by lobaz » Fri Jan 16, 2015 12:13 pm
Wiki image is correct. Width of the beam does not change focal distance, and green is focused closer to the lens than red. If you need to focus red+green to the same distance, you have two choices:
- Use an achromatic doublet, i.e. two lenses cemented together. These two lenses have their chromatic aberrations too, but in opposite direction so that they cancel each other for specific wavelengths. You can also use an achromatic objective that has other aberrations corrected too, but more complicated setup = more light loss.
- Change spread of one of the beams. For example, put a weak concave lens into the green beam. This makes the beam divergent a bit, so the main convex lens focuses it a bit farther.
Wiki image is correct. Width of the beam does not change focal distance, and green is focused closer to the lens than red. If you need to focus red+green to the same distance, you have two choices:
- Use an achromatic doublet, i.e. two lenses cemented together. These two lenses have their chromatic aberrations too, but in opposite direction so that they cancel each other for specific wavelengths. You can also use an achromatic objective that has other aberrations corrected too, but more complicated setup = more light loss.
- Change spread of one of the beams. For example, put a weak concave lens into the green beam. This makes the beam divergent a bit, so the main convex lens focuses it a bit farther.