Tuesday, August 20, 2013

Light Fall-off & the Center Gradient Neutral Density Filter in the 90mm Fuji GX-617

Panoramic Photography, which produces panorama images, can be achieved in different ways. In general, panoramic photography is normally thought of as being wide angle photography (that is, with a field of view that is wider than what we perceive to be "normal" with our eyes). In addition, this effect is heightened by an image that is wider than the traditional 3:2 or 4:5 ratio of the image height to the image width. (This is speaking of a horizontally formatted panorama, which is the most common orientation - vertical panoramas are also possible.)

Taking a Fuji GX-617 for example. The widest lens available for this camera is a 90mm f5.6 SWD EBC

This lens was designed to cover a 5x7" negative and to do so in a camera that is capable of both swings and tilts. The diagonal of a 5x7 rectangle is 8.6". To allow those movements, it has to have an even larger-than-8.6" image circle. If we look up the specifications of this lens, we find that it has a 236mm image circle, which is a bit over 9.25".

Optical design is matter of choices and trade-offs. Large format lenses are concerned most with correcting for curvilinear distortion and lateral color. They achieve this at the cost of persistent light fall off. This means that more light is hitting the center of the image circle than the corners. With the 90mm GX-617, this affect is so pronounced that a special filter was required for film: a 2x neutral density center gradient. In effect, when you photographed with the lens set at f/11, the outside of the image circle was getting the full amount of light, while the very center portion was getting the light it would have received if the lens was set at f/16.

SLR wide angle lens design has to take into account a mirror that flips up and would strike the lens if it came too close to the film/image plane. For this reason, SLR lenses use a retrofocus design, which allows the exit pupil of (for example, a 6x7 lens with a focal length of 55mm) to be much farther away from the film plane than 55mm (where the reflex mirror would hit the back of the lens). A side benefit of the retrofocus design is that light fall-off is minimized (but not completely eliminated). You have never seen a center gradient neutral density filter needed for this reason on a 6x7 or 35mm SLR camera.

This is one factor that (I believe) makes designing a panorama camera around a 6x7 SLR lens is preferable to designing it around a large format camera lens. It is the main reason I chose to build the Pentax HippoCam around the Pentax SMC 6x7 55mm f4. (Pentax also made a 6x7 Takumar f3.5, which by all accounts seems to also be a very good lens, but it suffers from much greater weight and an almost impractically large filter size. (100mm filter size, meaning even a UV filter is going to cost you approximately $80. The f/4 design requires the much more common and affordable 77mm filter size.)

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