Posted by Verto Network on August 27th 2008 to
Fashion Choosing your lenses Most lenses today tend to be plastic rather than glass, because plastic is much lighter than glass and won’t smash if you bash them or drop them like glass will. Plastic is therefore safer for driving as well. Sometimes, when extremely thin lenses are required for very high prescriptions, then a high index glass lens can often be the best choice. Lenses have a “refractive indexâ€Â, which is a guide to how much the lens material “bendsâ€Â or focuses the light rays to a single point on your retina at the back of the eye. Plastic has a refractive index of 1.5 and the higher this number the more the lens can bend or focus the light rays, enabling the lens to be made thinner (and therefore also lighter) from higher index lenses of the same power. So basically, the higher the index the thinner the lens will be. 1. Standard Plastic (1.5) Ideal for most standard prescriptions (up to about + or – 3.00DS). Anything stronger than this and the lenses start to look thick. 2. Plastic (1.6) For prescriptions up to about + or – 5.00DS this will thin the lenses down nicely, being about 25% thinner than standard plastic, but stronger prescriptions will again start to look thick. This is the lens of choice for most prescriptions up to about + or – 5.00DS where you want to have nice-looking thin lenses. 3. Plastic (1.67) This is the most popular high index lens used today, being about 40% thinner than standard plastic but also 25% flatter. It is also stronger than standard plastic, which is ideal for glazing rimless frames where you have to drill into the lens (standard plastic tends to crack around the screw holes in a rimless frame – so beware of cheap rimless spectacles where they might only be using standard plastic 1.5 lenses or 1.6. Make sure rimless specs are made with 1.67 lenses or alternatively polycarbonate, which is also very strong). So,1.67 lenses are great for most moderately powered prescriptions. Being thin and flat, they will look great for powers up to about + or – 7.00DS. 4. Plastic (1.74) This is the thinnest plastic lens available, being around 50% thinner than standard plastic1.5, and 30% lighter than the equivalent glass lens. Plastic 1.74 is great for higher prescriptions from say + or – 7.00DS up to about + or – 10.00DS. For lower prescriptions than this they will only be a little thinner than the 1.67 lenses, and so don’t waste your money on a 1.74 for lower powers when a 1.67 is more than up to the job. 5. Glass (1.7) A good lens for high minus prescriptions up to about -10.00DS. However, it will be heavier than plastic, and so we tend to use a plastic 1.74 lens in preference. Don’t even think about high index glass lenses for + powers as they will be really heavy. 6. Glass (1.8) Good for high minus prescriptions, but we don’t use these much. Also rather expensive. 7. Glass (1.9) The thinnest of all lenses, and excellent for very high minus prescriptions, but I would recommend visiting a good optician if you are over -10.00 as you really need to have your glasses properly measured by an experienced dispenser. These prescriptions need a lot of care and attention to produce a well fitting pair of spectacles which are comfortable to wear and look good. Which type of lens? There are basically three types of lens to choose from :- single vision, bifocal or varifocal. Single vision These can be used for just distance, intermediate or reading. However, some people can see both distance and near vision with their single vision lenses – it all depends on your prescription and your age. When you reach the age of around 40 to 45 or so (like me!) the lens in your eye starts to loose its flexibility and so focusing on near objects becomes increasingly difficult. This is when you start to need reading glasses, or a separate strength of lens for reading compared to distance. So you now need two different strengths of lens – one for distance, and another strength for reading. If you are short-sighted you can sometimes be able to take your glasses off to read. A single vision lens has a “singleâ€Â focus, so it has the same power over the entire lens. Bifocals These can be thought of as distance lenses with a reading segment added to the bottom of the lens. You can see for distance, and then as your eye turns down to read it passes into the reading segment with the different power for closework. Although you get a line on the lens at about the height of your lower eyelid, bifocals have a wide reading area which makes reading easy – however, some people don’t like the line getting in the way of their vision. The two main types of bifocals usually dispensed today are:- The “D28 segâ€Â (meaning D-shaped segment bifocal) looks like a letter “Dâ€Âon its side with the flat part at the top, just at about the height of your lower eyelid. The number 28 refers to the width of the bifocal portion in millimetres. The “C28 segâ€Â is exactly the same, except the top part of the bifocal segment is slightly curved upwards in a convex shape, rather than being completely flat as in the D seg. Again, the width of the bifocal segment is 28mm. Both lenses are very similar to look through, it’s just that the “C segâ€Â has a slightly curved top. As a rule we position the top of the bifocal to be 3mm below the centre of the lens, although that might vary depending on the frame chosen. Varifocals With these lenses you can see for distance through the top part of the lens, as in a bifocal, and then as you turn your eyes down to see closer objects such as reading, the lens power gradually increases as you move into the reading portion of the lens. As there is a gradual transition from distance to near, there is no line on the lens, so they look nicer than a bifocal. However, some