Finding the Focus Plane

[chung]

Very insightful link for understanding the S priciple and H rule

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

 

[chuck94022]

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.

 

[Chuck_P]

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...............

 

[Chuck_P]

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.

 

[chuck94022]

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.

 

[Chuck_P]

Keep studying.....

I will, as I'm more concerned about getting it right than just being right...

 

[Chuck_P]

Then the essential question to be answered is this assertion made by Bond:

"...a tilted plane of focus moves parallel to itself when the focus knob is turned, just as it does with a rigid camera."

I don't pretend to know anymore about it, it's intriguing though.

 

[chuck94022]

Then the essential question to be answered is this assertion made by Bond:

"...a tilted plane of focus moves parallel to itself when the focus knob is turned, just as it does with a rigid camera."

I don't pretend to know anymore about it, it's intriguing though.

First to your previous comment: I'm not spending time on this just to "be" right. I am interested in this, it is complicated, This forum is read by thousands, and it is worth the effort to make sure folks get the right information.

Bond's assertion directly conflicts with the Scheimpflug principle. If the focus plane moved parallel to its previous angle, the three planes would no longer intersect on the Scheimpflug line. Simple as that. You can draw this out for yourself on graph paper if you can't visualize it.

So, sorry, but Bond is clearly and unambiguously wrong. Merklinger's contribution to this provides the answer to the question left unanswered by Scheimpflug: around what point does the focus rotate, because it surely does. And it must regardless of which standard is moved.

 

[Chuck_P]

First to your previous comment: I'm not spending time on this just to "be" right.

I believe that.

 

[chuck94022]

Chuck, I'll try this with my own crude figures.

Consider the following situation, with forward tilt. The Scheimpflug Line (S Line) is one of the blue dots (a line seen end-on is just a dot. It sits at the intersection of the Film Plane, the Lens Plane, and the Focal Plane. The Hinge Line (H Line) is also a blue dot. It sits a fixed distance vertically from the lens, and its distance is determined by the amount of tilt of the lens:



Now we will rack forward the front standard. Bond claims the focal plane will remain parallel to the former focal plane. But that would result in the following:



Clearly that can't be true, because the focal plane must intersect at the S line (the blue dot). The S line is in a different place, because the front tilted lens has changed position relative to the film plane, because it has moved forward. Thus the intersection is lower now. Thus, the focal plane has to rotate as seen in this correct result:



(Forgive the crude graphics, its the best I could do on short notice. Click the images to see them full size.)

 

[Steve Smith]

It's good that the hinge rule is being discussed. A few years ago when I first got into LF I found a lot of discussion of the Scheimpflug rule with lots of people saying that it was the way of working out where the plane of focus was. I couldn't see it myself as it only shows a single point about which the plane of focus rotates.

It wasn't until I found the article on the hinge rule (on the website I linkled to earlier) that it all made sense.


Steve.

 

[tih]

If you look at the diagrams, and picture the pieces moving, you will find that every single isolated configuration change will move the two lines (Scheimpflug line and hinge line) relative to each other. Since the focus plane is defined by these two, the only single configuration changes that could move the focus plane into a new position that's parallell to the old one are a) one that would move both the same distance in the same direction, and b) one that would move one of the lines along the focus plane.

In practice, the only way to move the focus plane parallell to itself is to change both the angle of the lens, and the distance from lens to film plane. (Well, that, and the singular case where everything is zeroed, and the two lines are off at infinity, of course.)

 

[tih]

I find that http://www.trenholm.org/hmmerk/VCFaDOF1.pdf is very instructive, by the way.

 

[FiatluX]

If you can find the Sinar Photo Know-How, A self study course of sophisticated view camera technique By Carl Koch you´ll find all the answers you need in there.

It was last published in 1983 so the bay (search: Sinar photo know-how) would probably be easiest, great self study book for learning the possibilities of large format!

 

[Chuck_P]

Chuck, I'll try this with my own crude figures.

Consider the following situation, with forward tilt. The Scheimpflug Line (S Line) is one of the blue dots (a line seen end-on is just a dot. It sits at the intersection of the Film Plane, the Lens Plane, and the Focal Plane. The Hinge Line (H Line) is also a blue dot. It sits a fixed distance vertically from the lens, and its distance is determined by the amount of tilt of the lens:

View attachment 51402

Now we will rack forward the front standard. Bond claims the focal plane will remain parallel to the former focal plane. But that would result in the following:

View attachment 51403

Clearly that can't be true, because the focal plane must intersect at the S line (the blue dot). The S line is in a different place, because the front tilted lens has changed position relative to the film plane, because it has moved forward. Thus the intersection is lower now. Thus, the focal plane has to rotate as seen in this correct result:

View attachment 51404

(Forgive the crude graphics, its the best I could do on short notice. Click the images to see them full size.)

