View Full Version : How is the height of the lens above the plane of sharp focus measured?
Eugene H. Johnson
In Harold Merklinger book "Focusing the View Camera" he speaks quite convincingl y about the significance of the hinge rule. However, what is still unclear to me is how the distance "J" or the height of the lens above the plane of sharp focu s is measured in practical terms. This is especially difficult to comprehend whe n the "Parallel to Film Lens Plane" intersects the Plane of Sharp Focus below gr ound level. I would be most grateful if someone could shed some clarity on this point. Thanks in advance
I use different terminology than Merklinger. Specific heights are not important to me. What is important when applying the Schiempflug principle (aka: "the hing e rule") is that three planes intersect in a line for the magical 'from near to infinity' phenonomena to occur. These three planes are the film plane, the lens plane (a plane perpendicular to the axis of the lens and containing the nodal point of the lens), and the subje ct plane. whether this intersection occurs above ground level, at ground level, or below ground level is immaterial, the important thing is that these three pla nes intersect in a line. For wide angle near/far compositions this is important. For long focal length landscape photography it is irrelevant.
I hope your path is now more brightly illuminated.
Merklinger refers to this "J" distance quite a bit in how to calculate the tilt in the tables, etc. Like you, I have found that it is difficult to estimate this distance esp. when typically 10 to 30 ft under the camera. Basically, I don't use his "practical" method. I select 2 points near and far on the selected plane of sharp focus. Tilt until both are in sharp focus on the ground glass. Think about it, you would likely want to do this anyway after you set the tilt angle per his tables just to confirm you got it right. I still think his book(s) are valuable for the theoretical treatment of the subject.
Eugene, I worked through Merklingers book very diligently to discover some interesting things also. Your quesiton is a good one, and very hard to answer without a drawing. I will try to explain what I learned. Since there is two variables that determine where the Plane of Sharp Focus (PSF) lies, most of the time it is very difficlut to adjust the tilt angle to determine when both near and far come into focus. The other variable being lens to film plane distance has just as much control of the PSF as the tilt does. He does not emphasize this point in his book enough. So to answer your question, the way to properly calculate the distance J needs to be below the lens, I am forced to do a quick scaled drawing of the scene. This scaled drawing gives you J very quickly, from there you apply the respective tilt angle. Then you simply move the focus in and out till PSF falls right where you need it. The drawing takes me less than 1 minutes to do, and that is much faster than trial and error for sure. What really got me to comprehend his concepts is his web site which is linked to the front page of this forum. On his web site there is a 10 second movie showing the all variables working together and the relationships becomes very clear. Check it out, best of luck.
I still don't understand the point of this series of calulations. It seems that it is awfully inefficient and takes your concentration off of the image itself. Can somebody please explain it to me in a non-jargonic style? Is this technique more useful for axis tilt cameras? How does it differ from other methods?
Take a look at the following diagram:
The mathematics and idea as to why it works are simple, but applying the hinge rule in practice is harder than the Scheimpflug principle. I find it rather difficult to extend an imaginary plane that is one focal length away and parallel to the lensboard and find its intersection with another imaginary plane that is parallel to the film plane and crosses the center axis of the lensboard. In contrast, for the Scheimpflug principle, one simply extends the actual film and lensboard planes.
Most of the hinge rule has applications for table top work(macro) and not so much landscape. The easiest way to get from here to there is to look at the ground glass. It never lies. I use the one half the distance rule and I never fail.
Like Bill, I use Merklinger's "hinge rule" to estimate the J distance. I find that I can estimate the tilt angle in my head using Merklinger's approximation that the angle is about the focal length in mm divided by five times the J-distance in feet (actually, it's the arcsine of the focal length divided by the J distance in the same units, but you need a calculator or tables for this). As Merklinger points out, Scheimpflug is not as tight of a rule as the hinge rule, as you have the additional variable of lens plane to film plane distance, which affects your tilt angle. In other words, you can have the lens to film distance close to the focal length of the lens and use only a little tilt, or have the bellows racked way out with lots of tilt. Both situations satisfy Scheimpflug, but only one satisfies both Scheimpflug and the hinge rule for the desired J-point. In the former case, the J-point is longer than the latter case, assuming the film plane is in the vertical position.
Me approach in applying the Scheimpflug's rule in practice is different and dependent on what kind of camera I'm using and what kind of pictures I'm taking. Sinar/Linhof or flat bed?, far away or close up?.
I know Harold Merklinger's book and articles and have found very useful as a teaching tools with my students because of his optically and geometrically more than correct concepts.
I think too, that such a technical review constitutes a departure from most common people taking or trying to take pictures with a view camera outdoors.
If we are using a Sinar or something like that, we must take advantage of specific aids provided and that's all. Finally, we have payed for something.
In my case, I take lanscape with a K.B.Canham 45DLC, that don't provides any kind of special device in order to calculate the tilt and/or swing angle. The solution is to draw four (two verticals a nd two horitzontals) dashed lines in the ground glass like in Sinar models. Verticals are used for swings and horitzontals for tilts.
The only "mistery" to solve is what's the correct separation between each pair of lines; with a 60mm separation (30mm both sides of ground glass center), the system functions like in former Sinar cameras.
1/ Focus on a desired plane of sharpness foreground point coincident with the dashed line. 2/ Note the back standard position (mark in the rail, millimetrated scale, etc). 3/ Focus on a background point of the same plane still coincident with de other dashed line. 4/ Note the difference in rail displacement. 5/ The amount of displacement in mm equals the number of degrees of tlt or swing needed in such a situation.
The figure is valuable enough only for the back standard tilt or swing and can be transferred to the lens standard applaying a little trial and error correction.
The basics of this technique is plain geometry and free of any other tings than camera controls; no distances impossible to mesure, no complicated calculations or tables to consider.
In my opinion, one of most importants questions in practical photography is to try that technical skills, obviously necessary, don't disturb the magic, intense and unique lapse of time previous to the shoting .
In my last posting, there is a typing error. In the seventh paragraph "1/Focus on ..." they must be changed the order of "foreground" and "background". Focus in first place the background point and then the foreground point. It's only a more practical convenience question.
Sorry and thanks
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