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# Thread: Finding the Focus Plane

1. Originally Posted by PKM-25
rjbuzzclick, your sig is hilarious, I think I have about a dollar in nickels spread across all my camera bags, back packs, etc...
Thanks! You're the first person that's commented on it (or maybe it's just not as funny as I think it is...).

2. Originally Posted by chuck94022
I also understand, based on my reading of Merklinger, that as you shift your focus, the plane actually swings around what Merklinger names a "hinge" line, located below the lens, rather than "moving parallel to itself" as Bonds was quoted as saying above.
The way I understand it----racking the camera back forward or backward from the lens causes the plane of sharp focus to rotate about the hinge line, but moving the lens forward or backward will move the plane of sharp focus parallel to itself. THis seems to make sense---picture the lens parallel with the film plane, moving the lens forward or backward moves the plane of sharp focus forward or backward parallel to itself, it seems that relationship holds if the lens is tilted.

3. Originally Posted by CPorter
The way I understand it----racking the camera back forward or backward from the lens causes the plane of sharp focus to rotate about the hinge line, but moving the lens forward or backward will move the plane of sharp focus parallel to itself. THis seems to make sense---picture the lens parallel with the film plane, moving the lens forward or backward moves the plane of sharp focus forward or backward parallel to itself, it seems that relationship holds if the lens is tilted.
Maybe. I'm no expert. But I don't know how, mathematically, moving the front or the back standard (without changing any other movement) is any different (optically or mathematically). The one change the front standard movement incurs is the distance between the lens and the object. But the object itself is not involved (optically, mathematically) in the relationship between tilt and plane of focus. So I don't understand how one would keep the plane parallel and the other would cause rotation. But I keep reading the book, maybe one day I'll get it.

4. Originally Posted by chuck94022
Maybe. I'm no expert. But I don't know how, mathematically, moving the front or the back standard (without changing any other movement) is any different (optically or mathematically). The one change the front standard movement incurs is the distance between the lens and the object. But the object itself is not involved (optically, mathematically) in the relationship between tilt and plane of focus. So I don't understand how one would keep the plane parallel and the other would cause rotation. But I keep reading the book, maybe one day I'll get it.
I found this illustration, http://www.trenholm.org/hmmerk/HMbook18.html, I've never really understood the hingeline concept until I found this. There is no mention of the movement of the lens forward or back, but it only stands to reason (for my understanding) anyway. When a rigid lens is focused, the plane of focus moves forward or backward parallel to itself, tilting the lens should not change that relationship (it seems anyway), so I tend to believe what Bond is saying, it's enough to get one cross-eyed though.

5. ## Amazing.......

I practiced focusing the camera today and, through finally understanding the Hinge Line rule, and understanding that the Hinge Line rule is not independant of the Scheimflug rule, I have cut my time clean in half in finding the tilt angle and adjusting the orientation of the plane of focus with the camera back. Something definitely clicked for me today when it comes to using a LF camera. Viewing those Quick Time movie clips for the first time that put the whole process in motion is what did it---I just failed with the diagrams alone without the motion. Using Merklinger's table for finding the tilt angle is a snap as well. Amazing.

6. Very insightful link for understanding the S priciple and H rule

http://www.trenholm.org/hmmerk/VuCamTxt.pdf

7. Originally Posted by CPorter
I found this illustration, http://www.trenholm.org/hmmerk/HMbook18.html, I've never really understood the hingeline concept until I found this. There is no mention of the movement of the lens forward or back, but it only stands to reason (for my understanding) anyway. When a rigid lens is focused, the plane of focus moves forward or backward parallel to itself, tilting the lens should not change that relationship (it seems anyway), so I tend to believe what Bond is saying, it's enough to get one cross-eyed though.
Actually I believe it reinforces what I stated before. If the distance between the film plane and the lens changes without a change in tilt, according to these rules the plane of sharpest focus must rotate around the hinge line regardless of whether the change is due to front or rear standard movement. Think about it. If the plane of sharpest focus must touch both the H and S lines, and the lens tilt doesn't change (which is the only way the H line moves vertically) the result must be a change of angle of the plane of sharpest focus, unless the lens is not tilted relative to the film plane, because the vertical position of the S line will change.

So Bond is incorrect.

8. Originally Posted by chuck94022
Actually I believe it reinforces what I stated before. If the distance between the film plane and the lens changes without a change in tilt, according to these rules the plane of sharpest focus must rotate around the hinge line regardless of whether the change is due to front or rear standard movement. Think about it. If the plane of sharpest focus must touch both the H and S lines, and the lens tilt doesn't change (which is the only way the H line moves vertically) the result must be a change of angle of the plane of sharpest focus, unless the lens is not tilted relative to the film plane, because the vertical position of the S line will change.

So Bond is incorrect.
Bond is referencing a tilted lens that is focused accurately but on the wrong plane-----so, moving the lens to increase or decrease the distance of the lens from the camera's back moves the plane of focus parallel to itself. But moving the camera's back to increase or decrease the distance from a tilted lens causes the plane of sharp focus to rotate at the hinge line. My study on it is taking me to a different place, I guess I have to disagree with you. But I'm still studying on it, so things could change...............

9. Originally Posted by chung
Very insightful link for understanding the S priciple and H rule

http://www.trenholm.org/hmmerk/VuCamTxt.pdf
Yes, that is the other link that I have. I wasn't able to view the motion until I downloaded Quick Time for MAC and PC, perhaps others may need it too.

10. Originally Posted by CPorter
Bond is referencing a tilted lens that is focused accurately but on the wrong plane-----so, moving the lens to increase or decrease the distance of the lens from the camera's back moves the plane of focus parallel to itself. But moving the camera's back to increase or decrease the distance from a tilted lens causes the plane of sharp focus to rotate at the hinge line. My study on it is taking me to a different place, I guess I have to disagree with you. But I'm still studying on it, so things could change...............
Keep studying, because what you wrote, with all due respect, makes no sense.

It is not physically possible to keep the plane of sharpest focus on both the Hinge line and the Schempelflug line if the distance between the film plane and lens changes, without changing the angle of the plane of sharpest focus.

If Bond were right, the plane would no longer touch the S line at all.

Sorry if you disagree, but that doesn't change the optics. Bond was wrong.

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