Bond is claiming that a tilted plane of sharp focus (refer to as the focus plane) moves parallel to itself, not the focal plane. The focal plane is a line parallel to the lens plane but one focal length in front of the lens------so, to obey the H-Rule, the focal plane, the focus plane, and the parallel-to-film lens plane must converge at the H-line. Taking part of your suggestion, as I draw a line parallel to the focus plane, it continues to intersect the S-line and the parallel-to-film lens plane, but comes off the focal plane line, giving the appearance that an adjustment of the tilt angle is needed to re-establish the H-Line.

By this, then, it appears to me that, while the S-Rule is being obeyed with parallel movement of the focus plane , the H-Rule is not. Merklinger says that is possible, but the camera will not be in focus until both the S-Rule and the H-Rule are obeyed simultaneously. When re-reading Bond's statement, he is either:

- wrong, as you say
- he is right but fails to convey it properly in the article
- he is right, but I have totally misinterpreted his statement

I have to assume that when he says "....a tilted plane of focus moves parallel to itself....when the focus knob is turned...." that he means the front standard. Because, moving the back standard "rotates" the focus plane at the H-Line, and I view a rotation movement as different from a parallel movement.

 

[chuck94022]

Bond is claiming that a tilted plane of sharp focus (refer to as the focus plane) moves parallel to itself, not the focal plane. The focal plane is a line parallel to the lens plane but one focal length in front of the lens------so, to obey the H-Rule, the focal plane, the focus plane, and the parallel-to-film lens plane must converge at the H-line. Taking part of your suggestion, as I draw a line parallel to the focus plane, it continues to intersect the S-line and the parallel-to-film lens plane, but comes off the focal plane line, giving the appearance that an adjustment of the tilt angle is needed to re-establish the H-Line.

By this, then, it appears to me that, while the S-Rule is being obeyed with parallel movement of the focus plane , the H-Rule is not. Merklinger says that is possible, but the camera will not be in focus until both the S-Rule and the H-Rule are obeyed simultaneously. When re-reading Bond's statement, he is either:

- wrong, as you say
- he is right but fails to convey it properly in the article
- he is right, but I have totally misinterpreted his statement

I have to assume that when he says "....a tilted plane of focus moves parallel to itself....when the focus knob is turned...." that he means the front standard. Because, moving the back standard "rotates" the focus plane at the H-Line, and I view a rotation movement as different from a parallel movement.

For what it is worth, when I wrote "focal plane" above I meant "plane of sharpest focus". My bad for trying to save some typing.

The bottom line though as can be seen by the figure is that the light doesn't know whether it is the front or read standard that moves. Both movements accomplish the same optical result, of change the distance between lens and film, as shown. I think it is plain to see that the same figure represents the result of either a front forward movement or a back rearward movement.

 

[Chuck_P]

For what it is worth, when I wrote "focal plane" above I meant "plane of sharpest focus". My bad for trying to save some typing.

The bottom line though as can be seen by the figure is that the light doesn't know whether it is the front or read standard that moves. Both movements accomplish the same optical result, of change the distance between lens and film, as shown. I think it is plain to see that the same figure represents the result of either a front forward movement or a back rearward movement.

I have come to that conclusion, thanks. I also believe that Bond is saying the same thing, but I've just interpreted his words entirely wrong, that's my opinion on it anyway.

Nevertheless, been practicing some more with finding the proper tilt angle, using Merklinger's simple equation for tilt angles less than 15 degrees [angle = focal length / (5 x j)], and it just works like a charm, it puts the angle so very close, it may need just a little tweaking. I had trouble at first deciding how to determine how much of the distance "j" may lie below the ground to intersect the H-line. Simply, I stepped outside the cloth and just envisioned it, plugged in the number and set the angle of the lens plane, I found that each time it needed only the slightest tweaking. This is going to make using my LF camera so much more enjoyable, it had not been fun for a long time due to focusing problems---I believe the proverbial light came on over the head.

Thanks for your help.

Chuck

 

[tih]

This is going to make using my LF camera so much more enjoyable

And that's the important thing!

-tih

 

[henk@apug]

Knowing the theory and math of a view camera is a good thing, but
in the field practical knowledge helps me out more.

I learned to focus the view camera in a practical way just by viewing this
video. It is in the video section I think of apug but here is a link anyway :

http://www.youtube.com/watch?v=gR4m70xr9mE

 

[Chuck_P]

Knowing the theory and math of a view camera is a good thing, but
in the field practical knowledge helps me out more.

I learned to focus the view camera in a practical way just by viewing this
video. It is in the video section I think of apug but here is a link anyway :

http://www.youtube.com/watch?v=gR4m70xr9mE

Yes, I've seen this video-----it's a demonstration of Bond's "Focus/Check" procedure that I referrenced earlier in this thread. In his example, the planes requiring sharp focus for both the tilt and swing are established by the box itself, so putting them in focus is relatively easy compared to when the best plane(s) of focus have to be found, at least that's my experience so far. My understanding of the Hinge Rule, though, has greatly facilitated the "Focus/Check" procedure.