Help to clarifying general doubts

Lenses
There is more to lenses than one might think. This material or that, single vision or bifocal, photochromic or plain white, hard coated, multi coated or anti-reflective and so on. We supply the highest quality lenses, produced using imported machinery/raw materials, that satisfy all International quality standards. Take your time to read up on the different choices that are available - we concern ourselves with delivering the best quality; the selection of type is in your hands. PLEASE NOTE: the price guidelines given below are guidelines – when we receive your order, we will calculate the actual price of your lenses. At source, lenses are priced according to the power, so the prices given are indicative of lenses upto – or + 3.0 with zero cylinder.
Lens material
Lenses traditionally are of course made of glass, but technological advances have given us a number of choices:

Glass: It is a breakable, hard material, which is generally thinner, heavier and cheaper than its alternatives. Particularly with high 'plus' powers, ie for long sighted correction, they can be quite weighty. With lower powers or high minus powers they still give the purest vision and the lowest price.

Glass lenses are supplied in a standard Refractive Index of 1.523 in the range of +/- 6.0 with a maximum cylinder of 2.0 High Index lenses go all the way up to the absolute highest of 1.90 ~ used for high minus powers upto -23 and/or cylinder of 6.0. Bear in mind that although thin lenses are cosmetically desirable, they are also more fragile - so below a correction of -3 or -4 a high index lens is often not required.

Because of its high density or hardness, glass cannot be tinted as such. Most powers can be processed using dark grey glass, or its photochromic equivalent.

Plastic: This is a 'softer', thicker, lighter and more expensive alternative to glass lenses. It is the most commonly used material, and usually coated to make it scratch resistant, and/or it anti-reflective etc.

The standard refractive index for plastic lenses, because of its low density is 1.498. Users with a correction above -2 or -3 might consider the high index lenses of 1.56. The visual and weight advantage of high index lenses is less evident in plus power correction lenses.

Plastic lenses are easily tinted prior to hard-coating – thus opening a world of possibilities. Photochromic lenses, by comparison are surprisingly expensive in plastic – however, the science has its price and the benefits are easily apparent when you wear your glasses on a bright or sunny day.

Polycarbonate:A more expensive alternative to both the above, which is both thinner, stronger and more expensive. Often referred to as 'bulletproof', it is still breakable but generally more rigourous than the other materials and is the material of choice for children’s lenses particularly. It is both denser and lighter than plastic and is supplied in a R.I. of 1.586.

Type of lens
Lenses can be subdivided into a number of different types, namely single-vision, bi-focal, progressives, plus one or two rarer multi focals or high power lenses.

PLEASE NOTE: Price guidelines ARE only guidelines. All lenses’ prices vary according to their power, as we purchase directly from the manufacturers, and we pass on these actual variations to our clients. Lesser powers can result in lower prices than those given below, likewise for seriously higher powers the lens price may exceed those given here.

Single Vision:Simply this is a lens with one, uniform correction over the whole lens surface, be it for near or distant vision. Compound lenses are still 'single vision' but have a 'cylindrical' correction, as well as the spherical power, which will be fitted at a particular axis or angle.
Single Vision: Guideline pricing:
Glass - £6, Plastic - £12, Polycarbonate - £24 (h/c only)

Bi-Focals:These, as their name suggests, have two different power corrections within the same lens. They are subdivided into three types based upon the manufacture and distribution of the two different power 'regions'. Kryptok is the name given to a lens that has a semi-circular portion for the near vision power, the transition between the two powers is gradual thus leaving a blurred 'aura' around the reading portion - it is the cheapest but least satisfactory bifocal from a technical users point of view. UnivisD has a near vision area in the shape of a reclining capital D, where the transition between the two powers is immediate. Lastly, the older form of bifocals, the executive bifocal: this lens has two different powers either side of a horizontal separating line – usually supplied only in glass.
Kryptok Bifocal
Guideline pricing: Glass - £9, Plastic - £17, Polycarbonate n/a
UnivisD Bifocal:
Guideline pricing:Glass - £24, Plastic - £20, Polycarbonate £29 (h/c only)
Executive:
Guideline pricing:Glass - £12, Plastic – £42, Polycarbonate n/a

Progressives [varifocals]:The multifocal of choice. This lens has both near and distant vision areas with a gradual progression from one to the other [called the corridor]. With practice this allows the user to vary the degree of correction to focus clearly on objects in middle distance also. Cosmetically it is also preferred as the transition from one to the other power correction is not visible to anyone except the user.
Standard progressive
Guideline pricing:Glass - £43, Plastic - £29, Polycarbonate £85 (h/c only)
Short Corridor progressive:
Guideline pricing: Glass - £78, Plastic - £34, Polycarbonate n/a

Lens coatings and  tints.
Each of the lens types can be supplied in different tint shades [glass lenses cannot be tinted, but are available in grey or dark grey], graduated, photochromic and/or with coatings to improve its durability.

