Whats the practical difference: doublet and triplet?

[kirkster501]

Yes, I know, doublet = two and triplet = three.... So whats difference in the view etc and whats the motivation to use the triplet type please? I notice the triplet types are very much more expensive....

Steve

[ollypenrice]

To be tuly apochromatic the lens has to focus the whole visible spectrum (and a bit besides at the blue end to please the imagers) to the same point. Strictly speaking you need a triplet to do this.

In the real world there's a bit of give and take and a premium doublet doesn't just beat a bad triplet, it can beat quite a good triplet as well. But F ratio plays a hand. The faster the focal ratio the harder it is for the doublet to compete. Very fast apos have to be triplets or quadruplets. And aperture plays in here as well. The larger the objective the harder it is to colour-correct at a given focal ratio.

If you were lucky enough to have a Takahashi doublet for visual use you would not be in the market for a change of scope, believe me!

Where budget is a constraint you have to reflect on the cost of making triplets; you not only have two more optical surfaces to grind and coat but you also have more work in the lens cell and in collimation. Does this mean that you would be better, on a budget, to go for a better made doublet than a less well made triplet? I don't think you can say. It depends on the individual scope. If the end use is visual I might bet on the doublet. If imaging I'd want to try them both. Trying them both has to be the way.

Olly