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by Kim on Sat Feb 23, 2013 6:45 pm
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Here is a link to a new method of aligning lens for accurate focus. I think it looks very promising and seems to work well on my D800. It is Free and requires no fancy equipment.


http://www.youtube.com/watch?v=7zE50jCUPhM&hd=1
 

by DChan on Sat Feb 23, 2013 9:30 pm
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It's the same one that was posted here:

http://www.naturescapes.net/forums/view ... 7&t=228807


And the link to the thread on the FM Forum which describes the method and shows the video:

http://www.fredmiranda.com/forum/topic/1187247/0
 

by Connor Stefanison on Sun Feb 24, 2013 4:35 am
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thanks for posting this!
Its the same type of method as aligning an infrared camera trap system.
 

by neverspook on Sun Feb 24, 2013 8:22 pm
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Thanks for posting this, Kim. It looks like it might be a helpful method that I will have to try.

I have tried several different methods for micro adjusting. My preferred method is to have EJ do it for me. :)

But I have also recently tried Lens Align, Focus Tune and Reikan Focal on my new 500II.

It seems to me that Lens Align does something slighly different than Focus Tune, Reikan Focal and the DotTune method Kim has drawn attention to. With Lens Align, as I understand it, the goal is to set your MA so that the DOF is equal front and back of the point where you have focussed. The other methods result in an MA setting that produces greatest sharpness at the point of focus. As I understand DOF, with shorter lenses, setting your MA to maximize sharpness at the point of focus will result in DOF extending 1/3 in front of point of focus and 2/3 behind. That would mean that the MA set using one of these non-Lens Align methods would produce an MA setting that would appear back-focussed using the Lens Align method. Am I correct here?

And if I understand DOF, for longer lenses, DOF tends to be close to 1/2 in front and 1/2 behind the point of focus, so the various methods should produce similar results to each other. Is that correct? Though I am not quite sure at what focal length that 50-50 DOF arrangement occurs. If anyone has the answer, would love to know.

Assuming I am correct in what I have written above, is it better to set an MA that has maximum sharpness at the point of focus, or does it make more sense to set all lenses to have equal DOF in front of and behind the point of focus? What are the advantages/disadvantages of each method for shooting wildlife and fast action?

Does using the Lens Align method to equalize DOF in front and behind the point of focus, since maximizing sharpness at the point of focus is not the goal, result in some undesirable lack of critical sharpness at the point of focus (e.g. the eye of an animal), especially when shooting wide open? Or is that generally covered adequately by the DOF anyway?

The DotTune video Kim posted does say that if you want to set things up for equal front and back focus relative to the point of focus, you can do that using the DotTune method with a 3-D target such as the Lens Align target. But I am having trouble figuring out how to do that using the DotTune method. Any help here would be appreciated.

Also, I have noticed that there is, with some lens/camera combinations in particular, a noticeable difference in what MA setting any of the methods I have tried produces depending on whether I defocus between shots toward infinity vs toward MFD. Does the DotTune method solve that issue?

And when defocussing, is it best to just defocus a small amount or a fair amount (which is often the case in real life). I have generally defocussed quite a bit when using Lens Align. But FocusTune says to just defocus a small amount in one direction (but does not suggest which direction). And watching what Reikan Focal did when trying its automatic method with my 500, it seems to defocus just a very tiny amount.

Any answers to these questions will be much appreciated. Despite all the tools, I still find MAing very frustrating, especially with long lenses (and especially with extenders added).

Thanks.

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by DChan on Sun Feb 24, 2013 8:57 pm
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neverspook wrote:The other methods result in an MA setting that produces greatest sharpness at the point of focus.
Errr...the point of focus is always the sharpest point since day one and by definition whether you get there by manual focusing or auto focusing. That's the way it should be as that is where the focus is. Everything else within the depth of field looks sharp to your eyes but is never as sharp as whatever that is at that focus point.
 

by Glenn NK on Sun Feb 24, 2013 9:15 pm
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It is my understanding that the amounts of DOF in front of and behind the point of sharpest focus is not the same?

IOW the DOF is not centered about the POF.

Is this incorrect?

Glenn
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by neverspook on Sun Feb 24, 2013 9:24 pm
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DChan wrote:
neverspook wrote:The other methods result in an MA setting that produces greatest sharpness at the point of focus.
Errr...the point of focus is always the sharpest point since day one and by definition whether you get there by manual focusing or auto focusing. That's the way it should be as that is where the focus is. Everything else within the depth of field looks sharp to your eyes but is never as sharp as whatever that is at that focus point.
The point of focus is NOT always the sharpest point - otherwise no one would ever have to worry about micro adjusting at all, in the first place.

