Many frames, one view, and the pivot point that matters

Why stitch at all

Three reasons, and only the first is obvious.

Wider than the lens. The traditional reason: a landscape or an interior that no lens covers.

More resolution than the sensor. Shoot a subject in nine overlapping frames with a long lens and stitch them, and you have a file several times the camera’s native resolution, with better per-pixel detail than any single wide shot. This is how large prints of buildings and landscapes are made.

Less distortion. A stitched panorama from a 50 mm lens has the perspective character of a 50 mm lens across a very wide field, without the stretched corners of an ultrawide. For architecture this is a genuine quality advantage.

How stitching works

The software finds control points (features visible in two overlapping frames) and solves for the camera orientation and lens parameters that place every frame consistently on a sphere. Then it projects that sphere onto a flat image and blends the seams.

That means two things you can act on. Overlap must be sufficient for features to be found: 30 to 40 per cent between adjacent frames, more in a featureless sky. And the scene must contain findable features: a blank wall or a plain sky gives the algorithm nothing, which is why panoramas often fail at the top of the frame and succeed along the horizon.

Parallax: the one that actually ruins panoramas

Rotate a camera about the tripod’s screw and the lens moves through an arc. Objects at different distances shift relative to each other between frames (parallax) and no stitcher can reconcile that, because the frames disagree about the scene’s geometry.

The symptom is unmistakable: distant objects line up and near objects are doubled or sliced, usually along a seam. A railing in the foreground with a ghost of itself.

The fix is to rotate about the no-parallax point: the entrance pupil of the lens, which is somewhere inside the lens barrel and is not the sensor or the tripod screw. A panoramic head positions the camera so rotation happens about that point. Finding it takes ten minutes: line up two objects at different distances, rotate, and adjust the camera’s position back and forth on the rail until they stop shifting relative to each other.

Without a panoramic head:

• Distant scenes are fine. With nothing closer than about twenty metres, parallax is negligible and hand-held panoramas work.
• Rotate about your own spine, not by swinging your arms, if hand-holding. It is closer to correct than pivoting the camera.
• Keep the foreground out of scenes you intend to stitch without a head.

Projections

The sphere has to be flattened, and the choice changes the picture.

• Rectilinear: straight lines stay straight. Correct for architecture. Stretches severely beyond about 100 degrees of field, so it is unusable for very wide panoramas.
• Cylindrical: verticals stay straight, horizontals curve. The standard for wide landscape panoramas.
• Equirectangular: the full sphere, used for 360-degree virtual tours.
• Mercator and others sit between cylindrical and equirectangular and are often the best compromise for a 180-degree scene.

If a stitched panorama looks oddly stretched at the edges, the projection is usually the reason, and switching from rectilinear to cylindrical fixes it in one click.

Shooting for a stitch

• Manual exposure. Automatic metering changes between frames and the sky ends up in bands. Meter for the brightest part and lock it.
• Manual white balance. Same reason.
• Manual focus, locked. Refocusing between frames changes the framing slightly through focus breathing.
• Shoot in portrait orientation for a horizontal panorama: more frames, but much more vertical coverage and less need to crop.
• 30 to 40 per cent overlap.
• Level the tripod, or the panorama arcs and you crop away a lot of it.
• Include a frame of your hand at the start and end of each panorama. It makes finding the set among 300 files trivial.

Free software, which is the best software here

Hugin is free, open source, and more capable than most paid options. It exposes the control points, lets you add and delete them by hand, offers every projection, and includes tools for exposure correction across frames and for masking out moving objects. Its optimiser can solve for lens distortion from the images themselves, which means it produces better results than software relying on a generic profile.

It is not friendly. The assistant tab does most jobs in three clicks; the rest of the interface is a photogrammetry application and looks like one. It is worth the afternoon.

Underneath it, enblend does the seam blending and enfuse the exposure fusion, both usable independently from the command line.

Photoshop’s Photomerge and Lightroom’s panorama merge are easier and adequate for straightforward cases. Phones stitch in-camera and do it surprisingly well for casual use.

What fails

Moving subjects. A person who walks through two frames appears twice, or half of them appears once. Hugin lets you mask which frame wins in a given region, which is the reliable fix.

Water and foliage. Waves and leaves move between frames, so the seam through them never matches perfectly. Place seams away from them where you can.

Close foregrounds without a panoramic head, as above.

Featureless regions. A plain sky or a blank wall gives no control points. Hugin lets you add them manually, or you can constrain the solution using the known geometry.

Scenes where the light changes during the sequence. A cloud crossing the sun between frame two and frame seven produces a band you cannot blend away. Shoot fast, and reshoot if the light moves.

The limit

A panorama is a single viewpoint rendered wide. It is not a wider scene: you cannot see round a corner, and nothing hidden behind a foreground object is revealed. And every projection distorts something, because a sphere cannot be flattened without distortion. Cartographers have known this for four hundred years, and the choice of projection is the same choice they make: which distortion you are prepared to accept.