Chris,
There is no such thing as 'semi APO'. Either a lens is an APO, or its not.
There are achromats - which bring 2 wavelengths to the same focus, or apochromats, which bring 3 wavelengths to the same focus. For some lenses designed for astronomy, one of the 3 wavelengths may outside the visible spectrum.
Until about 10 years ago, to make an APO meant either the lens had to be a triplet, or for a doublet, one element was fluorite - which is very fragile, easily damaged by moisture, as well as expensive.
In recent years the use of ED glass has allowed doublet designs to achieve apochromatism, and are frequently diffraction limited at f/7. ED doublets that are apochromatic are a huge improvement over achromatic doublets. However a well designed triplet or four-element lens should be even better especially when the size of the field f view and field curvature are taken not consideration.
The other issues to consider are
- whether the lens is truly diffraction-limited,
- the amount of'secondary spectrum' and
- lateral chromatic aberration off-axis,
- the amount of field-curvature present, and whether to use a field flattener to correct it.
With three of four elements a lens designer can do a great deal to control all these aberrations over a pretty decent field of view For example
http://www.vixen.co.jp/en/index.html
...
In larger apertures however, the best of all however are all-reflecting optics where all wavelengths are focused at exactly the same point from UV to infrared, or those with thin correctors such as the maksutov-newtonian or Riccardi-Honders.