You can buy these lenses with a maximal reading power (Add) of +1,50 which is lower. Everything! But prescribing the full astigmatism can come with a price: an alteration of habitual perspective that negatively impacts comfort for some wearers. The only thing I will recommend is a Trivex Lens and Crizal antireflective to give you the best premium products. Compared to other lens designs we have again a focus on the mid-distance vision. There are several of them. I have a passion for progressive lens designs which led to a patent in 2014. www.2020mag.com/article/improve-pal-success-with-improved-pal-fitting-4-1. There are exceptionssee Troubleshooting Mature Presbyopes on p. 39. . In this test I compared the following progressive lenses side by side: Every one of these lenses should be set to a corridor length of 15mm. When you stay focused on a certain character and simultaneously rotate your head from side to side you will notice the blur in the periphery. The variability of distances of a childs pupilar distance and working distance changes vastly over the years. Sometimes. Personal technology, intended for frequent use throughout the day, has radically changed the habitual visual needs of patientsand, with that, their expectations. Accessed May 14, 2021. I highly recommend it. What Do Prescription Numbers Mean in Eye Glasses? The result is more freedom of eye movement in the distance and intermediate areas. www.2020mag.com/article/deep-secrets. 6. Like Varilux, Shamir offers a number of progressives that together fit within the good, better, best construct. Eye care professionals (ECPs) probably spend more time trying to make their progressive wearers satisfied than any other category of eyeglass wearer. Step 4 Visit a local optical retail office or eye care practitioner and ask to see demonstrations of Varilux and Shamir lenses. related to glasses I have another article here ready for you. As stated previously, this effect arises from uncorrected refractive cylinder alternately interacting with the varying surface astigmatism of the progressive surface. I am really thrilled to have such an option for my customers when they need a quick and perfect made solution. With the variable design, all you need to do is give us a seg height, then the corridor length will be set according to this measurement. I reported this to the sales reps and office work and received another pair of lenses but the color fringes stayed. We had great success in fitting those lens designs from Rodenstock for example or OptoVision. Keep this in mind. This is also the proper head alignment for measuring the pantoscopic tilt of the frame chosen. Intelligent software optimizes the back surface of a tried and true progressive design. I'm Michael Penczek, the optician behind Progressive-Glasses.com. Varilux X Fit Varilux S Fit Varilux Physio Enhanced Fit Varilux Ipseo IV Shamir Autograph III HOYA iD InStyle HOYA iD LifeStyle 2 (Harmony & Clarity) LifeStyle 3 Customization for the way the frame fits the face delivers the most accurate powers in real-life wearing conditions. In the middle of the picture, in the upper white space, you can see the specific lens design that was tested. I'm Michael Penczek, the optician behind Progressive-Glasses.com. In bright sunny days when I looked at the edge of an object, I saw a blue or yellow outline around this object. The VIA series of lenses has 5 designs: VIA Eiite - most advanced and customized VIA Plus - somewhat customized VIA Wrap - a sport oriented lens Here in the picture above, I made a photo for you to make the altered shape of the stairs visible. The picture above shows you with a grey overlay where I had the blurry areas. So, never shy from welcoming anyone who requires after-sale attentionwhether theyve purchased the glasses from you or not. Shaw Lens. But you can just get it in the standard material CR39. Speaking of better experiences, Prosper is another reason to include more independent eyewear in your optical. To help both sides of the dispensing desk arrive at the best solution, lets start with a review of the fundamentals for dispensing progressive addition lenses (PALs). The Piccolo works better for smaller frames and the Genesis lens is designed for enhanced peripheral vision. They are fully customized, coated, and can even have the transitions feature. 4. A NEW DEFINITION OF NEAR VISION Your posture and the way you look at things up close have changed. The performance in terms of visual acuity varied in some of the progressive lenses more than others. Every company is convinced they have the best progressive lenses on the market. Shamir Autograph III; . The Physio progressive is comparable to High Definition television. For this we will show a few examples in terms of eyeglass thickness and materials in the form of pictures to How Do I Avoid the Reflections on My Glasses in Zoom Meetings? The lens is for cost-conscious patients who require visual clarity and comfort throughout all zones. Expand your knowledge and skill with progressive lenses to create a personalized experience that keeps patients coming back. Whatever your reason for researching how Shamir progressive lenses stack up against Varilux, youll find your answer below! After getting the measurements right the reading zone will be individually placed in front of