Free for guru question

[gatorbait]

FREEFORM Guru Question(Sorry about the typo)

How is it that a lens can be freeform technology but yet you cannot have a specific base curve. The one lab that I have found that this is untrue is Digital Eye Lab. I have tried Zeiss, Essilor, and Hoya Freeform lenses and cannot get specific base curves but I can with the Digital 5.0 Mod Pal? What gives?

[shanbaum]

FREEFORM Guru Question(Sorry about the typo)

How is it that a lens can be freeform technology but yet you cannot have a specific base curve. The one lab that I have found that this is untrue is Digital Eye Lab. I have tried Zeiss, Essilor, and Hoya Freeform lenses and cannot get specific base curves but I can with the Digital 5.0 Mod Pal? What gives?

Different Lens Design Systems behave differently with regard to base curves, and for that matter, "donor" blanks. For a given Rx, some require that specific blanks be used (usually blanks provided by the same company as the LDS); some define a narrow range of acceptable base curves; some define a broad range of acceptable base curves.

[sharpstick777]

Some companies limit the base curve selection per RX to keep the number of possible point files to a minimum. Not all FF lenses are calculated individually, the point files are already run and set. It just save computer time and reduces the risk of something going wrong.

[Darryl Meister]

As Robert notes, it varies depending upon the Lens Design System, even between lens brands from the same manufacturer.

SOLA free-form lenses, for instance, allow more flexibility in choosing a base curve compared to ZEISS free-form lenses, which are generally allowed to deviate by no more than 2.00 diopters from the recommended base curve for a given prescription. This is primarily because the initial ZEISS free-form lens designs rely on additional fine-tuning by a lens designer using optimization points and weights that depend somewhat on the initial selection of lens form or base curve. This results in more accurate and precise optical optimization, but at the expense of some flexibility in base curve selection.

There are additional issues associated with the choice of base curve for free-form lenses as well, which I describe in this thread: Base curves of free-form lenses.

Best regards,
Darryl