DotTune: AF tune technique without taking photos

I have a proposal for a new AF tune technique that doesn't require taking photographs. At least I think it's new because I haven't read it anywhere else yet. If anyone knows of a reference that describes this already please let me know.

Nikon DSLRs have an electronic rangefinder in the viewfinder that shows you when:

• Subject is in focus (green dot)
• Focus is behind subject - green arrow pointing left, indicating focus ring should be turned to the left, away from infinity
• Focus is in front of subject - green arrow pointing right, indicating focus ring should be turned to the right, toward infinity

The rangefinder's determination of the above conditions is based on the phase-detect AF system's evaluation of focus. This system includes a configurable AF fine tune value which can be adjusted on a per-lens basis or globally for all lenses that don't have a per-lens value configured. Most would assume that this AF tune value is an "output" bias that the camera adds to all lens movement commands, so that instead of telling the lens "focus to X" it instead tells the lens "focus to X + tune value". In actuality the AF tune value is an "input" bias that feeds into the PDAF's sensing logic, altering when the camera believes it has acquired focus. You can read more about this here.

Since AF tune affects the camera's PDAF evaluation of what's in focus, it also affects the electronic rangefinder indication of focus. You can demonstrate this by establishing critical focus on a subject (green dot), changing the AF tune value, then without refocusing, observe that the rangefinder now shows a left or right arrow indicating the camera thinks the subject is no longer in focus, even though you didn't adjust focus.

Proposed AF Fine Tune Technique

1. Enter Live View and establish critical focus on a high-contrast subject. For cameras with poor LV clarity (D800/E), you may want to take a photograph and evaluate it on the computer to confirm critical focus
2. Set the body to MF. You could set the lens to MF instead but you may jostle the focus ring accidentally if you do so.
3. Look through the viewfinder to see if the camera thinks the subject is in focus. You'll need to half-press the shutter while doing this to keep the metering/rangefinder from going to sleep. If you see the green arrow pointing to the left, then the camera+lens is back-focusing, so you need to decrease the value of AF tune (negative adjustment). If you see the green arrow pointing to the right, then the camera+lens is front-focusing, so you need to increase the value of AF tune (positive adjustment).
4. Repeat step 3 until you get a green-dot from the rangefinder.
5. Depending on focal length and subject distance there will be a range of AF tune values which produce green-dot confirmation. For example on my D800 w/50G f/1.4 @ 5' subject distance, I get the green dot from -6 to -14. Conceptually the optimal AF tune value should be the value that's the middle of the range, which in my case is -10 (I independently verified this using a LensAlign). So you'll want to establish the range of green-dot values by progressively increasing the AF tune values until the camera stops showing the green-dot (it'll alternate between the green dot and an arrow when it's at the margin of the range)...and repeat that procedure in the opposite direction to find the other end of the range.

Some notes:

• When establishing the precise range of green-dot AF tune values you may need to examine the rangefinder for several seconds. I've found that for the extreme margins of the AF tune range the camera may take a few seconds to flicker the arrow/green-dot.

Questions and suggestions welcomed!

Thread title edited by moderator per the O/P's request

I don't know if this will work accurate enough or not, and I'm sure you'll get numerous replies on why it will or will not work, but hats off to you for coming up with this.

Hi Horshack, I have a question regarding steps 1 and 2. Do I establish critical focus in LV using MF and if that is so is step 2 before step 1? Or am I confused (not an uncommon occurance in my life)?
Thank you, Pete.

I tried it with my lenses. One I was slightly unsure I had the best adjustment. I was one adjustment off on it and the others were right on. Can't say if this is better than the other focus methods but it costs nothing to try and for me it seems to be as good as Focal and other ruler tests I have tried.

For me it seems technically correct. If the green dot is on then the camera thinks it is in focus. It should agree with the focus when it searches focus and settles at what it thinks is correct.

Such an algorithm would allow Nikon to introduce AF fine tune in the entry level 24 MP DX cameras - where it is sorely needed - and also increase the precision available to advanced photographers.

Horshack wrote:

1. Enter Live View and establish critical focus on a high-contrast subject. For cameras with poor LV clarity (D800/E), you may want to take a photograph and evaluate it on the computer to confirm critical focus

Since LV allows the aperture to be stopped down, I would think doing this wide open would shorten the DOF in many cases to the point where critical focus would be pretty obvious. Right?

Kudos to you Horshack for your lateral thinking.  Sometimes there really is a return from reading these forums! As another poster noted, it seems so obvious in hindsight.

I have two new camera bodies that I am just about to fine tune.  I suspect you might just have saved me a lot of time.

Regards,

David

Instead of having to switch the camera into MF after achieving critical LV focus...  Set the camera to AF-ON Only for the duration of the testing.  Now when you press the shutter button I think you will still get the rangefinder arrows and green dot but you won't be activating the AF.

What a brilliant idea !!!

....inclusive the follow-up idea to make it as an "in-camera" automated firmware !!

I hope this thread comes to the ears of Nikon techs' and they would make the effort to implement it soon !?!? One can dream , can't one ?

J-P.