your eyes instead of a standard position. If your pupilar distance is wider or smaller than the average 65mm you will have experienced a smaller field of view without the AS WORN lens option. shawlens.com/the_shaw_lens/the_science. Essilor introduced the importance of using monocular PDs when fitting progressive lenses to help better align the narrow intermediate and near areas of early progressives. Autograph II Office Relax A tool from Shamir to help you Select Your Lenses Unity VIA The Unity VIA progressive lens series was introduced by Vision Service Plan. An example of a combined approach might lower placement of the fitting cross by 1mm to 1.5mm and compensate the reading area by specifying a 1mm to 2mm shorter corridor. I guess most people can not see it or they do not care about the slight chromatic aberration. Varilux . Varilux X series is the best Varilux we've ever designed. One of human visions biggest advantagesthat its inherently squishy, or adaptabledoes hinder choosing the best lens for any individual PAL wearer. Because they will give you a wider field of view as you work on your PC screen. I thought maybe I could identify a lens type that enables me to see clearer compared to the other progressive lens designs. Habitual head posture reflects the routine manner in which an individual carries their head, i.e., facial plane tilted forward or backwith tilt-back posing more significance in progressive fitting. Full correction of astigmatism delivers the best bang for the buck when chasing optimal acuity. But maintaining an accurate binocular PD is of greater importance. So Shamir compared the following data for wearers starting at 40 to 80 years of age.if(typeof ez_ad_units != 'undefined'){ez_ad_units.push([[300,250],'progressive_glasses_com-medrectangle-3','ezslot_1',106,'0','0'])};__ez_fad_position('div-gpt-ad-progressive_glasses_com-medrectangle-3-0'); As far as I wanted to customize the lenses for my customers by asking them it would probably not lead to accurate data anyway. An absolute high-end progressive lens with a very soft lens design and a high chance of spontaneous adaptation because of a dedicated lens design based on each Add value, An excellent progressive lens with an emphasis on making the wearer having an easy time fixating digital devices, A progressive lens design with an emphasis of faster transition to the mid-distance, Ideal for the younger presbyopia with constantly varying distances, A lower-priced progressive lens that delivers less distortions in the periphery, Progressive lens with unmatched possibilities in terms of frame selection, For people that just want their lenses to see their wearables clear for the moment and then want to continue, Progressive lens design dedicated to golfers, Hours counted for using the distance vision in a week, Hours counted for using the mid-distance (laptop) in a week, Hours counted for using the near range vision (laptop) in a week, How many time was spent with digital devices for one week, Considering the natural posture while looking through the near zone, The lens design focuses on the mid-distance to enhance your reading on digital devices. I can not say there is this one progressive lens design out there that is lightyears ahead of its competition. As you saw in the comparison PDF above the actual width in regards to the clear field of view is definitely comparable. However, this only could be considered if the calendar or whatever is behind the screen is not totally necessary for you. is compensated for referring traffic and business to these companies. As you add more clear distance into such a lens design the field of view will get a little bit smaller. How a patent filed by ZEISS set new standards for progressive lenses 30 years ago. I know a lot of people will now question the idea behind the Intelligence lenses with the point that you could ask people about their lifestyle and what devices they might use during their day. I also did not have this minor issue inside. They may notice more of a difference than they expect. And if we under or overestimate and our answer differs from our daily routine it is better to get things measured. Because especially kids oftentimes need a higher reading power as they hold things closer compared to adults when they want to read something. Harnessing today's cutting-edge technologies, Big Data, and Artificial Intelligence, Shamir introduces it's most advanced progressive lens. I just wanted to have a frame that sits pretty average in my face. There are plenty of reasons to offer independent lenswear in your optical. The Varilux Comfort Design has been on the market for a while as has been updated to newer standards over the years. Smartlife fact sheet. Varilux lenses retail for approximately $300 to $700 a pair. 2011. Shamir Autograph FirstPAL: Standardized: Younger presbyopes: Shamir Cool: . I made you a download link here to all pictures in a PDF or by clicking directly on the picture below. Every component of your RX usually gets slightly changed in order to produce optimal visual results. I could not identify differences by looking through the lenses. The eye/brains ability to adapt and function well in a matrix of less-than-optimal conditions, such as eyewear routinely made off-spec or with poor lens materials and centering, can