people experience problems with varifocals as they only have a small reading area and also give distorted vision when looking around to the sides, which can make you feel a bit “woozyâ€Â. Therefore these lenses need to be measured and fitted accurately with you wearing the frames. The measurements need to be millimetre perfect, which can only be achieved by seeing you in person. Lens coatings 1. Hardcoat This is a colourless coating, often called a “scratch resistantâ€Â coating which covers the lens surface. It forms a protective layer which helps to stop little scratches forming through normal wear and tear, and also makes the lenses easier to clean. We highly recommend to have this coating added to your lenses as it helps to keep the lenses in good condition. Always have this coating for reading glasses which tend to get more wear and tear from taking them on and off. 2. Multi Anti-Reflection coating (MAR) This helps to reduce the reflections that can be troublesome on the lens surface. It improves the appearance of your spectacles by making it look as though you don’t have any lenses in your frames. It also makes night driving much easier by reducing the glare from on-coming headlights, and also helps to reduce the glare that you can get from computer screens. It gives the surface of the lens a slight bloom (like oil on the surface of water), but has no tint so the lens is still completely clear. The lenses can be slightly more difficult to keep clean, but just use a special microfibre cloth. DON’T use washing-up liquid to clean an MAR coated lens because it will damage the coating, and never use a normal cloth or handkerchief – they may feel soft to the touch but are actually quite abrasive for your coated lenses, which will soon scratch unless you use the proper microfibre cloth (it has a silky feel to it). 3. Hard Multi Anti-Reflection coating (HMAR) This is a combination of a hardcoat and an anti-reflection coating. Most anti-glare or anti-reflection coatings tend to come in this form, because you have both the benefit of the MAR coating with the increased hardwearing qualities of the hardcoat. Always have this coating if you are going to be doing any night driving or computer work. Tints There are essentially two types of tint that you can have on your lenses – a fixed tint, which remains the same darkness at all times, or a photochromic tint (such as transitions), which will vary in darkness according to the brightness of the sunlight. 1. Fixed tint The tint is added to the lens after manufacture and can come in various colours and darknesses. We describe the darkness of the tint by the percentage of light that is allowed through the lens. So an 80% tint allows 80 per cent of the light through the lens, whereas a 25% sunglass tint allows only 25 per cent of the light through (and so is darker in appearance). A lens with a transmissibility of 100% is completely clear with no tint at all, and one with 0% allows no light through at all (you would not see anything through it – only darkness). Most tints tend to be either brown or grey (black). I, myself, prefer a grey (black) tint because I think the colours appear more natural, but other people prefer brown tints (which do help to enhance contrast), but its all very personal and up to the individual. If unsure, have a look at an old pair of sunglasses that you liked and see what colour tint they have. Sometimes people like to have a light yellow tint (which again helps to improve contrast), or even a light blue tint, which would make everything have a cooler look. There are essentially 2 main shades of tint :- Comfort 80% tint This is a light tint which will give the lens a very slight colouring which helps to take away glare from bright lights. However, you would still have to screw your eyes up in bright light. Sunglass 25% tint This is a darker tint, like an average pair of sunglasses. It gives a good combination of being dark enough to take away the glare in bright sunlight, and yet not too dark so that you can still see well through it on one of our typical cloudy but bright days that we often tend to get. 2. Transitions These are a make of “photochromicâ€Â lens, which means all lenses that change colour (“chromicâ€Â) with the light (“photoâ€Â). The first well known brand to be marketed was the “Reactolightâ€Â lenses, and transitions are simply a modern improved brand of photochromic lens. They come in either brown or grey, and have a very light tint when you are indoors or at night time (the lens is almost completely clear), and then go darker as the intensity of the sunlight increases when you are outside on a bright sunny day. This makes transition lenses very flexible, and so you could use just one pair of glasses for everything – indoors and out. However, they do not go as dark as sunglasses, and do not go quite so dark when in a car, so on a very sunny day you might still have to screw your eyes up. Having said that, many people are very happy with transitions – it all depends on how dark you want them to be. So, if you want a light tint that will vary with the sunlight to take away most of the glare, the transitions are great. But, if you want them to go as dark as sunglasses (for use in very bright sunlight with your eyes completely relaxed), then you would be better off with a separate pair of sunglasses. Stephen Jenkins is a UK optician who qualified in 1989 and has his own high street practice in Chesterfield, Derbyshire (Watson and Jenkins Opticians). The practice also sells designer sunglasses online and is an official supplier for Oakley, Rayban, Gucci and most other brands. Advice is available on ultraviolet light and how to choose the most suitable tint for your sunglasses. 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