (if I understand correctly) goes through the mirror and hits a second mirror that points the image down to an AF sensor. This AF sensor detects whether light is in phase in specific focus points, or not, and if not how much it is out and which way. In the context of an AF-D lens the camera would pic a spot (or maybe just the one that the user selected) and wind the focus motor screw either anti-clockwise or clockwise until best phase is acheived. Is this correct? So I rebuild a 180mm f2.8 AF-N lens myself last year. There was a shaft with a cog on each end that goes from the focus screw on the mount to the focus ring. A previous owner had (I guess) forced the focus ring and busted the shaft. So I repaired it. It works now. I have been wondering whether this lens was spot on or not as focus seemed a little soft, and then I read Ken Rockwell's review of this lens saying that focus was off so he sent it back, so I had a go at playing with the AF fine adjust (on a D750). The D750 has a +/-20 fine AF adjustment, and I found that focusing about 2.5m away that it needed all +20 points to get it right. By default it was about maybe 1/2 or 1 centimetre out. I repeated on my 24-120 f4 and that is about right at about +2. So I would say that there's probably nothing wrong with the camera. I don't understand why this should be. Surely if the focus is out wouldn't the sensor detect this and turn the screw? Surely either the image focuses on the sensor, or it doesn't? There is no electronic link between the position of the focus ring and the camera. All it can know is whether it's too far forward or two far back and turn that screw? What does this AF fine tune actually do on an AF-D lens? What am I not understanding about this? Is there anything I can adjust in this 180mm to make it calibrate better? [/QUOTE]
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DSLR focusing and AF fine tuning on old AF-D lens question?
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