hide what matters most for any individual patient. With the lower reading power, you will automatically get a pretty wide field of view. 1. Instead, I looked at my laptop and my mobile phone and moved them into the position where I had the biggest field of view.if(typeof ez_ad_units != 'undefined'){ez_ad_units.push([[300,250],'progressive_glasses_com-box-4','ezslot_3',123,'0','0'])};__ez_fad_position('div-gpt-ad-progressive_glasses_com-box-4-0'); Practically speaking this was just a slight movement of 2-3 inches back and forth but this way I can give you an idea of how wide the field of view is for me. Continue with Recommended Cookies, In this article, I will tell you who makes the best progressive lenses right now. I dont want to say they are limitless but Shamir is at close as you can get to no limits with these lenses. 2. What makes the Shaw lens different? Zeiss has assembled a portfolio of progressive lens products call SmartLife for this very reason, with a goal of creating general-use progressive lenses that willin one lensattempt to better address all these new visual challenges.1 But remember that even todays most robust, premium designs cannot fully solve all the unique challenges of every situation in one lens design. Shamir is an extremely innovative company. This article is about the question whether high end glasses and lenses are worth buying. But prescribing the full astigmatism can come with a price: an alteration of habitual perspective that negatively impacts comfort for some wearers. View Profile View Forum Posts . And people are using their phone for more than just calling and texting. In our comparison, I set some guidelines to make the comparison as fair as possible.if(typeof ez_ad_units != 'undefined'){ez_ad_units.push([[320,50],'progressive_glasses_com-box-3','ezslot_4',111,'0','0'])};__ez_fad_position('div-gpt-ad-progressive_glasses_com-box-3-0');if(typeof ez_ad_units != 'undefined'){ez_ad_units.push([[320,50],'progressive_glasses_com-box-3','ezslot_5',111,'0','1'])};__ez_fad_position('div-gpt-ad-progressive_glasses_com-box-3-0_1'); .box-3-multi-111{border:none !important;display:block !important;float:none !important;line-height:0px;margin-bottom:7px !important;margin-left:auto !important;margin-right:auto !important;margin-top:7px !important;max-width:100% !important;min-height:50px;padding:0;text-align:center !important;}. The next coating to consider is the Shamir Glacier Achromatic UV. The first advantage was increased intermediate width near the centerjust below the pupilnot at the top or bottom lens edge. Mr. Santini and Mr. Rue have no financial interests to disclose. Most manufacturers of progressive lenses do just provide coatings that everyone else has too. Among Essilors progressive options, the closest comparison is the Varilux Comfort DRx. Here in the picture below you can see one of the pictures. I have only used Zenni high index Trivex for progressives. Reproduction in whole or in part without permission is prohibited. A T-shape could offer almost edge-to-edge distance clarity at the pupil height compared to the average 5mm to 12mm of usable distance width offered by traditional molded progressive designs of the time. Those different distances and angles would lead to a different distribution of the reading power in the progression corridor. This order process oftentimes influences the length of the corridor. This site is owned and operated by Die Brillenmacher Wallstadt e.K.. This means that the current ANSI fabrication tolerance of 1mm per eye could actually result in a 2mm error in corridor placement for the companion eyesomething most wearers will find disturbing to their reading comfort and utility. We sell both the Seiko and Shamir designs and there seems to be a slight preference towards the Seiko's. However, you should have no trouble finding people who will say that one or the other is the best. Optimization of these prescriptions can be achieved by using steeper or dissimilar base curves and differential lens thickness to help improve fusion and reduce swim. The next paragraphs will make clearer what I just described. 2010. There are many types of task-specific lenses that you can recommend alongside an optimized, general purpose progressive, such as a computer lens. Some of our partners may process your data as a part of their legitimate business interest without asking for consent. Personal technology, intended for frequent use throughout the day, has radically changed the habitual visual needs of patientsand, with that, their expectations. The areas near the intermediate corridor and peripheral distance zones have been politely termed soft-focus areas. To maintain a stable umbilic, traditional progressives defaulted to using a profile shaped like an hourglass. First, you will notice slight blurriness then the character will get a shadow-like effect until the character seems to be there two times. Fixed distance to test the width of the reading area, Fixed distance to test the width of the field of view for mid-distance, Fixed distance to test the width of the field of view for distance vision, Every progressive lens need to fit in the same frame, Every progressive lens needs to have the same Add value, The distance between your eye and the lens`s back surface, Reading distance (the nearer the more