DotTune: Video Tutorial for AF tuning without photographs

Started Feb 18, 2013 | Discussions
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FtoDin5min Regular Member • Posts: 407
Re: DotTune: Video Tutorial for AF tuning without photographs

What would one think is an acceptable difference between side and center sensors AF Fine Tune numbers? Do we know roughly what a unit of AF Fine Tune corresponds to? Formulae/absolute distance?

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FTD5

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Kaj E Veteran Member • Posts: 9,247
Re: Why the middle of the range?

Knowing that the sharp focus is reduced more rapidly from perfect focus in the direction toward the camera than away from it , why do you propose setting the  focus at the middle of the acceptable range and not for instance at 1/3 from the closer edge of the range? I would suspect that the green dot goes to blinking more abruptly at the closer focus side. Have not tried your method yet, this is just how I would expect things to behave.

Have you done any test to confirm that the middle of the range corresponds to the best Live view focus?.

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Robin Casady Forum Pro • Posts: 12,898
Re: Why the middle of the range?

Kaj E wrote:

Knowing that the sharp focus is reduced more rapidly from perfect focus in the direction toward the camera than away from it , why do you propose setting the focus at the middle of the acceptable range and not for instance at 1/3 from the closer edge of the range? I would suspect that the green dot goes to blinking more abruptly at the closer focus side. Have not tried your method yet, this is just how I would expect things to behave.

Have you done any test to confirm that the middle of the range corresponds to the best Live view focus?.

Kaj,

We look forward to your testing of your hypothesis.

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TOF guy MOD Forum Pro • Posts: 13,123
Kaj's question is a good one IMO
1

Robin Casady wrote:

Kaj E wrote:

Knowing that the sharp focus is reduced more rapidly from perfect focus in the direction toward the camera than away from it , why do you propose setting the focus at the middle of the acceptable range and not for instance at 1/3 from the closer edge of the range? I would suspect that the green dot goes to blinking more abruptly at the closer focus side. Have not tried your method yet, this is just how I would expect things to behave.

Have you done any test to confirm that the middle of the range corresponds to the best Live view focus?.

Kaj,

We look forward to your testing of your hypothesis.

The 1/3 -2/3 rule is far from general of course. But Kaj's objection certainly would apply IMO in any situation where the DOF does not extend the same in front and back of a target.

Example:

  • Testing a 200 mm lens at 10 feet (about 3 meters). Distance of focus is centered (give or take a very small difference) in the range of distances within the DOF  (rule of 1/2 1/2)
  • Testing a 28 mm lens at 10 feet. Distance of focus is 1/3 away from the nearest distance  within DOF and 2/3 away from the furthest (rule of 1/3 2/3)
  • Of course there are many other ratios possible, up to "infinity" (far distance extends to "infinity")

For the typical distances used to test a lens one would most likely be within the range of a 1/2 1/2 rule (you're more likely to test the 28 mm with a target closer to the camera), but there may be a correction to apply in this test as Kaj suggests i.e. consider how far the DOF extends behind and in front of the focus plane and apply the necessary correction. See for instance this calculator(which I did use just prior to posting just to make sure ).

Very valid point The O/P should add a comment in his video IMO.

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Horshack OP Veteran Member • Posts: 5,803
Re: Kaj's question is a good one IMO

TOF guy wrote:

The 1/3 -2/3 rule is far from general of course. But Kaj's objection certainly would apply IMO in any situation where the DOF does not extend the same in front and back of a target.

Example:

  • Testing a 200 mm lens at 10 feet (about 3 meters). Distance of focus is centered (give or take a very small difference) in the range of distances within the DOF (rule of 1/2 1/2)
  • Testing a 28 mm lens at 10 feet. Distance of focus is 1/3 away from the nearest distance within DOF and 2/3 away from the furthest (rule of 1/3 2/3)
  • Of course there are many other ratios possible, up to "infinity" (far distance extends to "infinity")

For the typical distances used to test a lens one would most likely be within the range of a 1/2 1/2 rule (you're more likely to test the 28 mm with a target closer to the camera), but there may be a correction to apply in this test as Kaj suggests i.e. consider how far the DOF extends behind and in front of the focus plane and apply the necessary correction. See for instance this calculator(which I did use just prior to posting just to make sure ).

Very valid point The O/P should add a comment in his video IMO.

The center of the focus-dot confirmation range is a proxy for the where the camera itself would focus, so the balance of front/rear DOF should be the same vs. a body+lens combo requiring no fine tuning. In the 'additional notes' section of the video I discuss the option of using a 3-D focus target like a LensAlign for those who want to DotTune to a precise point within the DOF if they want control over the balance of front/rear DOF.

Horshack OP Veteran Member • Posts: 5,803
Re: DotTune: Video Tutorial for AF tuning without photographs

FtoDin5min wrote:

What would one think is an acceptable difference between side and center sensors AF Fine Tune numbers? Do we know roughly what a unit of AF Fine Tune corresponds to? Formulae/absolute distance?

