About Photomerge
The Photomerge™ command combines several photographs into one
continuous image. For example, you can take five overlapping
photographs of a city skyline, and then merge them into a panorama.
The Photomerge command can assemble photos that are tiled
horizontally as well as vertically.
To create Photomerge compositions, choose File > Automate >
Photomerge and then choose your source files and then specify layout
and blending options. Your option choice depends on how you
photographed the panorama. For example, if you’ve photographed
images for a 360 degree panorama, the Spherical layout option is
recommended. This option stitches the images and transforms them
as if they were mapped to the inside of a sphere, which simulates the
experience of viewing a 360 degree panorama.
Take pictures for Photomerge
Your source photographs play a large role in panoramic compositions.
To avoid problems, follow these guidelines when taking pictures for
use with Photomerge:
Overlap images sufficiently
Images should overlap by approximately 30%. If the overlap
is less, Photomerge may not be able to automatically assemble
the panorama. However, keep in mind that the images
shouldn’t overlap too much. If images overlap by 70% or more,
Photomerge may not be able to blend the images. Try to keep
the individual photos at least somewhat distinct from each
other.
Keep the camera level
Although Photomerge can process slight rotations between
pictures, a tilt of more than a few degrees can result in
errors when the panorama is assembled. Using a tripod with a
rotating head helps maintain camera alignment and viewpoint.
Maintain the same exposure
Avoid using the flash in some pictures and not in others. The
blending features in Photomerge helps smooth out different
exposures, but extreme differences make alignment difficult.
Some digital cameras change exposure settings automatically
as you take pictures, so you may need to check your camera
settings to be sure that all the images have the same
exposure.
Create a Photomerge composition
Do the following:

Choose File > Automate > Photomerge.
Under Source Files in the Photomerge dialog box,
choose one of the following from the Use menu:
Files
Generates the Photomerge composition using
individual files.
Folders
Uses all the images stored in a folder to create
the Photomerge composition.
Specify which images to use by doing one of the following
Select a Layout option:
1. Auto
Photoshop analyzes the source images and
applies either a Perspective, Cylindrical, and
Spherical layout, depending on which produces a
better photomerge.
Perspective
Creates a consistent composition by designating
one of the source images (by default, the middle
image) as the reference image. The other
images are then transformed (repositioned,
stretched or skewed as necessary) so that
overlapping content across layers is matched.
Cylindrical
Reduces the “bow-tie” distortion that can occur
with the Perspective layout by displaying
individual images as on an unfolded cylinder.
Overlapping content across files is still matched.
The reference image is placed at the center.
Best suited for creating wide panoramas.
Applying Cylindrical Mapping
A.
Original
B.
Cylindrical Mapping applied
Spherical
Aligns and transforms the images as if they
were for mapping the inside of a sphere. If you
have taken a set of images that cover 360
degrees, use this for 360 degree panoramas.
You might also use Spherical to produce nice
panoramic results with other file sets.
Collage
Aligns the layers and matches overlapping
content and transforms (rotate or scale) any of
the source layers.
Reposition
Aligns the layers and matches overlapping
content, but does not transform (stretch or
skew) any of the source layers.
Select any of the following options:
Blend Images Together
Finds the optimal borders between the images
and create seams based on those borders, and
to color match the images. With Blend Images
Together turned off, a simple rectangular blend
is performed. This may be preferable if you
intend to retouch the blending masks by hand.
Vignette Removal
Removes and performs exposure compensation in
images that have darkened edges caused by lens
flaws or improper lens shading.
Geometric Distortion Correction
Compensates for barrel, pincushion, or fisheye
distortion.
Click OK.