 All right, well, I better get started. So these notes come from Dr. Olafsson, who many of you love and admire. I'm still at the VA over there, but I doubt that I'm a little bit today. But this is gonna be talking about lenses and prisons. This was a trip we just took to Lake Powell over UEA a few weeks ago. And this is our screaming daughter who was happy like the entire time, except when we wanted to family pictures. But it is an awesome trip. We found this cool little island here with some hydrogen, some really sweet formations. Anyway, so start out talking about stigmatism a little bit. Pretty basic, but in regular stigmatism, the axes are gonna be 90 degrees apart as opposed to irregular stigmatism, I think here at a conus or ectages, things like that. So one thing to remember about regular stigmatism and if you think of it as like a loose lens that you pick up out of the trial drawers, that's one of the five diopter plus, so. The extreme values are 90 degrees apart, so along the axis, along where those two little arrows are, there's zero power along that axis. The power actually comes 90 degrees from that, so. And another thing that's interesting about those is, we did this in labs in optometry school, but if you do shine a light through it, as opposed to a spherical lens, which will make a point focus, these end up making a line focus. And so you'll actually see a straight line if you put a piece of paper behind the lens. So the power meridian is always 90 degrees from the axis. And then here's just a little example down here from this prescription, so this is exactly like what you would pick up out of the loose lens set. So plano plus five axis 45. The plus five is actually at axis 135, the plano is at 45. And have you guys done much with power crosses a little bit? So go over that a little bit. There's a few practice problems throughout here if you wanna try them out, that'd be awesome, but let's see, we'll keep going. So just as we were just talking about the plain cylinders, that means there's no sphere. The power in the axis is zero and the maximum power is 90 degrees from them. Yeah, we kinda talk about most of this stuff, but there is no line focus image formed by the axis meridian, just the power meridian. So this just demonstrates why you get a line focus from a cylinder. If you think of a cylinder as actually being a cylinder that you just kinda chop a part of it off, then you can see that no matter where the planes of light are coming in, they all get focused to a single point, but because you have an infinite number of those points, it ends up creating a line. And on top, this is a plus cylinder, so you would end up with a focal line behind the lens. Now, would this be a real image or a not real image? Virtual image. What's that? Real image? Yeah, so since it's to the right of the lens, we're gonna get a real image. So plus lenses tend to give you a real image behind the lens, whereas this minus lens are going to actually bend the light behind the lens. You can picture the light coming in from the left in all these pictures. So you're gonna get a focal line behind the lens. In this case, you'd get a virtual image. So a little bit about power cross. As a power cross is just a way that you can represent a glasses prescription, a contact lens prescription, so it can make sense in a picture to you to see exactly where the power is in each meridian. So this is the prescription that Joe has been given for his glasses. So how would we represent this on this power cross here? So we're gonna have one power along this vertical meridian, one power across this horizontal meridian. So does anybody wanna have a shot at what would go where? Aside from running back there. Okay. This is actually super helpful, especially when you're in Hoffman's clinic and doing retinoscopy. So the plus three go on the horizontal one? Great, great thought there. So actually, so the way to do it is this first number here, so your spherical power always goes on the axis that is listed here. So you just put that minus two right here. Let's see. Oh, wait a minute, axis 90, sorry. The, yes, I totally just, it's early in the morning. Sorry, the minus two is an axis 90, so it is this number, but that's not the horizontal, so that's right here. Okay, thank you. So you put the minus two up here at axis 90. So now the question is, does the minus three go here or what number goes here? So it's actually, or not the minus three, plus three. So you actually add these two together in order to get the number that goes on your opposite meridian. So we're gonna have a plus one there. That was all very confusing, I'm sorry. So you have this power, so you put the minus two at axis 90, and then you add these two together to figure out what your 90 degree power is for, it won't be along the horizontal. So what does that look like inside the eye? So this is their ocular correction. So we know that one of these is going to produce a horizontal line, one of these is gonna produce a vertical line, and it's actually the opposite of what is in the power of the lines, because of representation of what we had here. Let me show you here. So the vertical line, a minus two, is actually gonna produce a horizontal line in front of the retina, and the plus one will produce a vertical, I know I have these labeled as H and V, that's just saying which power they're coming from on the power cross. So this vertical line here, the minus two, will produce an image