One of my OD's gave me an Rx for a bifocal and asked for 2D, BI OU at near only. Does anyone know how to induce prism in the seg and not in the distance. Rx is -.75-.50X180, +.75-.25 X145, Add is +2.50. I'm thinking I'll need to use a FT35. Any advise would be greatly appreciated.
First thought that comes to mind would be fresnel prisms cut to match the segs on both eyes. Otherwise contact your lab and ask them if they can do prism only in the seg for a digital blended bifocal. Might be an option as the seg is cut as the lenses are being surfaced.
A Franklin would be the ideal solution with no optical compromise if you can find someone to fabricate it.
A FT 45 might give you a little more room with frame selection, but maybe not. FT 45's seem to have the seg centered in the lens.
May I suggest consulting with your surfacing lab? I've had good luck with tricky PDs for CRX bifocals, where the reading segs are inset at a minimum from the [intermediate] top "PD." I've also seen an OC ordered below the seg line. All this suggests it may be possible to fulfill your script in a similar fashion. (I'd be interest to hear George's input here!)
If a reassuring answer (and guaranteed commitment) weren't forthcoming, I'd stick with a simple SV reading pair with the prism correction and SV distance without.
Edit
Last edited by Bimbol; 01-25-2017 at 09:04 AM. Reason: Edited for miscaluation.
Not easily, prism is ground in the back of the lens and thus necessarily translates to the whole design. You could fudge a bit by playing with decentration of the segment itself. The near total power spherical equivalent is about +1.50/+3.00. If the segs are decentered IN 1.5mm OD and 1mm OS it should induce about 2.25D and 3D BI, a little more than you need but you can always back off the decentration a few tenths of mm. If you need to get exactly 2D OU you'll need to calculate exact powers on the 180, but I think getting within a diopter or so total OU is fine, since near PD is debatable anyway.
Three rivers optical did one for us...Originally Posted by rbaker
Yeah I always do franklins for these. Or rather the guys that TRO outsource it to do franklins for me.
On my bucket list is definitely making my own franklins, I just haven't had the cojones to do it for someone by myself yet.
Thank you to all who responded. That's what I love about this site!!
Use a FT 45 and decenter the seg an extra 8mm per eye will give you 2 D BI OU at near only. Example; PD is 63/60. Order lenses 63/44. Cut out may be an issue dependent on frame size.
Decentering the segs for near horizontal prism is not uncommon. We generally only consider the add power for calculating the amount of decentration needed, and in this case if we are asking for 2 diopters base in OU that would translate to additional 8mm of decentration for each lens. Minimum of FT35 would be recommended.
Trip
From first glance, executive may be the only way to go. The problem I see with flat top lens IN THIS CASE, is the minus sphere OD and plus sphere OS. The decentration needed on each seg would be different. About 16mm OD but only 7mm OS. Anyone want to back me up or correct me??
You only consider the add power to induce the prism required, hence 8mm's extra in per lens (from pt's normal NPD). You leave the DPD as measured.
Thanks "optical24/7" for the correction
I'll have to stew on that this weekend. I see what you mean, but it's gonna take a few minutes to totally sink in.
thanks.
Your OD he doesn't know what he asked for is virtually impossible?
Also, a measly 2^ on an add that high? Drop in the freakin' bucket. Bad plan.
Thank You for speaking the truth!!
Chadwick Optical did one of these for us a few years back too. A split/bonded lens, as we had a pt who his MD wanted 10 prism diopters on the OS near only. They were pretty quick too - about a week if memory serves.
Jrum, don't think total power in the add portion of the lens ( distance+add) only the add power of the seg. I've made the same mistake.
There's another long thread on here discussing the subject.
Wesley S. Scott, MBA, MIS, ABOM, NCLE-AC, LDO - SC & GA
“As our circle of knowledge expands, so does the circumference of darkness surrounding it.” -Albert Einstein
Maybe back when Nixon was president!
Good and plenty!
site:optiboard.com Prism at near only
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
I have a serious doozy that I am about to do this on. I am planning on using the FT45 method mentioned above because of the power of the lens.
OD +8.75 -4.75 @ 91 .5 BU
OS +9.00 -4.75 @ 89 .5 BD
in the near seg ADD +2.25:
OD 2 BI .5 BU
OD 2 BI .5 BD
My calculations are telling me to add 9mm to the inset on each eye inducing 2.03 diopters of BI for each eye, as well as the ground in prism in the back.
This patient sees an OD not related to our practice, and a few years ago after they couldn't fit him to get him seeing properly I got him working in a CR39 ST35 from XCEL, he is EXTREMELY sensitive to vertex distance changing and doesn't like change. That being said the last pair I cut for him had no prism whatsoever - I have half a mind to think this OD is messing with me because he won't get glasses there anymore, but I'm just paranoid.
Can any of you guys that know more than me help me look for pitfalls I might come into? (I can handle the edging, I just want to make sure the technique Optical 24/7 mentioned (one I've read darryl talk about in another thread) holds true here? I haven't read anyone talk about it being done with this much CYL.
Thanks, I'd buy you a beer if I could.
Last edited by Tallboy; 02-02-2017 at 12:23 PM.
Seriously? Another OD prescribing prism near only?
Hey, ODs: ALL PRESBYOPES ARE ABOUT 15^ EXO AND THEY DON'T NEED PRISM!!!!
With the vertical prism being apart of the near requirement I would look to a Franklin vs decentration, can still keep as a D segment to help manage thickness.
Trip
Yeah thats what I usually do when the vertical prism is in the segment but not the carrier lens. Since the carrier lens has the same vertical prism as the seg it should be okay to be ground on the back, no?
Also there are no ft45 available in 10 base so Im SOL. Im going to call the doctor and give her the options of a ft35 decentered (leaving 26mm useable) or a franklin, which costs quite a bit more when you are talking about 2 sets of lenses needing to be ground in those high powers, see what she says...
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