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Horshack wrote:

I have a proposal for a new AF tune technique that doesn't require taking photographs. At least I think it's new because I haven't read it anywhere else yet. If anyone knows of a reference that describes this already please let me know.

Nikon DSLRs have an electronic rangefinder in the viewfinder that shows you when:

• Subject is in focus (green dot)
• Focus is behind subject - green arrow pointing left, indicating focus ring should be turned to the left, away from infinity
• Focus is in front of subject - green arrow pointing right, indicating focus ring should be turned to the right, toward infinity

The rangefinder's determination of the above conditions is based on the phase-detect AF system's evaluation of focus. This system includes a configurable AF fine tune value which can be adjusted on a per-lens basis or globally for all lenses that don't have a per-lens value configured. Most would assume that this AF tune value is an "output" bias that the camera adds to all lens movement commands, so that instead of telling the lens "focus to X" it instead tells the lens "focus to X + tune value". In actuality the AF tune value is an "input" bias that feeds into the PDAF's sensing logic, altering when the camera believes it has acquired focus. You can read more about this here.

Since AF tune affects the camera's PDAF evaluation of what's in focus, it also affects the electronic rangefinder indication of focus. You can demonstrate this by establishing critical focus on a subject (green dot), changing the AF tune value, then without refocusing, observe that the rangefinder now shows a left or right arrow indicating the camera thinks the subject is no longer in focus, even though you didn't adjust focus.

Proposed AF Fine Tune Technique

1. Enter Live View and establish critical focus on a high-contrast subject. For cameras with poor LV clarity (D800/E), you may want to take a photograph and evaluate it on the computer to confirm critical focus
2. Set the body to MF. You could set the lens to MF instead but you may jostle the focus ring accidentally if you do so.
3. Look through the viewfinder to see if the camera thinks the subject is in focus. You'll need to half-press the shutter while doing this to keep the metering/rangefinder from going to sleep. If you see the green arrow pointing to the left, then the camera+lens is back-focusing, so you need to decrease the value of AF tune (negative adjustment). If you see the green arrow pointing to the right, then the camera+lens is front-focusing, so you need to increase the value of AF tune (positive adjustment).
4. Repeat step 3 until you get a green-dot from the rangefinder.
5. Depending on focal length and subject distance there will be a range of AF tune values which produce green-dot confirmation. For example on my D800 w/50G f/1.4 @ 5' subject distance, I get the green dot from -6 to -14. Conceptually the optimal AF tune value should be the value that's the middle of the range, which in my case is -10 (I independently verified this using a LensAlign). So you'll want to establish the range of green-dot values by progressively increasing the AF tune values until the camera stops showing the green-dot (it'll alternate between the green dot and an arrow when it's at the margin of the range)...and repeat that procedure in the opposite direction to find the other end of the range.

Some notes:

• When establishing the precise range of green-dot AF tune values you may need to examine the rangefinder for several seconds. I've found that for the extreme margins of the AF tune range the camera may take a few seconds to flicker the arrow/green-dot.

Questions and suggestions welcomed!

There was a thread similar to this where someone posted this link: Camera Micro adjust for free

http://www.northlight-images.co.uk/article_pages/cameras/1ds3_af_micoadjustment.html

It is for Canon, but works equally well for Nikon.

It is almost the same deal and but uses a pattern on your computer. When you see the beat patterns you are in focus. By switching back from LV to PD AF, you can see the error by watching the focus move or the green dot. Very, very accurate from what I can tell. Very easy to see the beat pattern for good focus.

I verified my microAF settings from FoCal with this method (it matched) and it cost nothing. It is very easy to find and dial in the correct AF setting.

Regards,

Horshack wrote:

Assuming the technique in the OP holds up to scrutiny, firmware could potentially automate the process of fine tuning via the following procedure:

1. User selects a menu item/button that reads "Fine-tune AF"
   
2. Firmware prompts the user to position the camera so that the center AF point is over a high-contrast target, preferably with the camera mounted on a tripod. Firmware prompts user to press a button to continue. Everything from this point is automated...
   
3. Firmware lifts mirror and performs multiple CDAF (LV) acquisitions to establish the maximum contrast possible, then performs a final CDAF series to re-achieve the maximum determined contrast.
4. Firmware drops mirror and samples the PDAF sensor to establish the split-beam direction (front/back focusing) and magnitude (delta) from what would otherwise be sampled as a centered PDAF reading. Firmware converts that delta to an AF tune value, rounding to the nearest integer if necessary. Preferably firmware would implement higher-precision AF tune increments than what is presently available, since the delta measured through this process would presumably be more precise than a user-measured reading.
5. Firmware stores the reading in nonvolatile-memory and the procedure is done.
   

The process would only take a few seconds, depending on how many CDAF samples are necessary to establish optimal contrast. It would fast enough where photographers could take spot readings in the field if they wanted, tuning the AF for specific lighting conditions/shooting distances/etc..

If this actually works why not send Nikon this idea. No idea if they would do it but worth a shot. Make it a heck of alot easyer to do .

The flaw in the method...

The technique is dependent on the left arrow and the right arrow boundary points being within the -20 to +20 range.  You're probably SOL with this method if you have lenses that require AF fine tune values above say -15 or + 15.