DSLR focusing and AF fine tuning on old AF-D lens question?

dieselnutjob

Senior Member
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?
 

hark

Administrator
Staff member
Super Mod
Contributor
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?

I'm not familiar with all the mechanics involved with how an auto focus system works, but the only way the camera can truly focus directly at the sensor is via Live View which is the most accurate.

Hopefully some of our other members can offer you more info on the mechanics - possibly @FredKingston or @nickt or others.
 

Fred Kingston

Senior Member
On the D750... Go to Menu F1 "OK Button", then select "Playback Mode", then select "Zoom on/off", then select "1:1(100%)" and press the OK button.

Now turn on the Beep for the D750, select "M" mode, and make sure both Menu A1 and A2 are set to "focus", and the auto-focus is set to AF-S or AF-A, NOT AF-C... Now when you press the shutter/back button to focus, the camera should Beep when it thinks Focus lock has been achieved.

Now, with the camera on a tripod, focus on a subject, now turn ON live view. Did the camera beep? NOW, carefully, press the center OK button on the wheel... The camera should Zoom in on the pixels... now carefully, adjust the focus ring on the lens until pixels are in sharp focus. <--Depending on the lens, you might have to turn OFF auto-focus to get the lens to free wheel... In which case, use the Green Focus lock, light in the viewfinder as an indicator that the camera achieved a phase detect lock. Depending on how much you have to turn the focus ring to see sharp pixels, will depend on how much the lens and camera are out of sync with their design specs. You shouldn't have to turn the focus ring but just a small tweak...

The phase detect system and the mechanical focus adjustment of a lens are NOT a hard fast linear connection... Nikon frequently makes adjustments to lens sent to them for service to bring a lens within the plus/minus parameters of the fine-tune system...
 

dieselnutjob

Senior Member
I completely dismantled the 180mm f2.8. I couldn't see anything in there that "adjustable" except for one thing, which is where the end stop of focus cam bottoms out. Essentially that would impact whether the lens can go beyond infinity and buy how much and whether it runs out of travel completely.

Maybe I should have a play with that and see if it effects the fine AF or not.

The other thing I have to try is whether it makes a difference if I go from infinity to my 2.5m calibration point, or whether I start closer and go out.
 

nickt

Senior Member
Ok, this is just a guess how it works, lol... I don't know specifics, but the camera camera drives the lens until the focus sensors determine perfect focus. Since the image sensor is a different path than focus sensors, focus at the image sensor may not agree with the focus sensors. So Camera calibration says get perfect focus, then take 3 steps forward. (or whatever was set). There is no feedback on this correction. I would think Nikon calibrates this way too when they build the camera. So its not a straight servo that ends in perfection. The sensors do determine focus, but then a correction of 'x' number of steps is delivered to the lens. Mechanical slop and lens calibration determines what the lens does with the correction steps sent to it. Again, this is just how I THINK it works.
 

Rotties

New member
Have you tried calibrating the lens? simple job and all you need is a cheap calibration card for
$6.00 (see DSLRKIT) on amazon.
 
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