Likely within a few points but I don't have my 24G with me to establish. Visually the difference should be small enough where both peripheral points produce acceptable and consistent sharpness when using the same tune value as the center, but I haven't translated that into precise AF tune points but I'd guess it's maybe 2-3 points max.

Horshack OP Veteran Member • Posts: 5,803
Re: Kaj's question is a good one IMO

Horshack wrote:

The center of the focus-dot confirmation range is a proxy for the where the camera itself would focus, so the balance of front/rear DOF should be the same vs. a body+lens combo requiring no fine tuning. In the 'additional notes' section of the video I discuss the option of using a 3-D focus target like a LensAlign for those who want to DotTune to a precise point within the DOF if they want control over the balance of front/rear DOF.

But admittedly using a 2-D target not only robs you of precise DOF balance but may also mean you're tuning closer to one end of the DOF extreme than you'd otherwise want to be, thus not centered on whatever the natural front/fear DOF balance would be for a theoretic perfectly tuned body+lens. Such is the compromise of using a 2-D target for any tuning method.

J2photo Regular Member • Posts: 239
Re: Kaj's question is a good one IMO

Thanks Horshack for your hard work in making this video!  This is why I visit these forums.  Many will benefit from your hard work.

clarnibass Senior Member • Posts: 1,955
Re: DotTune: Video Tutorial for AF tuning without photographs

Yes, same thing

By the way, I didn't have tons of time yet, but I briefly checked my 85mm lens yesterday. I set up a 3D target with a flat area that was significantly bigger than the focus point. I wanted the front and back objects to show where the focus is etc.

What I found so far is that the range was much bigger. 0 got the confirmation dot. I think -7 still got it (I'll check again later), but on the positive side it got to +20 and still confirmed.

Is it strange that the range was so big? You and others seemed to get smaller ranges. At +20 focus was deifnitely not great, but of course some of the back subjects were in better focus. Is it possible that the front and back subject were too close to the focus point, making the camera think it was in focus. Maybe I should start with a flat target first.

lock Veteran Member • Posts: 6,189
Is that true?

Are hardware-measured far and close edges of the in-focus range range the same as the far and close edges of the range of perceived acceptable sharpness ?

"When the circle of confusion becomes perceptible to our eyes, this region is said to be outside the depth of field and thus no longer "acceptably sharp."  Are we now saying that AF sensors are affected by the same effects as our eyes ?

lock

Horshack OP Veteran Member • Posts: 5,803
Re: Is that true?

lock wrote:

Are hardware-measured far and close edges of the in-focus range range the same as the far and close edges of the range of perceived acceptable sharpness ?

"When the circle of confusion becomes perceptible to our eyes, this region is said to be outside the depth of field and thus no longer "acceptably sharp." Are we now saying that AF sensors are affected by the same effects as our eyes ?

I'm not exactly certain where the peak AF phase alignment aligns within the front/fear DOF sphere, but I am pretty sure that the midpoint of the VF confirmation range aligns to where the PDAF system acquires focus, so the question is not specific to DotTune but to the PDAF system itself.

Horshack OP Veteran Member • Posts: 5,803
Re: DotTune: Video Tutorial for AF tuning without photographs

clarnibass wrote:

Yes, same thing

By the way, I didn't have tons of time yet, but I briefly checked my 85mm lens yesterday. I set up a 3D target with a flat area that was significantly bigger than the focus point. I wanted the front and back objects to show where the focus is etc.

What I found so far is that the range was much bigger. 0 got the confirmation dot. I think -7 still got it (I'll check again later), but on the positive side it got to +20 and still confirmed.

Is it strange that the range was so big? You and others seemed to get smaller ranges. At +20 focus was deifnitely not great, but of course some of the back subjects were in better focus. Is it possible that the front and back subject were too close to the focus point, making the camera think it was in focus. Maybe I should start with a flat target first.

I would first try the method again with the focus re-established, in case it was off by just a smidgen. I'd also try a 2-D target as well. If you still get confirmation at +20 try the workaround here. Regarding the width of the range, I wouldn't worry about it once it fits within the +/- 20 tunable range - I've written elsewhere that I think the slop in VF dot is by design; if it were more precise it might prove too difficult to use for manual focusing.

lock Veteran Member • Posts: 6,189
It uses a symetrical approach.

So yes, i would expect it to be in the middle. The dof principle is about how we perceive things as sharp. That is something different.

ARB1
ARB1 Veteran Member • Posts: 3,137
Re: DotTune: Video Tutorial for AF tuning without photographs

Excellent work, thanks for sharing this.

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Orionisf Forum Member • Posts: 60
Re: DotTune: Video Tutorial for AF tuning without photographs

Tried to fineture one of my lenses.

And it went fine.

Had to adjust the the lens focuspoint to -17, because it did a confirmed focus from -13 to -20.

Then I wondered can it be true.

I took another test without liveview, the good old way.

The result was completely different with a result at +8.

The focus was spot on.

Wonder why?

Any other has tried the good old way afterwards?

And what was the result?