in front of the retina that's a horizontal line. The horizontal plus one will produce a vertical image behind the retina. A little confusing there, because it seems backwards, but that's exactly what the lens is gonna do. Okay, so here's one of our practice quiz questions to work on. So a point source from Infinity site strikes a cylinder lens with this power, and it's in minus cylinder form here. I don't wanna apologize. At what distance and orientation will the images be from the lens? So start out by doing a power cross based on what we learned on the last one. And then this is just the lens. We're not necessarily worried about the eye. Here, we're just wondering at what distance from the lens will these images be produced? So the system's minus here, do you have to change the way you press it? It's actually the same, yeah. But when you add the powers together, that would be the one difference. And we'll talk about switching from minus to the plus cylinder as well. So anybody wanna venture a guess on either one of the powers? Okay. So just that, like, the horizontal line? So here's our power cross. I know, it's backwards. It seems totally backwards. Yeah, so we have a plus two and a plus four. So we have to convert that into a distance. So we're gonna have one over four, which is 25 centimeters or 0.25 meters. But because this is along the horizontal axis, the image it produces is actually a vertical image. Throws it 90 degrees. We'll also have, because of this plus two here, we'll have one over two or 50 centimeters. And this is gonna be, since it's a vertical reading here, we're gonna have a horizontal line or a vertical one. So we'll have a horizontal line, 50 centimeters. Now I love this representation because this shows why we actually prescribe cylinder for patients because is it really that useful to have a vertical image or a horizontal image for the patient? If the prescription's off one way or another, that's exactly what they'd see on the retina. But when we get the focal point, when we move the prescription, so it's exactly between those two cylinder powers, you can see how the image actually kind of takes shape. And so that right in between the two, it's actually like a point focus, which is what we have with spherical lens is somebody who doesn't have a stigmatism. So that's the point where we would want to put the retina is right where there's that point, the focal point there. But this represents what the image is actually doing between these two line focal points. Okay, so this one's already got a little bit of the work done for you here. So now we put a point. So before we had an image coming from infinity basically hitting that lens. Now we're actually having a point of light, 33 centimeters in front of the lens. So now we add a little bit of extra math here. So you might actually have to write a couple things down. So what is that gonna do to, where is that gonna put our images at this point? So we have our same power cross. So if life was coming from infinity, it'd be the exact same as it was before, but now we have to think of, we've shifted where the object distance is coming from now. So now it's just like a minus three? Minus three, exactly. So you, which is how we end up here. So one over 0.33, you're gonna go to minus three. It's because it's coming from the left. So you just add that to our power cross basically, which gets us the minus one and the plus one. You just add that to the check. Yep, check. So knowing that, knowing that we have a minus one and a plus one at different axes now, where's that gonna put our images? How about for the minus one? So what's an image distance for a minus one? Diopter. So it'll be one meter. Yeah. And do you think that would be a horizontal line or a vertical line? Oh, okay. So I ended up doing the plus one first. But so this one would be, so from this horizontal one, that's the one that just turned out with the first here. It's gonna be a vertical line, yeah, about 107 meters one meter behind, so that'll be a real image. And the other case, we have a horizontal line, 107 meters behind. So let's talk a little about spherical equivalent. There's just a basic formula for that. If you get a prescription, and spherical equivalent comes in handy a lot of times, especially in contact lenses, when you rotate through Dr. Meyer and myself. But the basic formula is sphere plus half the sill. And the sign does matter when we're talking about the cylinder. So in this case, we have a plus sill prescription here for action. So what would our spherical equivalent be in this case? So you have minus two plus 150 minus a half, exactly. So pretty easy, but when you get into the higher sills, you might actually have to pull out a calculator sometimes. So this is kind of one of the examples coming from the book here. So with a couple extra steps mixed in. So first, how would you place this on a power cross? So we have a different axis besides 90 and 180 here. So it's axis 120. So how would we actually draw that power cross? Somebody wanted to shout out, what would be at axis 120? Mm-hmm. And then what's our other axis gonna be? 90 degrees away. 90's four, plus one, exactly. So next question, what is the spherical equivalent? In this case. Yeah, awesome. Is that because there's just a sphere, this first number, plus half of the sill. That's 150. So now how do we convert this to minus sill? Does anybody know off the top of here? So I leave. So you add the minus