how to focus the view camera (original) (raw)

Part 2 of Setting up the view camera

by Q.-Tuan Luong for the Large Format Page

Summary: two visual, iterative methods to focus the view camera on a chosen plane of arbitrary location, assuming that you want to maximize sharpness, as in landscape photography.

• The procedure I is 100% visual. All assessments are done by looking at the ground glass. It can be used with any view camera and does not require any accessory. It involves a bit of guesswork (the estimation of the best plane) and a few simple and fast iterations (the actual adjusting of the tilt/swing). Variants of this procedure are used by many photographers.
• The procedure II requires that you have a metric scale, ideally attached to your camera rail or bed. Measurements on the scale are used to make assessments. This procedure finds the best plane instead of having you guess it, however each iteration is more complex. There are probably as many visual techniques for focusing when employing a Scheimpflug relationship as there are photographers doing so.

In practice, what I personally use is procedure I (focus on far, tilt on near) when photographing mostly flat/planar subjects. When the subject is tall, most of the time I use procedure II, going directly to step 2 (spread focus between far and near). If this results in a very small aperture (f45 or worse), I might try the full procedure II (minimize focus spread with tilt) to get a larger aperture.

I recommend using a metric scale. It is also necessary for the optimal determination of the f-stop.

Principles

The Scheimpflug rule (named after an Austrian Army officer who patented it in 1904), states that the film plane, the subject plane (plane of sharp focus), and lens plane (the plane through the optical center and perpendicular to the lens axis) must converge along a single line. See references.

We all know that when a rigid camera is focused, all the objects which are at a same perpendicular distance from the film plane can be brought in focus at a time. For a rigid camera, the film plane and the lens plane are parallel (or equivalently intersect at infinity). So the subject plane must also be parallel to these two planes.

In a view camera, the geometric relationship between the film plane (the back standard) and the lens plane (the front standard) can be adjusted. This makes it possible to focus on virtually any plane, be it receding or slanted.

To use the Scheimpflug rule, here are a few points to keep in mind:

• The Scheimpflug rule can be used to help you figure out the camera movement you need. For example, if your camera back is vertical, and you want to focus on the ground plane, it tells you that you need to tilt the lens forward. Moreover, by picturing the intersection of those two planes, you get an approximate idea of the amount of tilt needed. For example, the lower your camera, the more tilt you need. The longer your focal length, the more tilt you need.
• When you rack the standards back and forth, the subject plane rotates along the "Hinge line". The Hinge line is the intersection of the plane through the center of the lens and parallel to the film with the plane perpendicular to the lens axis and one focal length in front of the lens. Harold Merklinger (see references) elaborates a lot on this.
• In camera advertisements they often show off the camera with a 45 degree tilt, however often, except for macro work, a small amount of tilt (5 to 10 degrees) will be sufficient.
• The depth of field area is a volume around the subject plane. When tilts and/or swings are applied, this volume is not constant as it depends also of the distance from the camera. The closer you are to the camera, the smaller this volume is. You can picture it as a sort of conical volume. When you focus on the ground plane, the top of the flowers which are next to the camera might be out of focus, while the top of the distant mountain will be in focus.

Which controls to use ?

• Which standard to tilt ? Most cameras have tilts (and swings) on both standards. Esthetic considerations determine whether you use front tilt, back tilt, or a combination of both. However, if you use long lenses, you might find that front movements require monkey-sized arms to operate.
  • Front tilt preserve linear structure, in particular keeps the verticals parallel.
  • Back tilt causes the foreground to "loom" and look larger, making the near-far relationship more dramatic.
  • Front tilt requires more lens coverage than back tilt.
• Base or axis tilt ? Usually field cameras have base tilt and monorails have axis tilt. If your camera has both, its probably better to stick with one of the methods. It is a matter of personal preference, and doesn't affect the final image.
  • With base tilt, it seems clearer to me which areas are out of focus. I also find axis tilt confusing if the far point is not close to the axis of the tilt.
  • With base tilt and shorter lenses, you might find that you cannot get the two standards close together enough.
  • Axis tilt requires less refocussing and recomposing than base tilt.
• Which standard to rack ?
  • For close-ups, focussing with the rear standards will preserve the magnification unlike focussing with the front, and is therefore preferred if your camera has both.
  • For landscapes, this is equivalent, and I prefer to focus with the front because the dark cloth doesn't move.
• A note about TS lenses. Tilt and Shift lenses available in 35mm for Canon and Nikon systems don't let you use a tilt and a swing like view cameras. Instead you pivot the lens to determine the axis of the tilt. Place it on horizontal, and you get what is refered to in LF terminology as a tilt. Place it on vertical, and you get what is refered to in LF terminology as a swing. More generally, once you have determined your plane of focus in space, the direction of the tilt axis is given by the intersection of this plane with the image plane. It's pretty difficult to make fine focus judgement on a 35mm viewfinder, so usually one or two iterations of the procedure I will be enough. For specific information about using TS lenses, see this article by John Shaw.