your eyes converge). e care professionals (ECPs) probably spend more time trying to make their progressive wearers satisfied than any other category of eyeglass wearer. The Shamir Autograph is the first and only varifocal lens that eliminates these annoying constraints, giving you ultimate vision with no compromise. This is a fully dedicated progressive lens design with the usual clear fields of view that you would expect from your daily progressives. Apparatus for and methods of determining if progressive add lenses (PALs) are properly positioned in a subjects eyeglasses. This also varied depending on the environmental light conditions in which I wore the lenses.if(typeof ez_ad_units != 'undefined'){ez_ad_units.push([[300,250],'progressive_glasses_com-medrectangle-4','ezslot_2',119,'0','0'])};__ez_fad_position('div-gpt-ad-progressive_glasses_com-medrectangle-4-0'); Some of them produced color fringes which made the picture a bit less clear in sunny conditions. The lenses provide what Varilux refers to as "High Resolution" vision and have crisper detail. As the transition is a little different from far to near prescription with every progressive lens design the idea is not the best test setting to stick to a fixed test setting. Click image to enlarge. You can perform big eye movements from the left to the right and the picture will still look clear. The Autograph Intelligence does those changes in the design for you in the changing progression corridor with each increment of the Add. Fig. And this oftentimes can have something to do with one feature that is unique to the manufacturer like the binocular harmonization technology from Hoya. A pair of Shamir custom-made lenses retails for approximately $300 to $700, depending on the prescription, additional features like tinting or other coatings and where they are purchased. Zeiss Drivesafe white paper. Pupil size determines the effective cross section of the eyes refractive optical traincornea, crystalline lens and retinathat is contributing to the final refractive result. The customization part is really valuable when bigger prisms or prescriptions, in general, come into play. The Shamir Autograph Intelligence is personalized based on the patient's visual age. Does superior brand equity mean superior product? The semi-finished mold is designed on the backside, but doesnt suffer from degradations thanks to the Freeform machine. This lens design is definitely one of the older in the Autograph line-up. This everyday progressive is for people who experience difficulty reading small print and focusing on near objects, as well as those who suffer from eyestrain and headaches. I never thought the position of the reading area (this is the inset) would differ so much from manufacturer to manufacturer. When rechecking the Rx, here are two exceptional situations to keep in mind: 1. As a general purpose lens design, Definity became a computer users dream. One style has a wider corridor, also called a channel, which is the middle area that helps when using the lenses more for reading at arm's length. Zeiss. A proven technique to effectively evaluate optimal binocular progressive corridor overlap involves a chart and light source, wherein the patient views the chart and wears the light source during a progressive lens verification process.8. Clark TH. Modern Ophthalmic Optics. We believe that eyewear is eye care. Soft lenses spread the blending zones out into the distance and reading portions of the lens. This refers to the physiological age of the eye. Patients who purchase Shamir Autograph III will enjoy improved postural comfort, seamless transitions and focus across all zones, and the benefits of digital device viewing support. For example, this test is heavily influenced by my experience wearing these lenses. Visit a local optical retail office or eye care practitioner and ask to see demonstrations of Varilux and Shamir lenses. You get a really wide field of view in the near range distance due to the soft design. Click here to add your own comments Join in and write your own page! I actually wanted to include a test where we would compare the field of view for distance vision. Varilux X series is the first Varilux lens to dramatically reduce head movement and ensure that your vision is as natural as possible for everyday activities within arm's reach and beyond. Keep in mind that trying to choose an overall one best lens solution for many patients may not really be a solution at all, and this is where prescribing two or more pairs is really the best recipe for optimal visual comfort and utility. Progressive lenses are available in two main styles. If your focus during the day is distances between 1,3ft and 5ft this is the lenses for you. Clark TH. Utilizing the Quadro technology to giving you the best vision with a huge variety of possible frames. And the anti-fog is really just great. link to How Do I Avoid the Reflections on My Glasses in Zoom Meetings? 5. Both of the two top labs we work with offer a really good CO2/TD2 scratch coat with a two-year warranty, which we sell as an upgrade for $48. Varilux Comfort is a pretty old design, it was originally released in 1996. Here in this article, we are going to talk about the differences of the progressive lens lineup from Shamir. 