How To Use The Lensmeter – Verification of A Complete Bifocal Pair


– Hello and welcome back
to our ongoing series. Let’s learn how to use the Lensmeter. Today is kit number four. Kit number four is a complete pair of mounted lined bifocals. We’re going to use the Lensmeter
to read both the distance and the near portion of those lenses. (upbeat music) Really brief side note for today, Lensmeters are notoriously ignored. Most places buy the
Lensmeter when they open and the thing is never
touched, calibrated, or serviced from that day forward. Chances are excellent that where you work, you’re gonna end up
working with a Lensmeter that’s 20, 25, 30 years old. If it is time for
recalibration or refurbishment, there are places online
or drop me an email and I’ll hook you up with somebody. Kit number four is another mounted pair. May as well go and find
number four, double check your etched mark to make sure
you have the right frame, right lenses in the right bag. And you will notice when you look at it that it is a lined bifocal. We’re gonna build on what
we’ve already been doing and we’re gonna add in
a couple of new steps. One, we’re going to take
a near PD measurement or verification. We’re going to check our segment height and we’re going to learn
how to use the Lensmeter in order to read the bifocal power. Which is kinda weird so,
hang on we’ll get there. In the mean time, we’re not
gonna do anything different. Just what we’ve been doing. Our right is supposed to be a minus 2.75, minus one 1.25 at 68. Could turn my Lensmeter on,
I could turn my power drum to minus 2.75 and I could turn my axis wheel to 68. Right, right, right. And what I want you to pay
attention to up against the lens stop is that the
top of the line is below the opening of the lens stop. If you don’t do that and
you catch the line anywhere above that, you’re gonna get a little bit of distortion that you don’t want. So, just double check that
and then you can lower your lens holder and just as
always when you look inside, you should have good, crisp sphere lines. And they should be closed. Use your spectacle table. Get this lined up all nice. Move your chrome knurled sleeve around. Rotate your power drum
away from you til you have good, crisp cylinder lines. Work everything so everything’s lined up. You are at 2.75, you move
to 1.25 you should be at the power drum at minus four. And that lens is looking good. We can put our three dots on there. Without moving the spectacle
tables, we’ll go from our right to our left. And this time it says I’ve got a plus two, at 111. And let’s see what we got
there, we’ve go some nice crisp sphere lines, those look good. And then I can move my
chrome knurled sleeve around a little bit, help me
get everything lined up. And then it just says I got
a little tiny bit of seal just at minus 50, so I’m
gonna rotate that drum away from me just a little tiny bit. And sure enough, I’ve got
cylinder lines at plus 1.50. It’s good. And I can mark that lens up. As I mentioned, there’d be two additional things we’re going to do. First, obviously we’re going
to the distance between our two center white
dots and that gives us our distance PD. And I have got a beautiful, perfect 64 there. Then I’m going to measure my near PD. When your PD is that second number, my distance is supposed to be 64. My near is supposed to be 60. And I do that by measuring from inside edge to outside edge of my segment, as you seen shown on the screen there. And it is supposed to be a 60 and I’ve got about 60 and a half. So that’s certainly well within tolerance and what you would expect. So we’re good there. And last thing I would
need to check is my height, which is supposed to be 12 in each eye. And heights are measured from the deepest point in the frame. And I have got about 11 and a half. And about 11 and a half, which is perfect. No problems there. Now, we’re going to make
sure that our bifocal power is the correct one and
this is where things get a little bit weird like I said. Why do I say that? I say that because bifocal
segment areas, the add power on a lens is often read with
the frame turned away from you, as if you were wearing it
with the segment portion against the lens stop. Let’s just talk about that
for a few minutes alright? If you go online and you
look into this a little bit. You’re gonna see there’s
five, six different ways of doing what I’m about to show you. Just find one that works for you. Another thing you’re gonna
find online, there is general consensus that it doesn’t
really matter if you turn the pair around, until you
get up into higher powers. The higher the distance
portion of the script, the higher the strength the add power is, the more important that it
becomes that you turn it around. Another piece of this is that
if you go for a practical exam and it doesn’t matter
what the lens power is, if you don’t turn that pair
around when you’re checking your bifocal segment, you’re gonna fail. Those exam proctors are looking for that. Practice it. Get used to that turning it
around to read the segment. Experiment, find one method
that gives you the consistent results that you need. And just do it that way. Right lens, right lens,
right lens easy enough. I’m going to start with
my distance portion in the Lensmeter against the lens stop, just as if I had the glasses
facing the other way. I am going to rotate my power
drum around until I have good sphere lines in focus, in minus cylinder form, should just about the prescription that I was, but it may
very well be an eighth or a quarter difference. So just get the sphere lines,
and the axis doesn’t matter it’s going to move. So, just spin your axis wheel
until you get sphere lines, it doesn’t matter what number it’s at. Alright I’ve got those. Now, I’m gonna take my cheaters and I’m gonna read my power drum and I have got a minus three. And I’m actually gonna write that down. Now I’m gonna take my right lens and I’m gonna move it up into my segment and I’m gonna rotate that power
drum in the plus direction until I have sphere lines in focus. Now, what you’re gonna see in
here is that the cross hairs where the cylinders and
spheres cross over each other is way, way down at the bottom. That’s normal, that’s expected. It’s down around three. Just leave things alone. Move that power drum until
you have good, beautiful, crisp sphere lines and you’re good. Let’s see where we ended up. We ended up at minus 1.50. So that is a plus 1.50
difference and sure enough my add power in my right is
supposed to be a plus 1.50. Great. So far so good. Let’s do that now with the left. Here’s my left, my left in the distance. Rotate my power drum as much as I need to. Rotate my axis wheel as much as I need to, make sure I’m minus cylinder form. I am at plus 1.75. Now I’m gonna bring
myself up into my segment and I’m gonna rotate that drum. This time I’m up around the two ring and I am rotating. I need to tweek my axis
wheel a little bit, til I get good sphere lines. That looks pretty darn good right there. Set those to the side
and I am now at plus 3.25 just difference of 1.50. I am good. My add power is a 1.50 in both eyes. The other and really far
simpler way of checking a standard lined bifocal
power is using your good old fashioned lens clock. And what you wanna do is
my right lens, right lens is clock your front base
curve, then you put your pin right on the white dot. Hold it, read it and I’ve
got a beautiful plus six. Then bring your clock
down onto your segment, aim right for the middle of it. Read the lens clock and I’ve
got exactly a plus 7.50. Low and behold, I’ve got my plus 1.50. Gonna do the same for my left lens. I’m gonna put my center pin
right on that white dot. I have got a plus seven there. And if I clock my segment
right up near the line, I’m probably about millimeter
below the line there. I have got 8.50 a difference of 1.50. My add in these lenses is a 1.50. By all means if they have a lens clock during your practical. Just to be sure you might
want to do both steps. If you’re checking jobs in, again guys bifocals, line bifocals are just
becoming kind of dinosaurs you may or may not even see
them anymore in your lifetime. But you do need to kinda know this stuff. When they do come along, just practice. Practice, practice. Again, it’s not gonna be perfect. This isn’t gonna be beautiful,
crisp on target stuff. Get used to the steps involved and when you get to any practical exam, you’re gonna be fine. That is lens kit number
four, a lined bifocal and next week we hit progressives. So, I will see you then.

10 Comments

Add a Comment

Your email address will not be published. Required fields are marked *