Why
Stitching is basically done for only one reason - to improve the quality of the image through using more pixels to make the image. This can be done on a full frame image because your camera simply doesn't have the pixel count to make the size of prints you want. It can be done because the shape of the image you want would result in a radical cropping of the image and thus throwing away some of your expensive pixels. A 3:1 panorama would cost you 50% of your pixels if you crop to get it, a square image costs 1/3 of your pixels.
Although it's true that some images enlarge quite well and even at 100 pixels per inch, make decent prints, but other images really do need 300 pixels per inch of print. Black and white, in my opinion; needs more pixels than colour.
Finding The Nodal Point
Is what you do when you use a regular fixed focal length or zoom lens for making the stitching images and you rotate the camera lens combination around what is referred to usually as the nodal point of the lens. There is a point in any lens in which light rays cross from left to right, from top to bottom and result in the typical upside down image on film or sensor. Anyone using a view camera knows about that upside down image, the rest of us never get to see it, but it's there none the less. While in theory if you knew the optics of the lens you could predict this nodal point, in practice we generally either guess it or ignore it or test it, depending on the situation.
Finding the nodal point is only of importance when there are both near and far objects in the image, lined up with the near in front of the far. If you rotate the camera NOT around the nodal point, you create parallax error - that is, the images won't line up properly. Objects located further from the camera than 20 feet with a normal focal length lens are sufficiently far that the nodal point doesn't need to be considered. If everything is at the same distance, the nodal point can be ignored (eg. photographing art or a flat surface). If the subject is arranged on a flat plane from near to far with nothing sticking up you can also ignore the nodal point.
In practice this basically means that when photographing distant landscapes, you don't need to worry about the nodal point. In fact you can hand hold the camera and rotate it quite casually and still get a good result of stitching. For near far situations however, you need to find the nodal point. There are rare lenses which have this point marked but most of the time you have to do your own work, or do some searching on the net to find the work done by others - often on forums sponsored by the stitching software manufacturer.
Remember that many (but not all) zooms change nodal point as they change focal length (ie. zoom), often in the opposite direction to what you'd expect (ie. closer to the camera body at longer focal lengths and further with shorter ones). With zooms you will have to test several focal lengths (usually three or four is enough).
To do the testing, you will need a 'nodal slider'. As you need it for the stitching too, this is ok. My first one was a wooden block with camera clamp at one end and a groove to fit my Arca Swiss type ball head. You could use a wooden block with a slot down the middle and a 1/4X20 thread bolt through it to position the camera. The other end would be attached to the tripod head (you can get 1/4X20 inserts which have teeth to grip wood and allow your tripod head knob to screw up into the insert (which is mounted from above so as you tighten the knob, it just pulls in tigher to the wood. I lost my wooden slider and replaced it with a Really Right Stuff Nodal Slider but this requires using Arca Swiss type clamps and plates on your tripod head. There are a number of sources for nodal sliders, some which handle all the rotation of the camera both horizontally and vertically.
What I do is to set my camera horizontal on my tripod and via my slider, with the tripod head rotating horizontally (no tilt or lean). I use a stick in the ground about three feet from the camera as my near point (a garden cane works nicely but so do coat hooks unbent).I either set up a second stick 10 feet away, or use a vertical line which is 10+ feet away (like the corner of a wall). I start with the slider so the rotation point is closest to the camera body. I focus on the near stick and swing the camera back and forth while looking through the view finder to check for movement of the close stick against the far line. I'm looking for the position of the slider rotation that stops movement of the two against each other.
I did find that if I let my eye move to the side of the eyepiece, some parallax becomes visible no matter what, so it's important to keep your eye centred in the viewfinder as you rotate the camera. If as expected the near stick moves against the far, then I move the slider or camera and try again, gradually moving the rotation point further from the camera until the two don't move relative to each other. That becomes my nodal point and I mark the place and note the distance between the rotation point and the film/sensor position of the camera, or for more convenience, the tripod socket. This then tells me the right slider position for all future shots with this lens at this focal length.
