I find a basic understanding of the theory helps me understand what’s going on, and as a theory is supposed to do, it helps me predict what might happen.
The light circle is the circle of light (containing the image) thrown by the lens, through the bellows, onto the back of the camera, onto the surface of the film. The dimensions of this circle are specific to the lens being used.
In these tests, I am using a Nikkor-W 150mm f/5.6 lens, which casts a light circle of 174mm in diameter.
The difference between the size of the film and the circle represents how much movement is possible (e.g., sliding the front or backplane) before vignetting occurs.
The adapter has the ability to shift a maximum 20mm in either (horizontal) direction. Similarly, the camera’s lens board is able to shift up to 20mm in either (vertical) direction. The combination of these two movements enables one to cover a wider area of the light circle.
In theory, a 3×3 grid should be able to cover the 4×5 area.
The Magnification Effect
The design of the adapter moves the plane of the digital sensor back about 65mm from the plane of the film. As such, it acts like an extension tube, and thus imparts a magnification effect.
That is a 150mm lens effectively becomes a 211mm lens (150mm * (1 + 0.41))
Magnification factor = mf = e/f = 61.7 / 150 = 0.41
e is the extension (mm) imparted by the adapter
f is the focal length (mm) of the lens
The obvious way to confirm that the coverage of the grid equals that of the 4×5 film would be to compare against the image seen through the glass of the view camera. However, there are two problems here. First, and foremost, this approach does not take into consideration the zoom effect imposed by the adapter. Second, it is not always convenient to measure against an image on the glass.
However, I am able to emulate a 4×5 camera, with a lens of a specific focal length, using an app on my iPhone, as demonstrated in the pictures below.
While there are some minor differences between the two shots above, which can be explained by a slightly different shooting angle of the iPhone, they are close enough to confirm that with a 3×3 grid of shots we are able to cover the entire “zoomed in” area of 4×5 frame .
The results also show that the zooming effect is reasonably close to that estimated, thus not contradicting the theory that the design of the adapter acts like an extension tube.
With this test I also confirmed the appropriate positions on each of the adapter and lens board scales to allow a consistent construction of the grid: -20, 0, +20 on the adapter and 30, 0, -30 on the lens board.