Procedure I

1. Estimate the best plane of focus.
   This is the tricky part, and requires judgement. Once this is done, the rest of the procedure is mechanical.
   • When your subject is essentially planar, such as a tide pool, a flat landscape with a distant mountain, or the facade of a building, the plane of focus is clearly the subject, and a fairly large aperture can be used.
   • The difficulty comes when your subject is somewhat three-dimensional, and several planes seem to make sense. For example, imagine a scene in which you have a 1 meter high rock in the foreground and a 1000 meter high mountain in the background. On which part of the rock and which part of the mountain do you focus ? In this case, there is an answer: when you establish a Scheimpflug relationship, the subject plane (plane of focus) is one where depth of field behind that plane remains double the distance of depth of field in front. This is just as when the subject plane is parallel to the film. If your camera is above the rock, you would have the subject plane defined by two points (assuming that no swings are used), the first point about two thirds of the way up the rock, the second point two thirds the way up the mountain. However, if in addition, you have a tree in between the two, then there is no easy answer. You will have to stop down quite a bit, but less than if you hadn't used movements.
   • If no good plane can be found, because the subject is inherently three-dimensional (for instance a landscape with a tall tree filling the frame in the foreground), do not use tilts/swings. They result in a smaller DOF area. The only way to get enough DOF is to stop down, like with a small camera. You might have to stop down a lot. Some compositions can just not be focussed sharp entirely with the view camera. You will learn to recognize them and try to find another composition. Go directly to the step 3 (adjusting the focussing point).
2. Adjust the tilt and/or swing and focus
   • If the plane is not slanted, most common situation in landscape photography where you want foreground and background sharp, you need only to use tilt.
   • If the plane is vertical, you need only to use swing.
   • Otherwise, you need to tilt and swing. Do not adjust them simultaneously, but proceed sequentially. If a swing is to be applied after a tilt had already be applied, the left and right points need to be chosen on the same horizontal line. Reciprocally, if a tilt is to be applied after a swing had already be applied, the top and bottom points need to be chosen on the same vertical line.
     To adjust the tilt, use the following. To adjust the swing, replace "top/bottom" by "left/right".
   • Choose a near point (top of ground glass) and a far point (middle/bottom of ground glass) both in the plane of focus and with good contrast to focus on. In the rock/mountain example, this would be a point on the rock approximatively two thirds of the height of the rock and a point on the mountain approximatively two thirds of the height of the mountain. I place tiny flashlights (Maglite solitaire with reflector unscrewed) as focussing points on the ground when it is too dark. If you are going to use axis tilt, the far point should be close to the middle of the ground glass.
   • [FF] Focus on the far point using the focussing knob.
   • [TN] Make the near point sharp using the tilt. You will_augment_ the tilt. Image location is affected (unless the pivot point of the tilt coincides with the rear nodal point of the lens): as you tilt, you may need to use a little rise to regain your composition).
   • [EF] Evaluate now whether the far point needs refocussing. If so, you will have to refocus further, go back to [FF]. Otherwise you are done. Usually a couple of iterations will be sufficient. This procedure continuously increases tilt. The more tilt you need the more iterations you will have.
     Variations of this technique:
   • Some people prefer to focus on the near with the knob and on the far with the tilt. This might work better with axis tilts, while the technique I described might be better with base tilts. Experiment for yourself and see what seems more efficient to you.
   • Howard Bond's Focus-Check procedure. Instead of [TN] and [EF], you turn the focussing knob only in one direction and check the effect on the near point [CN]. Then depending on the effect (got sharper or blurrier), you add or remove tilt. He recommends coming out of the dark cloth and looking at your camera.
   • Some cameras (Sinar, Ebony) have asymmetrical tilts, where the axis of tilt is below the center of the GG. You focus on a far point on this axis [FF]. After [TN], the far point remains in perfect focus, because it was along the axis of tilt, and thus did not change its distance from the lens, so you are done in one iteration !
3. (Re-)adjust the focussing point
   Visually, you adjust this point so that the most blurry close point and the most blurry far point would be equally blurry.
   • If your estimate of the best plane of focus was good, you should have ended at such a point while adjusting the tilt/swing.
   • Since this is difficult to judge accurately, I recommend using step 2 of procedure II which follows, if:
     * you had maintained the standards parallel because the scene had tall objects.
     * you are unsure that your choice of plane of focus was optimal.