And as I understand it, using Lens Align to assign equal focus front and back of the point of focus will not necessarily give you the greatest sharpness at the target itself, which is what is focussed on. Lens Align instructions say, as I recall, to equalize front and back focus, but that is not necessarily the same thing as mazimizing focus at the actual point of focus (ie the target).

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by DChan on Mon Feb 25, 2013 12:59 am
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neverspook wrote:
DChan wrote:
neverspook wrote:The other methods result in an MA setting that produces greatest sharpness at the point of focus.
Errr...the point of focus is always the sharpest point since day one and by definition whether you get there by manual focusing or auto focusing. That's the way it should be as that is where the focus is. Everything else within the depth of field looks sharp to your eyes but is never as sharp as whatever that is at that focus point.
The point of focus is NOT always the sharpest point - otherwise no one would ever have to worry about micro adjusting at all, in the first place.
Hmmm....your understanding seems to be off to me. And you're the first person who would tell me the sharpest point is not the focus. But, I could be wrong of course :)

Well, the way I understand it, people do micro adjustment because their system is not focusing on where they want the lens to focus on. It focuses, but it's not just where the photographer wants the focus to be. If you line up say 3 AA batteries diagonally and take a photograph of them, with the intention to focus on the middle one. If your camera front focus, you could find the nearest AA battery to be in focus instead of the middle one. If it's back focus, the farthest AA battery would be in focus. All of this certainly depends on how off the AF is in the first place. See? There's front/back focus problem but the lens still focuses. There's always something in focus, only that it is not what you want to be in focus. And to adjust AF is to adjust it so that the lens can be focusing on where you want it to be all the time, ideally. And that's my understanding.
 

by KeBul on Mon Feb 25, 2013 2:46 am
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Probably just a mis-reading/understanding of the terminology used...

Roberta's "point of focus" being the point at which the camera/lens has chosen to focus.

DChan is referring to the plane of focus, which is for a want of a better term "a single line of true focus" (often curved depending on the optical design).

Kev
 

by Colin Inman on Mon Feb 25, 2013 7:30 am
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I was thinking the same Kev.

Robertas point about dof in front & behind the plane of focus is a good one, and I assume why they recommend a certain distance from the lens to target.
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by Neilyb on Mon Feb 25, 2013 7:40 am
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I have tried this method whilst sitting in my hide, on high contrast objects. It does work but the mid point of the adjustment values is never the sharpest point, I usually end up adding +2 or 3. (or is it minus.. either way..)
 

by neverspook on Mon Feb 25, 2013 2:28 pm
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Thanks for clarifying the confusion, Kev. You are correct there.

Neil, is it the DotTune method you tried?

And if so, I was thinking further about that method. Since I believe with short lenses that sharpest focus gives 1/3 front and 2/3 back focus, then selecting the midpoint of the MA range as DotTune suggests, you would probably end up doing more or less what Lens Align suggests, with equal front and back focus. If so, then picking the middle MA of the DotTune range, would effectively result in some back focus relative to the sharpest point (and to bring the focus forward again, you would do a minus adjustment, I believe.)

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by Kim on Mon Feb 25, 2013 4:30 pm
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The method has had some amendments/refinements in the past 48 hours with particular reference to Nikon cameras.


http://forums.dpreview.com/forums/post/50922834
 

by neverspook on Mon Feb 25, 2013 5:55 pm
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It seems that for shorter lenses, the 1/3-2/3 front-back focus does not apply when focussed on a close subject. This ratio actually varies quite a lot more than I expected with focus distance for shorter lenses. It does tend to be around 1/2-1/2 at all distances for long lenses, though.

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by KeBul on Mon Feb 25, 2013 6:58 pm
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I wouldn't consider MFA for centre of depth of field to be good enough, If I wanted focus on the eye of a subject - that's where I want it to focus, not with centred dof and the true plane of focus a few inches behind or in front.

