A Selection of Shift Lenses
From left to right: Canon 24mm TSE, Canon 45mm TSE, Canon 90mm TSE, CZ 35mm PC Shift, Olympus 24mm Shift.
Overview
There are two basic methods of capturing multiple frames for later combining into a larger single image. The first is called a flat or "shift" capture and the other -- and more common -- method is called the panned or "spherical" capture. In both methods of capture, the process of assembling the images into their final result is referred to as "stitching". The reason for doing this is to either increase the total image resolution available, or create a wider or "panoramic" view than your lens can capture in a single frame.
The spherical panorama is accomplished by pivoting the lens about its optical center or "nodal point" during the independent frame captures. The flat stitch is accomplished by keeping the lens in a relative stationary position, while shifting the camera parallel to the camera's sensor-plane during the frame captures. The major drawbacks of the properly-executed spherical stitch are first, you must deal with mis-alignments caused by lens distortions during the frame assembly and second, the act of panning itself will impart geometric distortions in regularly-shaped objects like the rectangular face of a building. Without going into excessive detail, the properly-executed flat stitch avoids those issues entirely and the resulting image has a more normal, "rectilinear" rendering to it. However, the total FOV (Field Of View) one can capture with the flat stitch is limited by lens design while the spherical stitch is not similarly constrained. Thus both methods have their place in the photographer's tool box.
Unfortunately, either method will invoke "parallax" error if imporperly executed. Parallax is the change in relative position between near and far objects as you change your viewing (or in this case shooting) position. Parallax will be visible with either method even if you only alter the lens' proper shooting position fractionally between the frames. The resulting parallax mis-alignments will make accurately combining the multiple captures during post-processing far more difficult.
To maintian a parallax-free capture in a spherical stitch, you must pivot the lens around its optical center or "nodal point" during the independent frame captures. There are numerous accessories available to aid the photographer in achieving that end, but it is beyond the scope of this overview to discuss them in detail. To avoid parallax with the flat stitch, you must keep the lens in its exact same shooting position during the independent frame captures. The following step-by-step picture-tutorial explains how I do my captures for flat-stitching using PC lenses. PC can stand for either "Parallax-Correcting" or "Perspective Control" and they are also commonly referred to as "shift" lenses.
Note that the tutorial below shows the Camera being used in its Horizontal orientation and the Lens-shift captures also executed Horizontally (C-H/L-H) for an approximate 5:2 aspect ratio final image with about 60% more resolution than the camera's native resolution. With an "L" bracket on your camera you can combine the different Camera and Lens-shift orientations for three other capture options. By orienting the Camera Vertically on the rail and doing your Lens-shift Horizontally (C-V/L-H), you can capture a more normal 4:3 aspect ratio image, but at nearly double the native resolution of your camera! Though a bit more difficult to execute with the equipment shown below, you can also re-orient the rail to operate vertically on the head, which will then allow you to perform a C-H/L-V capture for a higher-resolution 3:4 (portrait) capture, or orient C-V/L-V for a vertical 2:5 pano.
My primary flat stitching hardware is a Kirk geared rail plate on top of an Arca CUBE tripod head. For those interested I have
a review of the Arca CUBE online in the Gear Notes section of this site.
Kirk Geared Rail Plate on the Arca CUBE tripod head, oriented for making a Flat Capture
The "Flat" Captures
First, your meter reading can be adversely affected if the lens is shifted, so you want to set your camera to its "manual" metering mode and take a meter reading with the lens centered on the camera. You then keep this exposure setting for all three frames:
Lens CENTERED for meter reading
FRAME 1: I shift the camera fully to the right on the rail plate and shift the lens fully left on its shift mount then take my first exposure. This frame will be the full-left side of the final stitch.
Camera full RIGHT
Lens full LEFT
FRAME 2: Now I shift the lens 11mm right, getting it back to centered, and shift the camera 11mm left, which returns the lens to its same relative shooting position as in the preceeding frame, then I make my second exposure. This frame will become the center of the stitch.
Lens returned to CENTER by shifting it 11mm RIGHT
Camera CENTERED by shifting it 11mm LEFT
FRAME 3: Finally I repeat the above steps, shifting the lens 11mm more to its full right position and shifting the camera 11mm left and make my third and final capture. This frame will be the full-right side of the stitch.
Lens shifted fully RIGHT
Camera shifted 11mm more LEFT to its final position
~~~
Processing and Assembling the Captures
Note: The image assembly portion of this tutorial is currently under construction and not complete, but I wanted to get the following images and processing notes up so the quality of this capture method could be seen and more easily understood.
I capture these images in RAW mode so that when I process them I can apply the exact same settings to each frame. In this way all three frames will match in terms of exposure, white balance, hue, satuaration and contrast.
Here are the left, center and right images from the above capture:
Since the lens' shift-axis may not be perfectly in line with the sensor as you shift, you often end up with some stair-stepping where the frames line up in your final image. This is normal and depends on the quality of the lens and/or adapter used, but it is usually only a minor percentage of the total number of pixels across the entire stitch. For example, with my Zeiss 35mm PC lens mounted on my Canon 1Ds2 camera, I will normally only have to crop off about 30 pixels from the top and bottom of the three-frame stitch for the final rendering.
Here is a stitch of just the left and right captures to show the extreme of the stair-stepping before the final crop:
Here is a 100% crop of the actual seam in the above stitch. The full right image is sitting directly on top of the full left image and there is no blending mask between the layers, so you are looking at the actual hard seam of the right frame where it meets the left. You can see for yourself how well the frames line up using the above method. While the seam is certainly not perfect, in my opinion it is pretty darn good as-is and the seams will be even better if the middle capture is also used:
In practice I will of course also add the middle frame, which keeps the central portion of my image intact and pushes the seams toward the outer edges of the frame, thus making them even less prominent. Next I add a blending mask to hide imperfections like the slight mis-alignment visible on the wire and in some of the bricks. A crop to the desired rectangular proprtion finishes the stitch and the image is now ready for processing in Photoshop just as I would normally do with any other single capture.
The question I am asked most often about the above process is, "Is it really worth all that time and trouble?" Obviously the answer is going to be different depending on the photographer, but let me answer it in this fashion: Once the camera is set up for the desired framing and the manual meter-reading is obtained, I can usually capture all three images in less than 30 seconds -- and often it is closer to 15 seconds. If executed properly, the three images will line up so well that I have no need for a separate stitching program and can usually hand-assemble them in Photoshop in under five minutes. Add in another four or five minutes to create the blending masks and execute the final crop and I have spent a grand total of about ten minutes creating this new single and higher-resolution image, so it's worth it to me.
For additional information please also see Digital Outback Photo's stitch article.