four and the plus five, and that's your new sphere, so that'd be a plus one. And then the plus five, you just turn that into minus five, and then you just do 90 degrees from the 120s of the 30s. Exactly, perfect. So I have kind of outlined the steps here. But sometimes from us dumb optometrists, you'll get these minus sill prescriptions. Actually, I completely learned in minus sill, so I had to totally shift things when I got here. But even the four options are weird, but. Anyway, so yeah, that's exactly right. So if you get a prescription, somebody comes and they have their prescription from the optometrists. I mean, normally your texts will be kind of converting that for you. But it's important to know, like to be able to convert in your mind. Okay, this is plus five, and I want to do contact lenses. I need to go into minus, so how do I do that? Or I have this minus sill prescription. I need to convert it so that I can know what their prescriptions have been for the last several years to know if I can do the laser comment or the case maybe. So here's the steps. The first number is just the sphere plus the sill. Sign does matter once again. Change the sign of the sill. That's all you have to do with the middle number. And then change the axis by 90 degrees, so exactly. Plus one minus five, axis three. So these are, there's actually another way to write cylinder that is almost never used. Ryan, have you ever even seen this used anywhere? So I don't know, maybe it is. Is it? Yeah. So it's important to at least learn about, but in practice I've never, ever seen it used. So these are all the same prescription. So a minus two at axis 180. And then a plus one at axis 90. So if you did a power cross for this prescription up here, what would be at axis 90 minus two? And what would be at axis 180? Plus one. So the way you would actually draw this in a power cross, it's weird, but you actually put the minus two at axis 90 and then plus one at axis 180. Yeah. I don't know what the practical application of that is, but if it's an OCAPS. Well, it's in the last minute optics book that we used. Is it? It is. Yeah. It's on this lecture. So, is it? So I'm sure it has some practical application. I'm not sure it is, but it's on there for you, so. Okay, so just a little bit about the different types of stigmatism with the rule against the rule and oblique. So with the rule, the vertical meridian is steepest. So in plus sill, that's axis 90 and minus sill, that's axis 180. And this just shows plus sill between, so anything between 60 and 120 degrees would be considered with the rule, with the rule. Against the rule, anything between 30 and 150 would be considered against the rule. And that just means the horizontal meridian with the corneal steepest. And then oblique could just be whatever's left. So between 31 to 59 and 121 to 149 would be oblique. When it comes to contact lenses, I know Dr. Maro would talk a lot about contact lenses. Most brands tend to get a little skimpy on the oblique meridians just because they're not as common. But you can, between, on these more with the rule and against the rules, you can pretty much find every 10 degrees in those ones with almost every brand. It does a stigmatism. Excuse me. So here's the different classifications for, if you wanted to be really specific about what kind of refractive error a patient had, you'd give them one of these classifications. Simple mild, simple hyperal, and then on down the line. So the same, yes. Yeah, that's why it matters. Maybe you're gonna talk about why it matters. It probably isn't used that often, but it can be helpful so that you, if I told you that there was a, this patient had simple myopic astigmatism, you'd be able to know at least what one of the meridians of that prescription is without knowing anything else about the patient. If you know that they're a simple myo, you'd know that they had no astigmatism, but that they were just myopic. So, I mean, numbers-wise, it doesn't help too much because you still don't know what their actual prescription is, but I honestly, when ICD-10 was coming out, I thought that we'd have to be this specific, and I'm glad that we don't. But you can, by seeing one of these names, you can know exactly where the astigmatism is and where it's doing, at least on which side of the retina the images are. So, if I said this patient is a simple myopic, where would, what does that mean, anybody? Yeah, and is there one image, is there two images? Yeah, exactly. So, there's no cylinder in that prescription, they're like a minus three myo, so it's gonna be, the image is focused in front of the retina. Simple hyperopathy of plus three behind the retina. What about simple myopic or simple hyperopy of astigmatism? I'll show pictures of these in just a second. The horizontal line images are both, like, you know, in myopic, they're both in front of the retina. Actually, so that's compound. So, simple, so it's important to know, so you can just know her. So, simple would be that, so simple myopic astigmatism means that one of the meridians is actually on the retina. So, one of those would be like a plano minus three. Simple hyperop, it means one of the meridians is on the retina. Compound means they're both one direction. So, both images are in front of her behind the retina. And then, mixed astigmatism is where you have one in front of the retina, one behind. So, we'll do some picture representations of that. But that's one reason why