Procedure II

This method requires that you have a millimeter scale so that you can measure the difference, in millimeters, between the near and the far focus points. By the way, metric units makes calculations much easier. Most monorail cameras come with such a scale, and on several flat-bed cameras (including the Tachihara, Technika, and Canham KBC), it is easy to attach one. You just have to make sure that it doesn't slip during focussing, and that you have a reference point on your focussing rail from which to take measurements. Having a pointer (like on the Technika) helps in making precise measurements. If it is not possible to attach a scale, you can always take measurements with a ruler.
The procedure is described for tilt adjustment only. Same considerations as before apply for swing. The idea is simple: by successive trial and error, you will determine the tilt which is such that the focus spread is minimized (step 1). Then you determine the optimal focus point (step 2).
The idea behind step 1 can be used without applying the full procedure. For instance, you have a distant landscape with some tall trees in the foreground. Since this is not planar, you'll have to stop down. If you shoot without movements, focussing somewhere behind the trees, the trees and the horizon will be out of focus and require stopping down. If you tilt the lens, focussing somewhere on 2/3 the height of the trees, the bottom and top of the trees will be out of focus and require stopping down. Which of the two alternatives is the best ? The answer is given by measuring the focus spread for each of them, and seeing which one is the smallest.
1. Ajust the tilt
* Read the initial total focus spread.
* Focus on the nearest point. Note the position A of focussing rail which corresponds to the maximum extension for any point in the image.
* Focus on the furthest point. Note the position B of focussing rail which corresponds to the minimum extension for any point in the image.
* Read the difference D between (A,B).
* Make a guess about the amount of tilt needed, and apply it.One possibility, now that your camera is focussed on the far, is to tilt until the near is sharp.
* [F]Read the new total focus spread.
* Focus on the closest point (highest point above the plane of focus). Note the position A of focussing rail which corresponds to the maximum extension for any point in the image (you might have to try several points and note the maximum value).
* Focus on the furthest point (lowest point below the plane of focus). Note the position B of focussing rail which corresponds to the minimum extension for any point of in the image (you might have to try several points and note the minimum value). In the rock/mountain example, this is now the base of the mountain.
* Read the new difference D between (A,B).
* [T] Adjust the tilt by a small amount.
* If the difference D has decreased, add tilt, repeat [F]
* If the difference D has increased, remove tilt, repeat [F]
* If the difference D remains the same, you are done. Note that if the subject is planar, D will be zero.
2. Adjust the focussing point
* Focus on the closest point (highest above plane of focus), which you want to make sharp. In the rock/mountain example previously used, this would be the top of the rock. Note the position A of focussing rail.
* Focus on the furthest point (lowest below plane of focus) which you want to make sharp. In the rock/mountain example previously used, this would be the base of the mountain. Note the position B of focussing rail.
* Focussing at the median of (A,B) will make the closest point and the furthest point equally sharp.
Note that this works regardless of camera movements (tilts/swings, translations), focal lengths, and formats. To be exact, the general result (referenced in Paul Hansma's article, and detailed by Leonard Evens, see references) is that you focus on the point C such as the ratio of distances d(C,A)/d(C,B) is (1+MB)/(1+MA), where MA and MB are the magnifications associated with the close and far object. This point is always closer to A than to B. However, the only case when the median rule is not a good approximation is when the two magnifications are significantly different and at least one of them is comparable to 1 which is only the case for close-ups with very wide lenses, so in practice one needs not worry about this technicality.
