Our human optical system surveys a scene and transmits messages to our brain which are, in fact, the contents two simultaneous, two-dimensional images, spaced about three inches apart. In making a 3D image we need to convince our optical system that, just as in real life, the two flat images presented add together to make a stereoscopic (3D) picture.

In Victorian times 'bioscope scenes' viewed with a hand-held stereo viewer were very popular. Two pictures were taken of a scene and pasted together side-by-side for viewing through simple lenses. They were very good with excellent depth and clarity.

The first requirement is to take two pictures of the scene, one left, one right. Obviously it's best unless using a specialist stereo camera, to avoid scenes with movement.

The viewpoint for pictures for most normal scenes, using say a 35mm to 100mm focal length lens (35mm photography) need to be about 3 inches apart - this is the average distance between the left eye and the right eye. Special devices can be bought to do this with extreme precision, however it can be done quite successfully by the 'rock from one foot to the other' method. To do this take one picture (left) then rock from the left foot to the right so as to lean to the right - go about 3 inches (75mm) keep everything level and without twisting the camera, take the second shot (right).

The two resulting pictures can be viewed as a stereo pair. If they are slides, then a double viewer can be used to recreate the 3D scene - this is the preferred method. If prints, then I find that placing two 5" x 7" prints (one 'left', one right') side by side about 3 feet away and deliberately going cross-eyed to view them works crudely. Some people find that going cross-eyed is not at all easy - so it's not something that I would recommend.

A popular form of 3D picture is a two-colour image that recreates depth by making each half of the stereo pair a different colour, this is known as an 'anaglyph'. Usually red and green is chosen for this.

To achieve a result we need to make the left eye image red and the right eye image green. (This is the convention, it could just as easily be the other way around - provided we view it that way).

Digital imaging has made the production of anaglyphs simple.

What follows looks long-winded because I've detailed every step. In practice the whole process is very quick and I can run through it all, once scanned in, to a final 3D image, ready to print, in under two minutes - honest!

1. Scan the 'left' and 'right' images into Photoshop in the usual way.

2. Make any obvious corrections, bearing in mind that the two images will ultimately overlap each other to form a single optical composite.

3. Go to the 'left' image and de-saturate it. Do this via Image > Adjust > Desaturate.

4. Go to the Channels palette and click on the RED channel (it highlights in blue).

5. Go to the horizontal right facing arrow at the top right of the channels palette and click. In the menu that opens choose 'Duplicate Channel.'

6. In the dialogue box that opens go to the Document box and click on the drop-down arrow and choose 'New'.

7. In the 'Name' box (not the 'document' box), type in the name of the image you are working on. (If you've already saved the left and right originals then choose another title otherwise the system will have great pleasure in telling you that the file name is already in use!). Click 'OK'.

   A new monochrome image will now appear.

8. Minimise or close the 'left' original. Keep the new monochrome image running.

9. Go to the 'right' image and de-saturate it. Do this via Image > Adjust > Desaturate.

10. Go to the Channels palette and click on the GREEN channel (it highlights in blue).

11. Go to the horizontal right facing arrow at the top right of the channels palette and click. In the menu that opens choose 'Duplicate Channel'.

12. This time in the dialogue box that opens, go to the 'Document' box, click on the drop-down arrow and choose (click on) the file name you gave in step 7. Click 'OK'.

13. Go back to the 'right' original and repeat steps, 10, 11, 12 having selected the BLUE channel in step 10.

14. Minimise or close the right original.

15. With the monochrome image running, created from the three separate colour channels (red = left, green & blue = right), go to > Image > Mode > RGB and as if by magic, the monochrome image becomes a red/green (and blue) anaglyph.

16. Put on your stereo red/green spectacles and view in glorious 3D.

17. All should be well but some adjustment may be necessary. See 'tweaks' below.

18. Print the picture to matt paper as relections from a glossy surface may be distracting.

NB. To make 'global' changes to the image, change from the Channels palette to the Layers palette, otherwise you may find that you are adjusting one colour channel only!

• Sharpening: Use 'unsharp mask' at an appropriate level.
• Brightness: As viewing via red/green spectacles can darken the scene, extra brightness is often required. This is best done via an 'Adjustment Layer' (Layers>New>Adjustment Layer>Brightness & Contrast). 'Levels' or 'Curves' may suite you better, it depends upon what you need to do to the scene. Anyway, 'Adjustment Layers' are preferred for the task as they can be altered later without detriment to the image.
   
  Take care if you choose to erase parts of the Adjustment Layer mask by painting with black as it will be painting in two areas simultaneously!
• Re-alignment: Should the two images be mis-aligned then some adjustment is possible. This is best done with the red channel, as altering green would also mean altering its partner - the blue. Go to Channels and activate/highlight the red channel. Activate the 'eye icon' in the RGB channel - this automatically opens the green & blue channel eye icons too. Choose the shift tool and adjust by the fon' in the RGB channel - this automatically opens the green & blue channel eye icons too. Choose the shift tool and adjust by the four direction-arrow keys to the right of the 'qwerty' keyboard area to incrementally shift up, down or left, right. Shifting left or right will decrease or increase apparent special separation. Viewing with the 'grid' visible (via Window > Show grid) will help to accurately align. Choose 'Hide grid' to take it off.
  • Note for Photoshop 6 about re-alignment (above): the image can only be nudged / moved if the channel is 'active' and the total area selected therefore, first, Select > All (or Ctrl + A), then go to the Move tool.
     
    Problem? If the move/nudge doesn't operate and you get a 'warning' notice - return to the Layers palette and ensure that the image (probably background) is active and not another layer.

• Wide angle and telephoto considerations: If the original images are photographed using wide angle lenses then the depth will be exaggerated, sometimes unacceptably so. A shift of less than 3 inches may be preferred. Conversely, long focal length lenses usually need a wider separation to give an illusion of depth - in this case a separation of several feet may be required between left and right pictures. However, with greater separation it's more difficult to maintain correct registration.
• Why desaturate? The 3D picture can be perfectly well made without desaturation as outlined above, indeed in some cases better final contrast may result. However I have found that a more even image results if the three tonal ranges are the same to begin with. When starting with a colour original you may prefer to use only one of the channels available if that particular channel gives an improved monochrome rendering of the scene.
• What sort of pictures work best? In my experience the scenes that work best have many 'planes' of image information, are of a gentle contrast range and avoid large, dark, featureless areas. Dark areas frequently reveal ghost images from the other channel which can be a distraction.
   
  A great deal of experimentation is required to perfect the technique, however I hope that these notes will tempt you to have a go.

No spectacles? - A quick alternative ....... If you have red and green filters for B&W photography, try them (red - left, green - right).