it's helpful. It's like, if I know this person has mixed astigmatism, that means that, you know, one of the images is in front of her behind the retina. I guess it's just good to know. So, this top one here, how would we categorize this? We have a single image behind the retina. Simple hyperop. What's that? Simple hyperop. What about this one here? We have one image on the retina, one image in front of her. And is it simple? Okay, simple. Exactly, so simple. One of the images is on there, and so it's simple. It kind of throws me off that they use the word simple here. I kind of wish they wouldn't have, but yeah. So, this is simple myopic astigmatism. Now, one more thing to add to the mix here. So, just to review quickly. So, with the rule, the vertical meridian of the cornea is steepest. So, think about what kind of image that would produce. And looking again once at this simple myopic astigmatism. So, we end up having a vertical line in front of the retina, horizontal line on the retina. So, is this with the rule or against the rule? So, what kind of? The against the rule. Yeah. So, why would it be against the rule? Also, the cylinder is, so, is there any horizontal line? Yeah, so, if the horizontal part of the cornea is steeper, that means the image that's in front of the retina is going to be vertical. So, the h in the v once again, just remember, that's just telling you where the line is coming from on the cornea. Yeah, in this case, it's against the rule. Ah, crap. So, what's this one, guys? So, we have both images behind the retina. So, I already gave it a compound-hyperopic astigmatism. And is this one against the rule or with the rule? The image in front. So, the first image comes from the vertical, so that means the vertical part of the cornea is steeper. I mean, it's with the rule. And then this last one. So, it makes you have one image in front of and behind the retina. And really, in this case, you know how I talked about how the lines come to a point focus? So, it's kind of right in the, the retina falls kind of right in between these two images. So, they, this person who corrected would likely actually still be seen pretty well because the point focus would be coming right on it. Or even uncorrected. They'd still be seen at least decently well. Okay. This is something I'm not gonna spend too much time on, but basically it just says if you, you can induce astigmatism, like in a patient's glasses by tilting their lenses. We kind of do this on purpose. But not to induce astigmatism, but there, most patients' pair of glasses is actually tilted a little bit. But if you tilted a plus lens, well, so if you tilted a plus lens along the horizontal axis, so it'll be axis 180 and you'll actually induce plus cylinder if it's a plus lens or minus cylinder, if it's a minus lens. So not too important, except that if a patient's glasses are too tilted and they just have a spherical prescription and they're saying things are blurry, it might just be that there's this induced astigmatism from the correct differences. You don't have to like, go into like detail with all the formulas. There are certain formulas that we had to do, test some of that stuff, but that's a pump and pass shape. Okay, so combining cylinders. So what if you have two cylindrical lenses or like two lenses as you take out of the drawer and you need to actually combine them to produce a certain type of correction? Probably not gonna do that too often, but if we did have combined cylinders here, how would we do it? So first we have minus one axis 90 and plus one axis 180. So in this case, since it's axis 180, the plus one, it's just like this is saying plano plus one axis 180. So plano would go along the 180 and the plus one ends up at 90 at degree axis. The same over here. So it's minus one axis 90. So the minus one will actually go along the horizontal. So we've combined these two cylinders together. We pulled these two cylinders, a plus one and a minus one sill lens, put them 90 degrees apart. So the question is, is the entire power of this lens system now? Like how do we know what the power of that lens system is? If we point to do it at degree axis, say 45 degrees, what is the power gonna be at that point? So any guesses on what the power would be at? Axis 45. What do you think? Yeah, it is actually. So you basically just add these two together if it's 45 degrees apart from where the two acts, et cetera. So because it's halfway between a plus one and a minus one, adding those together, we're gonna put plano. So what if it's say along 30 degrees or 60 degrees? Any ideas what it might be? So it's a little bit closer to the minus one. Does anybody have an idea of what the power might be? At 30 degrees or how we would even calculate that? So it's a third of the way between plano and minus one. It's kind of a way to think of it. So we're gonna end up with about minus 0.37. Same with up here at axis 60. It's a third of the way between plano plus one. It's actually a plus 0.37. So it's a little confusing, but it's more, I think it works better just to think of it in a picture form than to try to think too deeply into it. It's just, it's a third of the way between plano and minus one. A third of that is minus 0.27. So here we have two different cylinder lenses. So plano plus one, axis 30. And plano plus one, axis 150. So we're wondering what is, if we needed to combine these and we wanted to know, we wanted like an axis 180 and an axis 90 here. So