Horshack OP Veteran Member • Posts: 5,803
Re: DotTune: Video Tutorial for AF tuning without photographs

Orionisf wrote:

Tried to fineture one of my lenses.

And it went fine.

Had to adjust the the lens focuspoint to -17, because it did a confirmed focus from -13 to -20.

Then I wondered can it be true.

I took another test without liveview, the good old way.

The result was completely different with a result at +8.

The focus was spot on.

Wonder why?

If you got -20 on one attempt (beyond range) and +8 on another then something isn't right. Which lens and what focus distance?

Any other has tried the good old way afterwards?

And what was the result?

Most who have compared DotTune to the results they arrived at using other methods have said they aligned within +/- a point. You can read some of their feedback here.

TOF guy MOD Forum Pro • Posts: 13,123
Misconception

lock wrote:

Are hardware-measured far and close edges of the in-focus range range the same as the far and close edges of the range of perceived acceptable sharpness ?

"When the circle of confusion becomes perceptible to our eyes, this region is said to be outside the depth of field and thus no longer "acceptably sharp." Are we now saying that AF sensors are affected by the same effects as our eyes ?

DOF is not defined as a range of visually perceived acceptable sharpness, it is defined by the circle of confusion i.e. as much blur is allowed in the final print.

As far as vision is concerned it has to be linear for the most part because the picture is projected on the paper (or screen) plane. A feature which should be sharp, but is very slightly blurred, may still look adequately sharp if the blur is smaller than the circle of confusion. Regardless of whether in reality the feature is behind or in front of the focus plane we still look at it on the picture at the same eye to print distance, which would be by far the primary factor.

There are non-linear factors. The best known example is: the picture looks sharp if the eyes of the main subject are sharp. We may pay more sensitive to sharpness in the center of the image (or the immediate surroundings of the main subject) than away from it. And yes when looking at a picture we may instinctively more sensitive to detail in areas of the image that we guess are closer to the camera which took the picture. I don't think it's much of a factor.

In any it's totally irrelevant (!), since as stated before DOF used an an objective (not perception based) definition. In other words it's based on what the camera sees, not our eyes.

The objection remains: if the DOF does not extend evenly behind and in front of the focal plane then Horshak suggestion to use the center of the area delimited as in focus by the dot will induce an error. That is unless the camera corrects for that, which I very much doubt. I suppose (but not tested) that the way the dots work is simply by using the AF sensor, without correction. If the AF sensor detects a line of constrast as separated on 2 sensors then it's out of focus. If they match it's in focus. It's in fact looking at the circle of confusion and the camera calls it in focus if this circle is small enough that the AF sensor cannot make a difference whether it can be even smaller or not.

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Orionisf Forum Member • Posts: 60
Re: DotTune: Video Tutorial for AF tuning without photographs

Horshack wrote:

Orionisf wrote:

Tried to fineture one of my lenses.

And it went fine.

Had to adjust the the lens focuspoint to -17, because it did a confirmed focus from -13 to -20.

Then I wondered can it be true.

I took another test without liveview, the good old way.

The result was completely different with a result at +8.

The focus was spot on.

Wonder why?

If you got -20 on one attempt (beyond range) and +8 on another then something isn't right. Which lens and what focus distance?

The lens is Nikon 200-400 F4 VRII from a distance of 10 meters and the focus is spot on no matter if it's 2 or 50 meters with the good old ajustment.

Any other has tried the good old way afterwards?

And what was the result?

Most who have compared DotTune to the results they arrived at using other methods have said they aligned within +/- a point. You can read some of their feedback here.

lock Veteran Member • Posts: 6,189
I doubt it.

"When the circle of confusion becomes perceptible to our eyes, this region is said to be outside the depth of field and thus no longer "acceptably sharp."

"When does the circle of confusion become perceptible to our eyes? An acceptably sharp circle of confusion is loosely defined as one which would go unnoticed when enlarged to a standard 8x10 inch print, and observed from a standard viewing distance of about 1 foot."

At this viewing distance and print size, camera manufactures assume a circle of confusion is negligible if no larger than 0.01 inches (when enlarged). As a result, camera manufacturers use the 0.01 inch standard when providing lens depth of field markers (shown below for f/22 on a 50mm lens). In reality, a person with 20-20 vision or better can distinguish features 1/3 this size or smaller, and so the circle of confusion has to be even smaller than this to achieve acceptable sharpness throughout."

So it is a somewhat loosely defined yet standardized concept, but basically it depends on our perception as desribed in the last two sentences.

Add to this: "...DoF concept: it only accounts for the total DoF and not its distribution around the focal plane, even though both may contribute to the perception of sharpness".

lock

source in this case: cambridgeincolour.com

Shotcents
Shotcents Senior Member • Posts: 4,472
works okay

So I followed the directions in the video and it certainly works okay.

I had actually been using a simple system with a row of AA batteries, focusing on the center battery and adjusting -/+ until I was happy.

Using the test in this thread I found the AF tune values agreeing with my own using the admittedly primitive battery test.

Good show!

Robert

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