Minimum Blank Size - Multifocals

[lensmanmd]

Time to put on your thinking caps.

We all know how to calculate MBS for SV finish. I have been scouring the web for a definitive method to calculate MBS for multifocals and have come up with nothing.

I know how, but want to present a WebEx for our opticians and lab staff without creating one from scratch. Videos, PDFs, Powerpoints, all are welcome.

Anyone up for the challenge to provide links?

[lensmanmd]

I hear the sounds of crickets mating. That quiet on this thread.

[lensmanmd]

Still hearing crickets. Anyone? Lab experts...ABOMs.....anyone?

[Hayde]

Lensman, to give the crickets some privacy while you wait on an informed answer--tell me: why would the calculation be different for a MF than an SV? Assuming a standardized OC from the line (which is as assumption I've seen invite debate here on OB far more often than among labs,) aren't the additional complications sorted out at frame selection rather than afterwards in blank selection?

[rbaker]

Perhaps this old graphic will aid you in determining minimum blank size.

[Speed]

Lensman, to give the crickets some privacy while you wait on an informed answer--tell me: why would the calculation be different for a MF than an SV? Assuming a standardized OC from the line (which is as assumption I've seen invite debate here on OB far more often than among labs,) aren't the additional complications sorted out at frame selection rather than afterwards in blank selection?

Seg locations vary and complicate the process.

[Hayde]

Seg locations vary and complicate the process.

Ah, so if NPD=DPD-{DPD/[1+W(1/s-F/1000)]}, then plus lenses will need a bigger diameter than minuses, as will wider PDs require more additional diameter (compared to his/her SV blank) versus the narrower PD.

A I on the same page now? Anything I'm missing?

[Speed]

Ah, so if NPD=DPD-{DPD/[1+W(1/s-F/1000)]}, then plus lenses will need a bigger diameter than minuses, as will wider PDs require more additional diameter (compared to his/her SV blank) versus the narrower PD.

A I on the same page now? Anything I'm missing?

Don't seem correct to me.

[optical24/7]

I'll take a shot at it. With an Ft, it's usually the inset that you have to worry about, not height ( though it can be if extremely high or low.) If a 70mm Ft blank is set 5 in and 5 down, you would effectively have an 80mm diameter horizontally. (5 in x 2). But if the seg needed out set rather than inset you'd loose 10mm in effective diameter.

For height, with seg drop below datum you'd increase the ED 10mm, above datum height you'd loose 10mm of ED.

With a frame 55/15, ED 57 and NPD of 58:

55 + 15 = 70 - 58 = 12 + 57 = 70 - 10 = 60mm minimum blank size for horizontal.

For height, our frame has a 45B and SH 30:

45/2 = 22.5 - 30 = 7.5 up. 7.5 x 2 = 15 + 10mm ( lost diameter since we are decentering up) = 25 + 45 = 70.

For this particular combo, a 70mm blank would be minimum. (At least that's my thoughts. There are way smarter people here to tell me I'm wrong) ��

[lensmanmd]

Let's assume frame is 56/18 with 48B and 62ED. NPD is 62. SH is 18. Decentration would be 6mm per eye. Vertical would be 6 drop on a ST.

SV MBS is 62+12+2 (if adding a safety margin) for 76mm on a FSV.

ST is 6/6, so horizontal decentration is null. Vertical will also be null as it is also a 6 drop.
Take these into account and now MBS is 62+2 for this ST. 64 MBS will result.

Now, if this SH is 21, instead of 18, changing the drop to 3, MBS will increase.

And unlike SV, if the NPD is wider, say 70, MBS will also increase.

Many different parameters to take into account. Writing and illustrating the various scenarios is a daunting task. It sure would help if there were instructions that are readily available.

Now you know my dilemma. No need to rewrite if something is already available.