[chris lewis]

I think the general consensus is that the Synta ED doublets that use the FPL-53 reach the visual criteria for 'APOness’ when it comes to color correction.
However I think that it is pretty impossible to get full correction with a Doublet. The ‘new’ 80 / 127mm is a triplet that uses FPL-51 glass and it is not marketed as an ‘APO’ as such. I do not think it is a true APO either but again visually it may reach the ‘APOness’ criteria. There does however appear to be more QA variation with the ‘low end’ triplets entering the market which is to be expected at that price point.The above scopes do not reach the same CA or SA correction compared to an AP, TEC or Tak or even a TV APO triplet. I think that a very good 'low end’ ED or triplet is great for visual, and fair for photography. For photography a 'high end' - read expensive - APO is best and the aperture is not as important for most objects.


This is just my opinion and others will vary.


Below is an interesting quote from Roland Christen. [APO lens maker].

The question being:


“The FPL51 -FDC1 would be more difficult to achieve color correction compared to the FPL53”



Reply: It has to do more with focal length than color correction. You can get almost perfect color correction with FPL51 in a 130mm size if you make an F15 doublet. The same perfect correction can be achieved in an F12 doublet using FPL53 because the dispersion number if higher by some 20%. In order to get an FPL51 glass 130 F7.5 lens with the same perfect color correction, you simply place two F15 doublets back to back and you have an F7.5 lens. If the inner elements are the FPL51, you can fuse them together (glue them) and you have a 4 element lens that is totally indistinguishable from a triplet where the inner element is one piece instead of 2 pieces glued together. So, now we have designed a 3 element FPL51 lens of F7.5 focal ratio with essentially perfect color correction. Can it be beat by an FPL53 lens? Yes, using this more advanced glass will result in either a faster focal ratio of around F6, or a lens of F7.5 with less spherochromatism. Since sphero-chromatism is the least noticeable defect in an optical system to the beginner, then it seems smart for a manufacturer to make triplets using FPL51 instead of FPL53 (or Fluorite).

Rolando