Having said that, I'm not convinced DotTune really has anything to do with what we generally consider depth of field, which is a function of perceived sharpness of picture given a particular viewing size and distance.
You have to remember that what we are trying to reconcile here is any difference between two light paths: the path through the lens and mirror box to the phase detect AF array and the path through the lens and mirror box to the focal plane or sensor, for accurate focus to occur the phase detect array must be in sync with the camera's focal plane, i.e. the light paths must match, so when the array sees something in focus that same point is also in focus on the sensor once the mirror is lifted and the image is projected onto the sensor.
So, having set a point of focus directly on the sensor using live view, The DotTune method is using MFA adjustment steps to move the array calibration/fine tune point, the focus confirmation light which is a function of the phase detect array confirms focus 'yes' or 'no' for each calibration/fine tune point providing, over the range of samples, the upper and lower limits of the calibration/fine tune points where the array itself finds focus, the tolerances and/or sensitivity of the AF array gives the range and it is considered that the centre of that range would be very close in perfectly matching the light path to the array and the light path to the sensor/focal plane. Obviously this requires a controlled test target to make sure both methods are focusing on the same object within frame.

A simplistic analogy would be a pair of eyes, 2 light paths viewing the same subject, if we say the right eye has perfect vision (liveview focus) and the left doesn't, (the phase detect array), an optometrist will try various strengths of corrective lenses (MFA adjustments) to correct the vision in the left eye, continuos feedback as to which lens is sharper (focus confirmation dot) should lead to a corrective lens that gives a matching sharp image in both eyes.

So to quote Roberta

"then selecting the midpoint of the MA range as DotTune suggests, you would probably end up doing more or less what Lens Align suggests, with equal front and back focus. If so, then picking the middle MA of the DotTune range, would effectively result in some back focus relative to the sharpest point"
- No not quite since the mid point selected is purely from the AF array's range of confirmed focus successes and this should match the live view sharpest point of focus.
I'm not familiar with it but I assume the lens align method looks at a resulting test picture, in which case for that method you are bringing depth of field into the equation and as correctly surmised if you set the MFA adjustment to equal front and back focus when the dof produced is a 1/3 to 2/3's ratio then you would actually be introducing a small element of front focus to the combination, i.e. the sharpest point would be at the 1/3 point not the 1/2 point.

There may well be other factors involved in the cameras/lens AF story but I think I'm pretty much correct in the above belief (although happy to learn more if I'm wrong).

Kev
 

by neverspook on Mon Feb 25, 2013 7:55 pm
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"I'm not familiar with it but I assume the lens align method looks at a resulting test picture, in which case for that method you are bringing depth of field into the equation and as correctly surmised if you set the MFA adjustment to equal front and back focus when the dof produced is a 1/3 to 2/3's ratio then you would actually be introducing a small element of front focus to the combination, i.e. the sharpest point would be at the 1/3 point not the 1/2 point."

Wouldn't that be back focus, since you are setting MA to be at 1/2 of the way back when it should be 1/3 of the way back for exact focus at the point of focus?

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by neverspook on Mon Feb 25, 2013 8:30 pm
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I had another look at the distance tool on the Lens Align website. http://michaeltapesdesign.com/lensalign.html

For most lenses, and certainly anything 200mm or longer, DOF is very very close to 1/2-1/2 front and back at the distances recommended for MA testing using Lens Align. So setting an MA based on equal front and back focus should be about right. For short lenses, the DOF front and back is less even, especially at the longer recommended distances for Lens Align testing. Of course, with those lenses, DOF is greater anyway, so MA accuracy could be argued to be less of an issue for that reason unless you are shooting wide open.

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by DChan on Mon Feb 25, 2013 9:34 pm
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KeBul wrote:Probably just a mis-reading/understanding of the terminology used...

Roberta's "point of focus" being the point at which the camera/lens has chosen to focus.

DChan is referring to the plane of focus, which is for a want of a better term "a single line of true focus" (often curved depending on the optical design).

Kev
I was simply saying it's a matter of fact that any subject situated at the point of focus is always the sharpest.
 

by DChan on Mon Feb 25, 2013 9:46 pm
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The DOF beyond the subject is always greater than the DOF in front of the subject. When the subject is at the hyperfocal distance or beyond, the far DOF is infinite, so the ratio is 1:∞; as the subject distance decreases, near:far DOF ratio increases, approaching unity at high magnification. For large apertures at typical portrait distances, the ratio is still close to 1:1. The oft-cited rule that 1/3 of the DOF is in front of the subject and 2/3 is beyond (a 1:2 ratio) is true only when the subject distance is 1/3 the hyperfocal distance.
From here:


http://en.wikipedia.org/wiki/Depth_of_field
 

by DChan on Mon Feb 25, 2013 10:00 pm
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Colin Inman wrote:
Robertas point about dof in front & behind the plane of focus is a good one, and I assume why they recommend a certain distance from the lens to target.
That's why the largest aperture (the one with the smallest f-number) should be used during AF fine tuning.
 

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