Digital conversion of a large format camera

Part 1: Sliding back

This is more of a private project and less science related. But then you never know what it will be used for in the future. After having experienced the convenience of a digital SLR, I wanted the same advantages for the large format system. I experimented with building a scan back from scratch and with modifying a Canon LiDE scanner, but eventually ended up with a MegaVision S3 pro 24 mm x 36 mm CCD sensor. While this sensor does not have the large image area of the large format camera (4" x 5" in the case of the Sinar F used here), it is significantly larger than the sensors found in most affordable digital SLRs. Compared to the SLR, the view camera offers full movements (shift, tilt, and swing of lens and film plane). At the very least the digital back can replace Polaroid proofs that are typically taken before a sheet of expensive film is exposed.

Figure 1: Complete camera setup with laptop running MegaVision's PhotoShoot 3.6 to acquire photos. The sliding back attaches as Graflok back and replaces the Sinar groundglas and plate holder.

The complete setup (Fig. 1) consists of a Sinar F 4" x 5" camera (the black/chrome thing) mounted on a tripod, an attached custom made sliding back (mostly grey and white) holding the viewfinder and sensor, and a computer to acquire and store the images. The sliding back is build to connect as Graflok (International) back and replaces the Sinar ground glass and plate holder.

Figure 2 left: Back with viewfinder in position to compose the image. The viewfinder is automatically centered when the back is in its right-most position. Right: Back with CCD sensor in position to take an image.
A click stop arrests the sliding back at the point the CCD sensor is centered in the light path.

To take an image, you open aperture and shutter, position the viewfinder in the light path, focus, and frame the image (Fig. 2 left). You then slide the sensor in the light path, set aperture and shutter, and expose the image (Fig. 2 right). The sliding back can be locked in any position by tightening a silver knob in the upper right corner of the back. The viewfinder is centered by sliding the back in its right-most position. The CCD sensor is centered by sliding it to the left until a click stop arrests it. The sensor can also be positioned to the left and right from the optical center to allow multiple shots that can be stitched together as a panorama photo.

Figure 3 left: A Hasselblad focusing screen is marked with center cross as well as frames for vertical and horizontal sensor position (too faint to see in this photo).
Right: A Hasselblad magnifying hood attaches in front of the focusing screen.

A Hasselblad 2 1/4" x 2 1/4" focusing screen is build-in on the viewfinder side of the sliding back (Fig. 3). It covers a much larger area then the 24 mm x 36 mm the sensor sees which aids in composing the picture. It is marked for framing with the sensor in horizontal and vertical position and mounted in precisely the same plane as the CCD sensor. A Hasselblad magnifying hood slides under the brass rails and allows a magnified (and diopter corrected) view of the screen. The hood is a restored older model and somewhat disappoints with respect to image quality and distortions. It is fine to roughly frame and focus, however, for more precise work I prefer to either use a ground glass loupe or to take test pictures and judge the image on the computer screen.

Figure 4 left: The sensor side of the sliding back features a MegaVision Hasselblad adapter. A Megavision S3 back can be mounted in horizontal (landscape) position (middle) or vertical (portrait) position (right).

The MegaVision S3 pro digital back is an older (early 2000s) digital back that is primarily used with professional medium format cameras. It attaches to the sensor side of the sliding back in a Hasselblad adapter which allows it to be quickly rotated from horizontal (landscape) to vertical (portrait) position (Fig. 4). The back uses a 24 mm x 36 mm DALSA color CCD sensor with 3072 x 2048 (6 M Pixel) resolution. The sensitivity can be switch between 100, 200, and 400 ASA equivalents. Twelve micron pixel size and 12 bit / color conversion provide a large dynamic range which results in beautiful images. The color response is in my opinion much better than that of current digital SLRs like the Nikon D2x. The age of the sensor technology shows in the noise: especially with longer exposure times noise gets very noticeable and I often digitally process images to reduce it at the cost of sharpness.

Figure 5: The sensor can be freely positioned along the horizontal axis of the back. It is arrested with a click stop when it is centered in the light path. The brown scale along the top of the back indicates the offset with respect to the optical axis of the camera.

