Let's say you have two images you want to stack. It is assumed that both images are about the same size (in pixels by pixels)
and that both are of the same object and taken under the same conditions (same exposure time, same film or CCD, etc.). The
instructions below should help you walk through the process.
Open Photoshop and open both images such that they are displayed in two windows. Choose one of them to be the base image.
Click on the second image to make it the active image. Use the selection tool to select the portion of the image you want
to stack ( if you want to stack the entire image, under Select pull down menu select ALL.
Copy the selected portion of the image to the Photoshop clipboard by going under the EDIT pull down menu and selecting
Now click on the base image to make it the active image ( you can close out the second image at this point ). Under the
EDIT pull down menu, click PASTE. This will basically genereate a new layer in the base image, the new layer
contains the data copied from the second image.
Now you have to align the images for a proper stack. In the LAYERS box select Difference as the layer mode.
Then, change the cursor to the MOVE mode.Place the cursor in the new layer and drag the image around until you see
total blackness ( or as close to it as you can ). When total blackness has been achieved, it means the images are aligned
properly. If you cannnot obtain a solid level of blackness, it means the images are at different scales, rotated with respect
to each other, etc. ( these problems are harder to deal with ). If you images where taken in sucession ( as is normally the
case when you plan to stack ) you should have minimal problems getting alignment. If you see some occasional "grain" noise
or spots, do not worry. Grainy noise is what stacking will reduce. Spots are probably dust or dirt in the individual images,
these can be removed later by other means.
Once the blackness has been achieved in the previous step, change the layering mode to NORMAL. Then slide the OPACITY
slider such that it has a value or exactly 50%.
In the LAYER pull down menu, select FLATTEN. This will combine the layers into one single image. Then, save
the image off to a file format of your choice. Additional editing can be done as desired.
The above procedure is for stacking two images. If you want to stack say 4 images, stack them in pairs as decribed above,
then stack the two composites. I have not found a mathematically proper way to stack more than two images at one time using
Photoshop ( there may be a way, but I have not discovered it yet ).