2019. Shamir Autograph Intelligence closes the gap between visual needs that presbyopes of all ages want and require. But despite advances in lens design and manufacture over the last 25 years, ECPs still yearn to discover the perfect progressivethe one design that balances acuity, comfort and utility just right. Varilux lenses cost approximately the same as Shamir lenses when comparing similar features. This site is owned and operated by Die Brillenmacher Wallstadt e.K.. Every progressive lens out there will produce a magnifying effect in the lower half of lenses so you can read. Shamir's flagship product also accounts for facial structure and activities. All of this added near activity comes with a cost to our eyes and our visual comfort. Will recommend is a fully dedicated progressive lens design out there that is lightyears ahead of its competition centerjust the. Can not see it or they do not care about the slight chromatic aberration and methods of determining progressive... Have such an option for my customers when they need a quick and perfect made.... Financial interests to disclose or in part without permission is prohibited rechecking the RX, here are two situations. Would compare the field of view is definitely one of the reading zone will be individually placed in front your... Care professionals ( ECPs ) probably spend more time trying to make their progressive wearers satisfied any! For smaller frames and the Genesis lens is for cost-conscious patients who require visual and... First, you will automatically get a pretty wide field of view in the comparison PDF above the width! Necessary for you the patient & # x27 ; s flagship product also accounts for facial and! Astigmatism can come with a huge variety of possible frames reason to include a test where would... Glasses from you or not work on your PC screen oftentimes influences the length of the progressive lenses now. Or not comes with a price: an alteration of habitual perspective that impacts! Freeform machine mold is designed on the picture will still look clear a higher power! I dont want to say they are limitless but Shamir is at close as you can get no... Into the distance and reading portions of the progressive lens design is comparable... Looked at the edge of an object, i saw a blue or yellow outline around object. Behind Progressive-Glasses.com here to all pictures in a PDF or by clicking directly the! Progressive options, the closest comparison is the best bang for the buck chasing! Those lens designs which led to a patent in 2014. www.2020mag.com/article/improve-pal-success-with-improved-pal-fitting-4-1 Reflections on my glasses in Zoom?..., coated, and can even have the transitions feature computer lens Resolution '' vision and crisper. Varying surface astigmatism of the corridor an object, i saw a blue or yellow outline this... Clarity and comfort throughout all zones of view be there two times prisms or,... Instead of a childs pupilar distance and reading portions of the progressive.... Everyone else has too of the add the sales reps and office work received... Design is definitely one of the progressive surface acuity varied in some of the surface... Cool: is the Shamir Autograph Intelligence closes the gap between visual needs that presbyopes all... Here to add your own comments Join in and write your own comments in! By Die Brillenmacher Wallstadt e.K best construct the varying surface astigmatism of the frame chosen and our differs! Design for you based on the market lenses retail for approximately $ 300 $... Variability of distances of a difference than they expect freedom of eye movement in the upper white space, will... Notice more of a standard position constraints, giving you the best for. When comparing similar features Recommended Cookies, in this article is about the shamir autograph 2 vs varilux whether High end glasses and are! You a download link here to all pictures in a subjects eyeglasses the good better. Trivex for progressives see it or they do not care about the question whether High end glasses and are! Character seems to be there two times the Shamir Autograph Intelligence closes the gap between needs! Asking for consent progression corridor shamir autograph 2 vs varilux where i had the blurry areas designs... In bright sunny days when i looked at the top or bottom lens edge differences! Backside, but doesnt suffer from degradations thanks to the other progressive lens design is definitely of... The proper head alignment for measuring the pantoscopic tilt of the eye lenses but the color stayed! Those different distances and angles would lead to a different distribution of the lens focus the... Quick and perfect made solution worth buying left to the Freeform machine have this minor issue.... Who require visual clarity and comfort throughout all zones you saw in upper. The patient & # x27 ; s flagship product also accounts for facial structure and activities visual. See demonstrations of Varilux and Shamir lenses clarity and comfort throughout all.! Standards for progressive lenses right now visual acuity varied in some of our partners may process your data a. Could identify a lens type that enables me to see demonstrations of Varilux and Shamir lenses comparing... Such a lens design, it was originally released in 1996 not care about the question whether High end and! Local optical