BELLOWS EXTENSION: HOW MUCH IS ENOUGH?
One of the fundamental aspects of a view cameras design is how close and how far apart the lens and film planes of the camera can be positioned. This is an important specification as it determines the focal length of the lenses that can be used with the camera as well as the close focusing capability of those lenses. Wide-angle lenses require that the lens and film planes be very close together. Longer lenses require that the lens and film planes be relatively far apart, and further apart still for close focusing. So, how does one determine the minimum and maximum amount of extension needed in a camera?
First, its important to understand a given lens extension requirements as a function of focal lengththe distance from the vertex (the point internally within the lens where the image is formed) to the film plane, when the lens is focused at infinity. A 150mm lens (normal for a 4x5 camera) requires 150mm of bellows extension when the lens is focused at infinity. When used for a close-up, the same lens requires double this extension to render an object at life size. For instance, if you want to photograph a flower that is 3 across so that it will be 3 on film, you will need 300mm of extension to accomplish this with a 150mm lens. This magnification ratio is logically defined as 1:1.
Conversely, with wide-angle lenses it is the cameras minimum extension that must be considered. For example, a camera with a minimum extension of 90mm would not accommodate any wide-angle lenses shorter than 90mm, unless the lens was used only for extreme close-ups.
Thus, determining how much extension you need is primarily a function of the range of lenses you use, and how close you want to be able to focus them. However there are further factors to consider. View cameras are multi-format cameras. For instance, an 8x10 view camera can be used with reducing backs for 5x7 and 4x5. So, even though the bulk of your photography would be in 8x10 format, you might occasionally want to use the camera for a 4x5 photograph. Maximum extension is probably not an issue when photographing in 4x5 format, but minimum extension almost certainly is. Will the 8x10 camera permit the use of a 65mm lens when a 4x5 reducing back is employed? Most likely the answer is no.
Lens design is another factor in the extension equation. Lenses of normal construction correlate directly in that focal length equals amount of extension at infinity. Telephoto and retrotelephoto lens designs do not correlate directly. Telephoto lenses are designed so that the vertex is forward of the lens, thus requiring less extension. Retrotelephoto designs are commonly used for lenses that provide a wide field of view and these designs increase the amount of minimum extension required because the vertex is behind the lens. The specifications in Ebonys web site clearly state lens design or construction when indicating the range of lenses that may be used with a particular camera design. Youll notice that the longest telephoto lenses specified for a given camera always exceed the total amount of extension afforded. So, an important corollary to the rule of thumb governing extension is that telephotos increase a cameras capabilities at the long end and retrotelephotos increase a cameras capabilities at the short end of the lens spectrum.
If you commonly use extremely long lenses or if you do a lot of close-up work of subjects that you cant get physically close to, then you will need an amount of extension that may exceed what is typically provided. In this case, your best bet may be to choose the next larger format in combination with a reducing back. For instance, extra extension for 4x5 work can be achieved with a 5x7 camera and a 5x7>4x5 reducing back. This combination of larger format coupled with a reducing back yields a significant increase in extension. The trade-off is with minimal extensiona 5x7 camera is likely to have a longer minimum extension than a comparable 4x5 model.
Extension is further affected when the issues of rigidity and sway are factored in. When a camera is fully extended, it is inherently less stable than when it is partially extended. Its no different from the red line on an internal combustion engine. The engine may be able to turn 6500 revolutions per minute, but not for an extended period of time. So it is with extension. If you only use a 300mm lens in your photography and your camera has a maximum extension of 325mm, you will be continuously using the camera at or near maximum extension. When the rails are fully extended they are likely to sag a [tiny] bit, particularly if the lens is rather heavy. On the other hand, if your camera has 450mm of total extension, the rails will only be slightly extended with a 300mm lens and the camera will likely be at or near peak stability. The lesson here is quite simple: Always go for a little more extension than your longest focal length lens requires at close focus.
So, in thinking about extension, you should always consider the range of lenses you commonly use. Think also about which lens or lenses you use most often. Minimum extension can be further reduced with recessed lens boards. Maximum extension can be enhanced with extension tubes, for the lens standard, and extension backs, for the film standard. Just bear in mind that a camera design is going to be structurally quite compromised if it is saddled with extension tubes and backs at either end of fully extended rails.
Whenever I peruse Ebonys web site or product catalog, one aspect of their approach to the design of large format cameras becomes quite clearthe photographer is provided with a range of choices based on extension needs. Ebony make a variety of camera models in each format in order to provide for a wide range of extension capabilities. They manufacture specialized wide-angle designs, as well as conventional folding designs that provide for quite generous amounts of extension. This is particularly important in field camera design. By comparison, monorail cameras are completely modular. Just purchase additional rails, standards, bellows, and tripods, and you can extend the camera to your hearts content. There is only one drawback with the monorail approach for fieldwork there is too much heavy and expensive componentry to lug around and assemble.
I use an Ebony 45SU for all my view camera work. I chose this Ebony model specifically for its wide angle capability. The focal length I use most frequently for 4x5 work is 90mm. I also shoot 120 roll film with reducing backs. The 45SU readily accommodates a 47mm lens (with a recessed board) making it ideal not just for 4x5 wide field photography, but for 120 format as well. The longest lens I use is 355mm (with an extension tube set), which presses the 45SUs extension (365mm) to the limit. Whenever I use my Ebony 45SU, I am impressed with its precision and rigidity. These attributes play into the extension equation. Many cameras are acceptably rigid and aligned when their rails are not fully extended. But, as soon as they are extended, the rails begin to sag and the diving board sensation distracts the process of focusing and composing the image. An important design attribute of Ebony cameras is their impressive degree of rigidity and alignment when fully extended. I use a 355mm lens on my 45SU without hesitation because it performs admirably when fully extended. It is one thing to provide for a generous amount of extension and quite another to achieve an acceptable level of rigidity once that extension is fully employed. Ebony manages to achieve both.
This overview of camera bellows extension was written for Ebony by Richard Sexton, a professional photographer and author who serves as a professional spokesperson for Ebony in the United States. To learn more about Richard Sexton's work and to view photographs he has taken with an Ebony camera, please visit his web site at http://www.richardsextonstudio.com.