in this case, with this lens by itself, along the horizontal, it's about two thirds away from the plus one, if that makes sense. When we have an axis 30. So it's like 30 degrees away from plus one. Or sorry, 60 degrees away from plus one. 30 degrees away from plano. So we're gonna end up with a plus 0.33 in this one. Along the vertical meridian, what would it be then? Any ideas? So it's like, it's basically just one minus the 0.33. So about 0.66. Now we're on this one. We just tilted the axis a little bit. But we'll end up with basically the same results here. So plus 0.33 at 180, plus 0.66. So now we wanna know, in this lens system, how much, if we combine these lenses, how much total power would we have at axis 180 and how much total power would we have at axis 90? So, any ideas on how we would do that? Just add them, exactly, you just add them together. That's exactly right. So we're gonna end up with plus 0.67 at 180 and 1.32 at axis, or at 90. So we've combined those two cylinders and we wanna know how would we actually write that prescription in plus cell form. So, I'm actually hoping one of you guys can do this. But one important thing to note here is that we're saying it's plus 0.67 at 180, which is different than axis. 0.67 on the horizontal. Goes on the horizontal. So maybe try to do, try to put this on a power cross real quick and then give me a plus cell RX for this. And it's not a nice round number, I apologize for that, but if anybody has an answer, you can just throw it out there. So you put it on a power cross, this is what you end up with in plus cell form, you always put the least plus number first, 0.67. And then you take the difference between those two, which is about 0.67. And then since the lower number is first, that's what our axis is, really good job. Okay, so the reason that we've been talking about these power crosses and things is that retinoscopy is a big part of that. So here's like a potential story problem you might get or you might actually find in a clinic. So you're retinoscoping an AFAKA down syndrome patient, the streak oriented horizontally, you get a neutral reflex at plus 22, or the streak oriented vertically, you get a neutral reflex for the plus 27. So first, just if you just take those two powers that I gave you in lower working distance and all that, how would you do a power cross for that? What number would be on the horizontal? What number would be on the vertical? Oh yeah, maybe the power cross is adding too much confusion. Super helpful for me though sometimes. So just remember, the streak is oriented horizontally. So what are you actually, you're moving that streak up and down to see what this axis is here. So our plus 22 is actually gonna be along the vertical because you're moving the streak up and down. Our plus 27 is gonna be along the horizontal. So now a working distance of 50 centimeters. So how much do we actually need to, what do we need to do to the prescription because of our working distance? You have one over 0.5 is gonna be two. So we just take minus two from both of those. So that's gonna give us a plus 20 and a plus 25. So what is our prescription now? So I think this is like an awesome potential problem you can get and you'll run into this all the time in Pete's clinic and if you need to ret somebody in clinic which we do all the time in contact. So now we'll kinda burn through a lot of stuff pretty quick. So spherical aberrations, this just means that the farther you get away from the center of the lens, it actually, the lens actually bends the light rays a little bit more. So instead of getting a nice crisp image in a plus lens, it actually bends the light a little bit more out towards the edge and it's actually focused a little bit more anteriorly. So why does this matter? So image quality can decrease with a larger pupil as basically what it comes down to. And this has, this can be a big issue with like night my OPU. I don't know if you have patients who complain that their vision gets worse at night and certainly with cataracts and things like that. But when you have a bigger pupil, the light can, it just makes a bigger difference because some of the light is now being able to be, well it will be focused more anteriorly to the retina and cause just a bigger blur circle for the patient. So the variable in all this is just the pupil size. If you have a smaller pupil, it won't matter as much if there's spherical aberrations in the answer. Yeah, yeah, it can. So sometimes, so actually this happens like all the time that somebody will have like a low prescription, like I mean it's like minus 50 plus a quarter or something like that, they get along just fine but their complaint is my vision at night when I'm driving sucks. I get so much glare and everything. So that's a prime opportunity to give them that prescription just for like a pair of night driving glasses because that's gonna help that light be focused in with the bigger pupil. And this also comes into play with refractive surgery. So because normally the cornea is flatter in the periphery that the body actually handles this by making the cornea flatter in the periphery. So it's not as noticeable. But then we go ahead and flatten out the middle of the cornea and it kind of takes away some of that. So that's why pupil size is such a big deal when it comes to refractive surgery and making sure that they take that into account. Because otherwise, if they have a larger pupil at the end