It takes longer to describe than it does to do, so don't despair. Repeat the process for other lenses or for other focal lengths.
If you use a full three dimensional rotation device that allows for both horizontal swing and looking up and down, then this is all you need. If, like me; you generally only do single row stitching and don't have one of those devices (large and expensive) that can do multi row stitches, then if you do your single row stitch with the camera aimed up or down, you have to adjust the slider so you have the nodal point of the lens rotating around a vertical extension of the tripod head. With a ball head, this means a shorter distance on the slider when aiming up, a longer distance than expected when aiming down (because the ball head leans back when aiming up, forward as it aims down.
When I get a moment, I will add some good stitching and nodal point and slider references to my list of links on the Links page of my website.
Flat stitching is what is done when you have either a lens or camera back that can shift left or right, up or down. This means you don't have to rotate the camera. Instead you rely on the excess coverage of the lens used to move the film or sensor over relative to the centre of the lens so you are capturing the information from closer to the edge of the image circle produced by that lens. For example I have a Canon 90 ts-e lens which shifts back and forth and if the camera is held vertically and the lens shifted horizontallly, the combination of a left shift image and a right shift image gives me an almost square image. As these lenses are few and far between and generally more expensive, this is a less useful and frankly less used method. None of these shift lenses are zooms and many manufacturers don't make them at all. As images stitched this way are at least in theory easier to stitch, there is a potential advantage. I personally find that I still have to use my dedicated stitching software and it can work just as easily with roatation so even though I have this lens, many of my stitches don't use this method, even with my shifting lens.
In theory, you can buy adapters to hang your digital slr of the back of a view camera and use the view camera's shifting capabilities to shift the camera, but my camera is so heavy that NO view camera has proven rigid enough while remaining portable to work this way. A lighter camera might work ok but frankly the hassles of using the view camera make it more trouble than it's worth.
Camera Settings
You can't afford to have any exposure or focus difference between shots so this means manual exposure and manual focus whenever possible. The stitching software can compensate for some exposure differences but not focus so if you have one of those auto auto cameras with no manual override, then only stitch things where the distance remains relatively constant and in which the lighting doesn't change significantly over the proposed image.
Stitching Software
At this point I work with Photoshop CS3 to edit my images and it does include a fairly effective stitching routine, under the FILE/AUTOMATE/PHOTOMERGE menu within Photoshop or directly from Bridge using TOOLS/PHOTOSHOP/PHOTOMERGE menu. I have had problems with it however, and at times the flawed stitching isn't always obvious. As good stitching software is relatively inexpensive, I usually go straight to the dedicated software to do the stitching, before loading the images into Photoshop.
I use PTGui, software available for both Mac and PC. The software works by taking pairs of images which will be next to each other and finds points that are common to the two images. It does this automatically and for every pair. In a 5 image single row stitch, this would be 4 pair, 1 and 2, 2 and 3, 3 and 4 and 4 and 5 image pairs.
Were I to shoot a 3 image by three row stitch, I'd have 2X3 horizontal pairs, but I'd also have 3X2 vertical pairs for a total of 12 pair of images. Good thing the modern software does this automatically.
You get a report after the points are set and the alignment is calculated, telling you just how good the fit is. Had you used perfect lenses with no distortions and asolutely perfect rotation around the nodal point, you'd always get 100% agreement in aligning the pairs of matching points. In the real world, good rotation and modest distance of the subject from the lens usually results in really good alignment. There are times though that you have to use the editing features of the software to assist the points placement, throwing out irrelevant point pairs.
You can ask for the image to be blended for you, or output as a series of layers, in which case it's easy to edit just where one image stops and the next starts, allowing you to move round things like blades of nearby grass that stick up and create parallax issues. A little practice and most stitches go very well. I have never had PTGui not make a good stitch without warning me first, unlike Photoshop which gives no reports and fumbles along as best it can, for better or worse.