* Making he closest point and the furthest point equally sharp is optimal if the far point is not at the horizon. If you had applied movements, this would always be the case.
* If you have maintained the standards parallel and the far point is the horizon, you should instead focus closer to infinity than to the near point (the two thirds of (A,B) is practical for determining the f-stop to use). This is because perceptually, a higher resolution is required for the horizon to appear acceptably sharp than for the foreground. Note however that Ansel Adams recommends, if sharpness has to be compromised that the nearest point be sharper.
### Alternatives methods and references
The two methods that I have described are visual and iterative. There are other approaches which try to nail down the correct tilt directly. All of them require a way to measure tilt angles.
One class of methods are based on measurements on the camera and computations. They require a calculator such as the Rodenstock tool or Bob Wheeler's Vade Mecum which runs on palm devices. Both are detailed in Bob Wheeler's Photographer's Aids: A survey. In particular, Bob Wheeler gives a in Notes on view camera geometry a practical rule for use in the field. Wheeler's rule states that the angle of tilt is 60*delta_focus/delta_GG, where:
* delta_focus is the focus spread, the distance on the focussing rail from A to B
* delta_GG is the perpendicular distance between the images of the near point and the far point used to define the plane of focus, imaged on GG before applying tilt to the camera.
There is also a method developed by Harold Merklinger in his book "Focusing the View Camera", and well summarized on his web site. This site in particular has a few brilliant animations which are very helpful in visualizing the Scheimpflug. Merklinger points out to a rule complementary to the Scheimpflug, called the "hinge rule". His method calls for the measurement of the distance J from the lens to the required plane of sharp focus, measured parallel to the film, and the use of a table to get the tilt value. Personally I think this a great geometric analysis, but that in practice it is not easy to apply because J is not that easy to estimate. For a discussion on the practical feasibility of this approach, see this QA forum thread andthis one. If you want to try to use this approach, thisExcel file might help.
For the historically inclined, Harold Merklinger's page has the 51-pageTheodor Scheimpflug's 1904 British Patent. For the geometrically inclined, I sketched an elegant (I think) geometric proof of the Scheimpflug's principle using Desargues theorem. An essentially identical derivation is given in Bob Wheeler's excellent Notes on view camera geometry. Generally speaking, these notes are some of the best exposition of the math behind view camera operation that I have seen.
Subsequently, Emmanuel Bigler sent me a new derivation (Scheimpflug's rule: a simple ray-tracing for high school ?) which requires only high-school level (well, French high school from the 60s) geometry. He then goes on to show geometrically, using an absolute minimum of algebra how to derive the position of the slanted planes defining the DOF area (Depth of field and Scheimpflug's rule: a minimalist geometrical approach).
A number of great technical/mathematical notes can also be found on this page by professional mathematician and photographer Leonard Evens. Of particular interest is the thoroughView Camera Geometry. Originally intended as a relatively short article for the American Math Monthly, it soon got too large for that, approaching book length at 105 pages (in the spirit of Wheeler's notes). It does address a whole lot of issues that arise in the use of a view camera from an analytic, mathematical point of view. That page has also the more focused, but formula-free (and therefore more accessible to those familiar with geometry)Depth of Field for the Tilted Lens.
Howard Bond describes with plenty of details his well-proven method in his article, Setting Up the View Camera (reproduced on this site). The idea of biasing the focus towards infinity in landscape photography is discussed by Harold Merklinger's The INs and OUTs of FOCUS, and in the article by Joe Englander, Apparent Depth of Field: Practical Use in Landscape Photography.

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