of the day at night, they're gonna get a whole lot of glare and halos. And it just has to do with where that light's being refracted to. And just to show how much of an effect it has doubling the pupil size increases the spherical aberrations by about 16 times, it's fourth power. And there's also chromatic aberrations that just means that light based on wavelength is bent either faster or slower. So the difference between like blue light hitting the eye and red light is actually about a half diopter which is quite a bit. This is just a representation of where light, different colored lights might actually, the same power hitting the eye might actually be end up being refracted. And we can use this in a couple different tests but just by showing that by using different lenses we can shift where the colors are actually visible on the eye. Okay, so another important concept I'm gonna get to is just this near point, far point. So it's kind of, it's a pretty simple concept when we throw in accommodation that's where it kind of can get confusing. So the far point is basically, what's the farthest point away from their eye that they can see? For a myo, is that image gonna be in front of the eye or behind the eye? Their far point, do you think? Uncorrected we're talking about here. So if they're myopic, the light obviously is showing up in front of their retina, but is there, at what point can myopes see a clear image? Yeah, it's up close, right? But it's still in front of their eye. Hyperopes on the other hand, if they have zero accommodation, they're kind of screwed because it's behind their eye so they can never see clearly if they have no accommodation. So we'll just, with a couple diagrams, we'll just demonstrate some of these. So the far point is the farthest point away from the eye that they can see clearly without correction. The near point is when they accommodate what's the closest that they can see. So the farthest away the eye can see clearly with accommodation entirely at rest is the far point. Or an object point imaged by the eye onto the retina in an unaccommodated eye. So why is this important? So if we know that they're a minus three mile and so their far point is 33 centimeters away from the eye, we wanna give them a pair of glasses that's gonna put the image right where their far point is. So minus three lens will put the image at 33 centimeters and make it seem like they're looking at optical infinity even though their eye's still looking at this point but the lens has put the image there for them and so they can actually see clearly far away. So we wanna put the correcting lens, have it image be at the eye's far point. That's what we wanna do. So in myopia, here's just the little demonstration here. So light coming from infinity in an uncorrected state ends up being in front of the retina. So the far point is maybe just a little bit in front. We put a lens there that has its secondary focal point right at where the patient's far point is and it puts that image right on there. So the accommodative amplitude is basically how much the patient is able to accommodate. We know that at age 40 it starts to decrease, really it's been decreasing a little bit throughout almost their whole lives but at age 40 is when it really starts to present itself. So in an uncorrected myopia, their far point will be in front of their near point but they're still pretty limited range depending on what their prescription is of clear vision. So the image that they can see clearly on the retina is the far point. With accommodation they can actually focus in to see where this near point is and then this distance here is their accommodative amplitude and the distance you can convert it into diopters. But they're both in front of the eye. So here's an uncorrected hyperrope. So their far point is actually behind the eye which sucks because that'll be really blurry. If they can't accommodate to get a near point then they can never see clearly. But their near point through accommodation can still be in front of their eye. Basically just shifts the image. But if they can't accommodate enough then both the far point and the near point might actually be behind the eye. But in a young patient, typically they can still see clearly at distance by accommodation. So this green line here is just showing so the image that's actually showing up on the retina is coming from a point source. Say 33 centimeters away. But then they can accommodate a little bit closer. So here's a little story problem here. So Paul's far point is 12.5 centimeters behind his cornea. So that's gonna be a hyperropic prescription. How much is it? So we just do one over point 125. It's a plus eight, a hyperrope. As a culminate of amplitude is five diopters. So what would his near point be? So with a culminate of amplitude it literally is just like additional subtraction. So he's a plus eight hyperrope. He can accommodate five diopters. So eight minus five. He'd still have three diopters of remaining hyperrope. Which is bad news, because that means that his near point is actually behind the retina as well. And it's still a plus three. So there's like no area of clear vision for this patient in an uncorrected state. Everything's blurry. So the near point's 33 centimeters together. Okay, so I'll let you just do this one real quick. And then we're almost done here. So Larry is an uncorrected mile with a far point of 40 centimeters. What is this refractory? Does anybody