Tinting:Many people forego power correction when they exchange their specs for a pair of sunglasses on a sunny day. This can lead to many difficulties, and put people at risk in some situations. All plastic lenses can be tinted to any shade or shades prior to the coating process.
Single colour tinting
Guideline pricing: Glass – n/a, Plastic - £3, Polycarbonate £3
Graduated tinting
Guideline pricing:Glass – n/a, Plastic - £5, Polycarbonate £5

Photochromic:This is a lens that will turn dark in the sun but return to clear when indoors, at night or in cloudy conditions. So, power sunglasses for when the sun comes out only! Usually available in photo-grey or photo-brown.
Photochromic lenses
Guideline pricing:Glass s/v – £9, Plastic s/v - £30, Polycarbonate n/a
Guideline pricing:Glass Dbf – £18, Plastic Dbf- £66, Polycarbonaten/a
Guideline pricing:Glass prgs – £24, Plastic - £69, Polycarbonate n/a

Coatings:These treatments add qualities to the base materials, and can protect the eyes as well as the lenses if chosen correctly. The hard coating to a plastic lens will make it scratch-resistant. The anti-reflective coating to a lens will diminish the surface reflection and reveal your eyes and protect from glare at the same time. Ordinarily the lenses are offered in three classes - uncoated, hard-coated and multi-coated (includes the anti-reflective treatment) - though, for example, glass has no need for hard-coating and some plastic progressives are not available without hard-coating. Please check availability with us at the time of ordering.
Hard or scratchresistant coating
Guideline pricing:Glass – n/a, Plastic - +£4, Polycarbonate n/a
Hard-multi coating with anti-reflective coat
Guideline pricing:Glass – £6, Plastic - +£9, Polycarbonate +£9

N.B. High Index or slimmer lenses vary from +20 to +40% of the standard lens price.Now lens choice is up to you. We suggest, where possible, to take the advice from a qualified ophthalmologist or optometrist both for the material, type and lens treatment.  Our staff are always prepared to offer educated suggestions, but only they have the opportunity to do a full medical examination and can come to know your actual physical/medical circumstances. We guarantee unreservedly for the quality of the lenses we supply, but Drishti Opticals cannot take responsibility for any negative side-effects of lenses incorrectly ordered.

Your prescription

Here you can find the information necessary to a) understand your prescription and b) select and order the lenses that suit your actual needs. Bear in mind that the prices on this website are simply guidelines. The prices vary according to power, type and coatings, can vary greatly one from another, even when one or two of the variables are the same - so we encourage you to submit your preferences and we will forward you a 'no obligation' quote by return email.

Spherical power
- this is what we usually think of when we talk of the 'strength' of our glasses. It will be plus [indicating long-sightedness] or minus [indicating shortsightedness] and for almost all users, between plus 5 and minus 8.

Cylinder & Axis
- with irregular shaping of the cornea, many prescriptions will mention a number under the heading of CYL and Axis. Either plus or minus 0.25 to 4.0 is within the 'nomal' range. Lenses are rarely prescribed above 4.0 cylinders, but where they are, the price and delivery will also be greater. The Axis measure is simply the degree of rotation required on the lens to match the irregularity of the corneal surface. The next measurement is particularly important in the case of high cylindrical correction.

PD - All prescriptions must include the PD measurement. PD stands for "pupillar distance" and varies between 50 - 74 mm usually. The focal centre of a lens must be aligned with the pupil, even more so with astigmatic correction, to give the maximum clarity. The axis rotation of a lens is made during the fitting once the lens and pupil centres have been matched. So, if it is not mentioned, be sure to ask your optician for your PD measurement.
Refractive Index- a measurement of the refractive-ness of the lens material. A larger number means a thinner lens. Standard R.I. for plastic lenses is 1.498, for glass is 1.523 and polycarbonate 1.586. What is called a 'High-Index' lens is a lens with a greater refractive index than standard, and thus will be thinner for the same degree of correction. Higher power will always mean thicker lenses, so to balance irregular corrections for left and right, to lighten the load on high powered specs, it is advised to use high index lenses.

Lastly it must be said that in certain optical or medical situations, Drishti Opticals will decline to supply glasses – for the reason that the precision required in centring the lens etc really is best done ‘in person’. As mentioned above, within the normal range of spherical or cylindrical correction, we can confidently supply corrective glasses of a high street quality at very affordable rates. For high astigmatics [ie high cylindrical correction] or prism lenses the cost advantages can be less convincing.

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