(Editor’s note: Guest contributor Steve Inglima continues his discussion of the view camera and how its aesthetic informs digital photography. This installment explains the circle of coverage in relations to the shift movement in the view camera and the DSLR equivalent, the tilt / shift lens. Here are links to part 1 and part 2.)
Shift & rise movements
Shift and rise are the other movement opportunities. Shift is varying the relative position left/right of the lens from the sensitize material, while rise is the up/down movement. Both are often referred to as “shift.” Both stages could be moved if the camera facilitates this (some might have only front standard adjustments). This is possible if the projected image circle of the lens is larger than that of the sensitized material. If it isn’t larger the image will fade or cut off when shifted. The image circle size determines how much movement is possible and still preserve full image illumination.
For typical 35mm SLR users, this concept might seem academic. But imagine that your full frame 35mm film camera, or digital camera could allow the inclusion of a small sensor in the middle of the 24 x 36 mm “full frame” area. This would then capture only a small part of the full image circle of your 35 millimeter camera. It would render the images as being “telephoto”, as the image are would be smaller than the lens can render.
Then imagine that you could magically move the sensor around within that image circle, and capture any particular part of that 24 mm x 36. That is what a view camera allows you to do! Then imagine a lens that would create a gigantic image circle, much larger than the 24 x 36 mm “full frame” area…and allow you to move within that to take advantage of the same movements afforded view cameras. Good news…that is very possible! What capabilities does this actually allow?
Imagine that we wanted to photograph a tall building whose face dimensions are a perfect rectangle (like the original World Trade Center in NYC, for instance). The top of the building would measure exactly the same width as the bottom of the building. However if we photograph it from the ground perspective looking up, the top of the building in the image would be very small compared to the bottom of the building. This is normal and expected, as things that are further from the camera are going to be less magnified than the things that are close to the camera. If we photographed the building from the ground with all stages parallel to the building, we might not be able to image the whole thing.
Tilting the camera up to include the top of the building changes the perspective. The top is smaller than the base.
If we could magically elevate ourselves 50% to the middle of the building, then we would image the dimensions as appearing more equal from top to bottom. Of course, that is not always practical or even possible to do. So how can we render the building of an image such as this the way we know the building to be, even though photographically it is correctly imaged as a virtual triangle? The answer is utilizing the view camera property of front rise. How is that possible? The magic resides in taking advantage of an image circle larger than that required by the sensor!!
It helps to envision how a lens renders the three dimensional world onto a flat two dimensional surface preserving focus across the entire field. At the principal plane of focus, the light path from center of the subject to the center of the image is one particular distance. However, the light from the edge of the subject through the center of the lens to the edge of the film or digital sensor is a longer distance. Typically we wish this entire plane to be rendered in focus, and corrected orthogonally (or having straight lines rendered as straight lines and proportionally analogous to the original.) Some optical gymnastics, with multiple lens elements (a compound lens) accomplish this goal. When corrected, the focus extends to the edges of the field of view. Without these corrections (actually modifications) and using a single element lens, the image would be subject to a curvature of focus, referred to as the “Petzval Curve”, after Joseph Petzval, the well regarded Slovakian mathematician, optical theorist and engineer. He had perfected a way to correct the focus curvature of field by essentially varying the focal length of the lens based on the radius of the glass itself and thus varying with the light ray angle. This is done by using different types of glass in concert, each with different refractive properties to compensate for what would be that curve. Essentially, this is how all modern photographic lenses work today.
Even though the light path from the edge of the subject through the lens to the edge of the image circle is longer than that of the center, acting in concert with a “corrected” lens and its variable focal length compensation; the result is focus that is maintained from center to edge of the image circle. This also results in magnification increases from the center of the image to the edge of the circle.
That allows us to “correct” the shape of the image to match the subject. Thus, even if we’ve positioned our camera at the base of that rectangular building and the image circle size is large enough, we can raise the front standard (rise) of a view camera to include the top of that building. Using the increasing magnification of the edges of the image circle; the lens combined with the rise movement renders the image of that building as dimensionally uniform! This is how buildings are corrected for architectural photography. In tabletop photography, this technique renders rectangular products positioned at an angle to the camera as still appearing rectangular in the image itself.
(Editor’s note: In the next and concluding post, Steve showcases solutions for bringing the physical view camera to the DSLR.)
Graphics by Steve Inglima | Photographs by Kevin Ames