focusing confusion

[nwilkins]

If you read the title of this post it will probably come as no surprise that I am a newcomer to LF. I have been doing some reading and I thought I was pretty much understanding what happened with tilts/swings.

But then I read the following (from Howard Bond's checklist):

"a tilted plane of focus moves parallel to itself when the focus knob is turned, just as it does with a rigid camera. Choose two targets that are in a plane that is parallel to the one you want to make sharp. After these target images have been made sharp with the help of a swing or tilt, turn the focus knob to move the focus to the desired plane. "

I thought that when you focused the camera it rotated the angle of the plane of focus along the hinge line? Or am I getting that wrong?

Thanks in advance,

Nick

[Heroique]

Hi Nick,

I'll try to offer a very, very simple answer, but you might also consult AA's The Camera for a great overview about LF focusing + very helpful illustrations.

Once you visualize your best plane of focus, you can "get there" (that is, move your camera's plane of focus to coincide with this "best" plane of focus) by using, say, front swings/tilts and/or the focusing knob. Using front swings/tilts moves the plane differently than the focusing knob. You normally hear the words "hinge lines" and "rotation" for the former, and "parallel movement" for the latter. It might take both to arrive at the best plane of focus, but a neutral camera (no movements, just focusing with the knob) can get you there, too.

Using DOF to satisfy your focus needs is a related (and very important) focusing issue.

[ic-racer]

am I getting that wrong?
You are getting that wrong . Keep reading and practicing.

[Bob Salomon]

Read pages 10 to 12.

http://linhof.com/wp-content/uploads/2015/03/why_large_format.pdf

[nwilkins]

You are getting that wrong . Keep reading and practicing.

so changing the distance between the film and lens does not change the angle of the plane of focus? Really?

[nwilkins]

Read pages 10 to 12.

http://linhof.com/wp-content/uploads/2015/03/why_large_format.pdf

thanks Bob but I didn't see anything about the effect that focusing has on the plane of focus in that pdf

[nwilkins]

okay I am realizing I didn't make myself very clear. so I'll try and explain again. I'm sorry for the confusion. new post coming in a minute here

[nwilkins]

Okay here goes:

I understand what happens to the plane of focus when I tilt the lens. It is now no longer parallel to the film plane. It now extends away from the camera at some angle, and is the shape of a cone.

Everything I have tried out on the ground glass (and read about) seems to indicate to me that changing the distance between the film and lens (let's call this x) changes the angle of the plane of focus. so at a given x the plane of focus might be parallel to the ground, but at a different value for x the plane of focus might start at the ground but move higher as it extends away from the camera.

So I thought that this was the rotation of the plane of focus (around what the Merklinger website calls the hinge line).

Is this correct? ic-racer says no but if it isn't correct then I am more confused than ever.

[Heroique]

…the shape of a cone.

You might try "wedge," not "cone."


I am more confused than ever.

Do you have a local library?

Excellent books on technique with awesome illustrations:
1) The Camera by Ansel Adams
2) View Camera Technique by Leslie Stroebel
3) Using the View Camera by Steve Simmons

You might also use the site's search function. Below are some really cool terms to learn about, from our very own armchair and practitioning experts:

Geometric plane
Film plane
Lens plane
Plane of focus
Best plane of focus
Front movements, back movements
Scheimpflug rule (or principle)
Geometric perspective
Viewpoint perspective
Depth of field (DOF)

[Bob Salomon]

Try it this way.
The depth of field, when shooting at normal distances, runs 1/3rd toward the camera - from the point focused on - and 2/3rds away from that point.

You use your tilts/swings to capture the plane of focus desired. You ensure the apparently sharp area desired by focusing 1/3rd into the desired area and stop down so the entire desired surrounding area is also sharp. Remembering that most lenses are diffraction limited to f22 so you usually do not want to exceed that stop.

Scheimpflug/DOF calculators like Rodenstock's help you to easily compute the proper angles and apertures to capture these shots easily and in expensively by measuring the difference between the near and far points desired when you focus on those points. You might want to look this pocket sized double sided calculator up. It works for all formats from 35mm to 810.

[Paul Metcalf]

nwilkins - changing the distance between the film plane and the lens plane using the focus knob does not change the angle of the focus plane

[ic-racer]

The practical focusing example in the OP works if the lens tilt is not so much as the hinge point is down near the center of the earth. You will see a rotation if the hinge point is near the tripod legs.
140124

[mdarnton]

You are right--focusing will theoretically tilt the plane of focus a bit when you are tilted (yes, if you focus the lens outwards .2mm, the junction of the plane of the lens and the plane of the film will move a tiny bit relative to the junction of the film plane and the subject plane), however, since you already got very close with the original focusing to be able to set the tilt, fine focusing won't change it enough for you to notice. If you set the tilt up for slanted ground infinity forward with your tripod high, then lower your tripod to the ground for a macro shot of the ground, yes, the plane of focus will certainly be wrong even though the ground and the film haven't changed their relationship, but you're not going to do that, are you? It's all about doing reasonable things, not theoretical ones. :-)

[Doremus Scudder]

If you read the title of this post it will probably come as no surprise that I am a newcomer to LF. I have been doing some reading and I thought I was pretty much understanding what happened with tilts/swings.

But then I read the following (from Howard Bond's checklist):

"a tilted plane of focus moves parallel to itself when the focus knob is turned, just as it does with a rigid camera. Choose two targets that are in a plane that is parallel to the one you want to make sharp. After these target images have been made sharp with the help of a swing or tilt, turn the focus knob to move the focus to the desired plane. "

I thought that when you focused the camera it rotated the angle of the plane of focus along the hinge line? Or am I getting that wrong?