have it? So suppose he looks at a book 25 centimeters in front of his eyes. How much accommodate accommodation would he need to see it clearly? This gets a little bit tricky. So how do we even set this up? So the amount of the power that he needs is a minus four because he's trying to look at something 25 centimeters late. Exactly. So if his natural refractive area is a minus 2.5 if he needs to get the minus four, how much accommodation would he require? Exactly. Yeah? So when you're accommodating, you're adding minus power. It's bringing it closer. So it's always gonna be a minus number unless you're relaxing accommodation. Exactly, you did exactly right. So we're gonna do one over four. That's four diopters required. He already has 2.5, so four minus 2.5 is one and a half diopters. So what if it's 10 centimeters in front of his eyes? What would just be like the required? Yeah, so seven and a half. Hopefully he has that much of an incident that close. So his refractive area is minus 2.5. That's just another way of asking a question. So if the maximum amount he could accommodate, the amplitude of accommodation is nine diopters, where would his near point be? So in diopters, before we even figure out the distance, what's the total power that he could produce with his eyes? So it's a minus 250. So no, it's actually, I mean, it should be, it's 2.5, it's plus 2.5 diopters in my eyes, so it's still a minus 2.5. So 11 and a half diopters. That's gonna be about 8.7 centimeters. So that's a close, so he's gonna be between 40 centimeters and 8.7 centimeters. That's the only area that he has clarification when he's having corrective. Everything else is correct. So 48 year olds, so now we're dealing with presbyopia here. So Marina's 48, four diopter, my oh. She's wearing bifocals of the plus two act, and she has one diopter of remaining accommodations. So what is the far point of her natural eye? So plus four. Where's that gonna put the far point? And is it in front of the eye behind that? So it should be like a minus 0.25 to the left here. Oh crap, and then what's the near point without her glasses? Five diopters, so one over five. So her clear vision range is from 0.25 to 0.2 meters. It's like five centimeters of clear vision when she's uncorrected. What is the range of vision with her glasses on them? So she goes from five centimeters to, so with her glasses on them, so she's fully corrected now. So what's her far point now? Infinity. Yeah, when she's corrected, she can actually see all the way to infinity, theoretically, without cataracts and all that. And then she, so she has a plus two add now, plus an additional diopter of accommodation. So you have about three diopters total that she can get up close. So three diopters in front of the eye. So what distance is that? Exactly. So she basically now with her glasses on, she goes from five centimeters to infinity all the way to 33 centimeters. And that just kind of shows how we got there, but. So big difference by that patient being corrected. So this just shows like different types of bifocals. Sorry, so I'm not patient though, her near point is not as near as the bifocals. So she could still take her glasses off. And oftentimes they do, like those, my other patients would just take their glasses off to read if they're within that range of vision. But with her amount of accommodation and her bifocals, she could get about, yeah, so you like the minus eight or something? Yeah. Yeah. Exactly. So with bifocals, I just wanted to show this because part of the lecture in the next one minute is about prison. With bifocals, it's important because if they have a line, because there's different prescriptions here, there's a little bit of image jump that happens. And that's important for patients when they're first learning bifocals. When you're prescribing their first pair of bifocals and they're like, I don't wanna pay for a P.A.L. I'm just gonna get the line. It can be good to educate them that, hey, be careful around steps and curves because when the bottom part of your lens, there's a little bit of an image jump there. So just when you're walking on the street, look down to go up your curves. And you can calculate this formulas to calculate exactly how much image jump there is. That's not really important for this lecture. But by going from one prescription to immediately jumping into the next. And it can vary depending on if there's a plus lens or minus lens as well, as far as the magnitude. So I'm gonna skip through. This just shows like round top. So there are a couple other things if we had time that's good to talk about, but. So hopefully that helps a little bit to know how to convert from plus sill to minus sill, how to work with accommodation to figure out what are these people actually seeing clearly with that glasses. And most of them are just gonna rescheduling glasses anyway, but if somebody calls and they've lost their glasses, it can, wow, you know, they're a plus six hyper open. They're 60, they can't see clearly anywhere right now. So, and so I know we're, it's about 757 right now, so we gotta go up to clinic, but any questions about any of this? There's a lot of stuff to talk about optics related, I'm certainly most excited about. If you have any questions about it, email me or seriously, I really have to help you. So there's a lot of that other stuff that's really fun and doesn't get used that much in clinic, but it's good for you. Thank you. No problem.