To make use of the large image circle of the lenses typically used with the Sinar F camera, the sensor can be freely positioned along the horizontal axis of the sliding back (Fig. 5). A millimeter scale along the top edge shows the relative position of sensor and back. To make a panorama photo, the sensor is moved to the left-most position and the first image is taken. It then moved by 20 mm to the right and the second image is taken. With the sensor in vertical position, frame centers are 24 mm apart and a 20 mm offset guarantees enough overlap to later stitch the images. After 6 images are taken, they are stitched together with suitable software to obtain a 29.5 M pixel panorama photo in a 127mm x 36mm format. The advantage of using a sliding back as opposed to rotating the camera is that there are no complications due to lens distortion or a misaligned nodal point when stitching the images. The sensor can also be shifted vertically with the camera's vertical shift to obtain multiple rows for very high resolution images.

Figure 6 left: A Schneider 75 mm Super-Angulon has the large image circle needed to make panorama images by moving the sliding back. The digital back is triggered by the X-contact on the lens's shutter. The standard bellows have to be almost completely compressed to focus the 75 mm lens and hence provide only minimal movements (a few millimeters). Right: To allow more camera movements, the bellows can be replaced with bag bellows.

For taking panorama photos or using camera movements, I use a Schneider Super-Angulon 75mm lens (Fig. 6). This lens is clearly not ideal - its optical resolution is likely to be lower than the resolving power of the digital back (even at 6 M Pixel). Also, its chromatic aberration is probably not ideal for a digital sensor that recovers the pixel's color information through Bayer-pattern calculation. Large format lenses are optimized for a large image circle and not for resolving power, and lenses of this vintage show the compromises. However, it's affordable! To use camera movements with a lens with such short focal length, the box bellows are replaced with bag bellows (Fig. 6 right). The sliding back was designed for minimal shading to occur when camera movements are used. Also, the S3 pro sensor does not incorporate a lens array which would require the light to hit the sensor at a relatively steep angle and therefore cause problems with the sensor not being in the optical center.

Part 2: Shutter and mount for Nikon lenses

To have a larger selection of lenses and to test how much of an impact the lens quality has on the overall image quality, I decided to build a lensboard that would allow mounting Nikon lenses on the Sinar F, i.e. a lensboard providing a Nikon F-mount and as well as a behind-the-lens shutter.

Figure 7 left: With this lens board Nikon lenses (here a 50 mm f/1.4 AIS lens) can be mounted on the Sinar F. Right: On the front of the lensboard are a port for the rubber bulb operating pneumatic shutter, a X-contact to trigger the digital back, a switch that allows two different exposure times, and a button to release the lens.

As the distance between the flange of the F-mount and the sensor plane is fixed at 46.5 mm, there is no room for camera movements (the small image circle of the 35mm Nikon lenses don't allow much movement anyhow).The lens is focused with the lenses focus ring and not like the Super-Angulon by varying its distance to the sensor plane. The lensboard and the sliding back are mounted on the same Sinar standard to be able to focus the lens at infinity. Bellows can however be used (Fig. 7) for close-up and macro photography and allow extreme magnification. A shutter that is flat enough to fit within the standard is attached to the back side of the lens board.

Figure 8: Front (left) and back (right) of the Nikon F-mount lensboard. A pneumatic Packard shutter is installed behind the F-mount. The shutter's cylinder is connected via the yellow rubber tube to the shutter connector. A switch on the top side provides the signal for the X-contact: it closes when the shutter is fully opened.

The shutter used is a restored Packard No. 6 Ideal shutter - a design that has been around since the late 1800s. It can be operated in "instantaneous" mode which gives about 1/25 second exposure time and in "Bulb" or "Time" mode (by pulling out the brass knob on the front, Fig. 8) in which the operator manually controls who long the shutter is open. The trigger is pneumatic, i.e. a rubber bulb pushes air into the cylinder on the shutter's back which in return opens the shutter leaves. A flash (X-) contact triggers the digital back.

Figure 9: Two test images taken with the digital back and a Super Angulon 75mm f/8.0 (left) as well as a Symmar 150 mm f/5.6 (right).

Overall, the Sinar F with sliding back and the Nikon lens board fill a niche between film based large format photography and digital SLR photography. The 24mm x 36mm format makes the effective use of the viewfinder difficult and a live-view in the PhotoShoot software (including a 1:1 view to check the focus) would dramatically add to the usability of the system. However, that's unlikely to happen as MegaVision has moved on to different generations of digital backs. While noise poses a problem with longer exposure times, I am still impressed with the color rendering abilities of the S3 pro back.