Thanks in advance,

Nick


Okay here goes:

I understand what happens to the plane of focus when I tilt the lens. It is now no longer parallel to the film plane. It now extends away from the camera at some angle, and is the shape of a cone.

Everything I have tried out on the ground glass (and read about) seems to indicate to me that changing the distance between the film and lens (let's call this x) changes the angle of the plane of focus. so at a given x the plane of focus might be parallel to the ground, but at a different value for x the plane of focus might start at the ground but move higher as it extends away from the camera.

So I thought that this was the rotation of the plane of focus (around what the Merklinger website calls the hinge line).

Is this correct? ic-racer says no but if it isn't correct then I am more confused than ever.


nwilkins - changing the distance between the film plane and the lens plane using the focus knob does not change the angle of the focus plane

Paul,

You and the others are wrong. Sorry :) I had to consult Merklinger to verify this, but Nick is correct. If a lens tilt is applied and you change the distance between lens and film (i.e., focusing closer or farther away, the Plane of Sharp Focus rotates around the hinge line. It gets closer to parallel with the ground as the bellows draw decreases (It can even go beyond parallel with the right lens) and vice-versa. So, Bond is wrong when he says that a tilted Plane of Sharp Focus moves parallel to itself when one focuses back and forth.

Go here if you don't believe me: http://www.trenholm.org/hmmerk/HMbook18.html Merklinger has great little videos that demonstrate just what the Plane of Sharp focus does when you change bellows extension with lens tilted, tilt the lens (both axis and base tilts) and tilt the back (again, both axis and base tilts). The first video tells the tale...

Nick, you've got it right! (The others above got it wrong...) How useful Merklinger's methods are in the field is another issue. Maybe I can expound a bit on focusing the view camera with a view to Nick's questions:

First, the Plane of Sharp Focus (PoSF) is exactly that, a plane. That means it's two-dimensional and has no depth. The Depth of Field (DoF) is the area on either side of the PoSF in which objects are acceptably sharp (since this latter depends on lots of things, it is not a fixed value; acceptable DoF for a contact print is going to be a lot more than DoF for a big enlargment).

When the lens and film planes are parallel, the PoSF is parallel to them as well and moves back-and-forth, parallel to them, when focusing back-and-forth. DoF is, in this case, parallel to the PoSF and located on either side of it. At closer distances, the DoF is shallower than at infinity, which results in an unevenly distributed DoF (shallower in front, deeper in back) at close distances. Note that this largely becomes irrelevant as we focus closer to infinity, since the DoF distribution becomes almost equal.

When we tilt the lens forward, the PoSF tilts forward away from parallel even more. The planes of film, lens and sharp focus intersect in a line called the Scheimpflug line. The DoF now is wedge, with the shallower DoF closer to the camera and the deeper farther from the camera. For extreme tilt (say we've positioned the PoSF parallel with the ground), the DoF extends above and below the PoSF, not in front and in back of it. For intermediate positions the DoF is similarly positioned, somewhere between above and below and in front and in back of (hope this is clear).

Knowing all of this helps one to visualize where the PoSF should go in a particular scene in order to get what one wants in focus and avoid unpleasant surprise areas of unsharpness. It also helps one to use the optimum f-stop for a particular scene (i.e., not having to stop down more than necessary in order to avoid diffraction and/or use a higher shutter speed). Usually, we choose focusing points in the scene that fall in the PoSF and focus on those in order to position the plane where we want it.

Bond, although he didn't understand exactly what the PoSF does when focusing back-and-forth, was explaining how to position the PoSF when such focusing points do not exist. Basically, he picks focusing points parallel to the position of the PoSF he wants, focuses on those and then adjusts focus. The amount of error introduced using this method (i.e., the amount the PoSF tilts from focus-point position to final position) is likely slight enough that the method works in practice. However, Bond is in error here and, if the two positions are really far apart, the amount of change in the tilt of the PoSF could be significant enough to throw something out-of-focus...

Choosing the appropriate f-stop to get the DoF one wants is the companion issue to this. I use and advocate the focus-spread method detailed here: http://www.largeformatphotography.info/fstop.html

Nick, I'd suggest you take a look at the "How to Focus the View Camera" article on the LF homepage as well ( http://www.largeformatphotography.info/how-to-focus.html ). There are a bunch of other great articles there as well.

Hope this clears things up a bit,

Doremus

[tgtaylor]

The quickest and easiest way to approach this (esp in the field) is from the mathematical definition of a plane which goes something like this (without the math):

1. A plane in space has infinite length and breadth but no depth (read depth of field). There are an infinity of possibilities.
2. Three points in space determine a plane. Photographically speaking a particular plane is determined when three points on that plane are in fine focus simultaneously.
3. With the three points in fine focus, you stop-down to give depth (think depth of field) to that particular plane.
4. If you move the focus (lens) further away from or towards the film plane, you are simply rotating the infinity of possibilities for planes in space. Remember, a plane has infinite length and breadth but no depth.

Thomas

[nwilkins]

thanks to everyone who responded, particularly Doremus. The diversity of contradicting responses certainly makes me feel better about having been confused! And thanks to everyone who went out of their way to explain basic view camera focusing principles - I already have a handle on them (and on a process for focusing in the field) but it's nice to see the community is so generous, even to complete beginners.

ps to Heroique yes I did indeed mean wedge and not cone