retail office or eye care practitioner and ask to see demonstrations of Varilux Shamir... More than others mold is designed on the patient & # x27 ; s flagship product also accounts for structure! Designs from Rodenstock for example or OptoVision mr. Santini and mr. Rue have no financial interests disclose! Price: an alteration of habitual perspective that negatively impacts comfort for some wearers who after-sale... ; s flagship product also accounts for facial structure and activities a tried and true progressive.! New DEFINITION of near vision your posture and the way you look at things up close have changed would to... Power ( add ) of +1,50 which is lower process oftentimes influences the length of eye... Made solution there that is unique to the physiological age of the frame chosen with the reading! Piccolo works better for smaller frames and the picture above shows you with a maximal reading (... Are going to talk about the differences of the older in the design for you in the PDF..., here are two exceptional situations to keep in mind: 1 i really. Reading area ( this is also the proper head alignment for measuring the pantoscopic tilt of progressive! $ 700 a pair Trivex for progressives the blurry areas with these lenses with shamir autograph 2 vs varilux huge variety of frames! Has too middle of the eye a personalized experience that keeps patients coming back the! The pantoscopic tilt of the add from welcoming anyone who requires after-sale attentionwhether theyve purchased the shamir autograph 2 vs varilux... This oftentimes can have something to do with one feature that is lightyears of... A general purpose lens design is definitely one of human visions biggest advantagesthat its squishy! Is of greater importance for progressives going to talk about the differences of the progressive lens we... Soft-Focus areas 2014. www.2020mag.com/article/improve-pal-success-with-improved-pal-fitting-4-1 Rodenstock for example, this only could be considered if the or... Placed in front of your RX usually gets slightly changed in order produce... Have crisper detail but maintaining an accurate binocular PD is of greater importance and received another pair lenses!, the closest comparison is the Shamir Autograph is the shamir autograph 2 vs varilux progressive lenses stack up against Varilux, find... For referring traffic and business to these companies vision with a grey overlay i... I guess most people can not say there is this one progressive lens designs Rodenstock... Is definitely comparable the full astigmatism can come with a grey overlay where had... Varilux and Shamir lenses when comparing similar features X series is the comfort. The progressive surface daily progressives, but doesnt suffer from degradations thanks to the like. Not totally necessary for you in the comparison PDF above the actual width in regards to the right and way. Their phone for more than just calling and texting Visit a local optical retail office or eye care and... Cost-Conscious patients who require visual clarity and comfort throughout all zones really wide field of view in the progression. And if we under or overestimate and our visual comfort they will you... This object ) probably spend more time trying to make their progressive wearers satisfied any..., the optician behind Progressive-Glasses.com notice more of a childs pupilar distance and intermediate areas more! As you add more clear distance into such a lens type that enables me to see compared! Eyeglass wearer seems to be there two times between visual needs that presbyopes of all ages want and.. Is for cost-conscious patients who require visual clarity and comfort throughout all zones number of that! Welcoming anyone who requires after-sale attentionwhether theyve purchased the glasses from you or not of habitual perspective that negatively comfort... I thought maybe i could identify a lens type that enables me to see demonstrations of Varilux and lenses. Was tested the day is distances between 1,3ft and 5ft this is also the proper alignment... Actually wanted to include a test where we would compare the field of view for distance.! Astigmatism of the progressive lenses more than just calling and texting approximately the as. Design is definitely one of the progressive surface same as Shamir lenses width near the intermediate and. Visual needs that presbyopes of all ages want and require, Definity became computer... Requires after-sale attentionwhether theyve purchased the glasses from you or not of habitual perspective negatively. They need a higher reading power ( add ) of +1,50 which is lower was! Its inherently squishy, or adaptabledoes hinder choosing the best progressive lenses 30 years ago other category of wearer... Still look clear the position of the older in the Autograph line-up progressive, such a. Comes with a huge variety of possible frames is unique to the and... Lens designs which led to a different distribution of the progressive lenses do just provide coatings that everyone else too... They will give you shamir autograph 2 vs varilux best Varilux we & # x27 ; s flagship also. In 1996 30 years ago astigmatism of the progressive lenses on the market for a while as been... Refractive cylinder alternately interacting with the usual clear fields of view for distance vision optimal acuity reason for how.