Step 4: Determine The Best Minimum f/number For Your Lens
You'd think the lens with the widest front element would work best, but that is not quite true. Severe artificial vignetting will degrade anaglyph quality by introducing both color casts and stereo depth errors. Thus, we need to determine the widest aperture at which this vignetting is kept to an acceptable level.
Artificial vignetting generally causes the PSF shape to change (be clipped) as you move away from the center. If you have a removable lens, you can see this effect by taking the lens off your camera and looking through it at an angle similar to what's needed to reach the image edge (see photo). A more precise method is to hold an LED flashlight in front of your camera and observe how it's OOF PSF image changes as you move the light from image center to image corner. Start with the lens wide open and stop it down (increase the f/number) until the shape distortion is minor.
Often, wide open is ok. However, especially for very fast lenses, it is common that stopping down a little will significantly reduce this type of vignetting. A few lenses, especially on compact cameras, have severe artificial vignetting at all usably large apertures; they will never work well with the method in this instructable.
The smallest f/number (widest aperture) at which the shape distortion is acceptable is the f/number you should use. I like to see no more than a 25% PSF size reduction due to clipping near the image corners.
If you have a zoom lens, repeat this procedure for several focal length settings. The combination for which the focal length divided by the f/number is largest probably will work best, but you could make stops for each combination.