Today we are more likely to use it in the following format as this allows the index of the lens to be taken in to account

The myopic patient needs to know how much thinner the lens will be and be able to relate this to the cost of the lens. Hyperopes tend to be less bothered by the 'value' for the thickness of their lens, and base their thinking on how thick they 'feel' the lens is. Telling a hyperope that the lens is actually 0.5mm thinner may be meaningless, if the lens still appears thick.

Most big lens manufacturers now have their own lens ordering softwear, and these often contain a lens comparison module, such as Essilor's 'Opsys' system. Figure 2 shows its use for a comparison between a -8.00 lens in CR39 and 1.8 glass. The Essilor Opsys system also enables you to digitally rotate a lens (to scale) so that the patient can view it from all angles (Figure 3).

For plus lenses you need to be aware of the minimum edge 'thinness'. A lens any thinner than 0.6mm will be dangerously thin, but in a plastics frame the edge should ideally be about 1mm and in metal a little thicker at 1.8mm. Exceptions to this are supras and rimless. A supra needs to be 2-3mm thick to take the supra cord. So care needs to be taken in dispensing a plus lens in to a supra, as the lens will need to be thicker than in a full rim equivalent. Be aware that if you dispense a 'thinner' aspheric lens into a supra frame this would also need to be made a little thicker. This may defeat the object of a thinner lens, but it would still be 'flatter', and consequently cosmetically better. In a rimless frame the edge needs to satisfy a minimum thickness, which may vary with the method of attachment. A useful guide would be to keep it at 2-3mm.

When calculating the thickness it is essential that you remember to include the edge thickness in plus lenses and the centre thickness in minus ones.

These approximate formulae are best used on the 'power' of the lens rather than each surface as the accuracy achieved by looking at both surfaces is negated by the inaccuracies of the approximate methods. Strangely the accurate sag calculations are just as easy as the approximations.

Sometimes a patient may enquire about the different lens options and how thick they may be. This is especially for a myope interested in paying more for higher index materials.

In this month's look at calculations in practice, Janet Carlton describes how best to calculate what is all imoprtant to most patients - lens thickness

More often that not, it is easier to compare the thickness of a new lens to that of the old. If you know what the patient is wearing you can tell them it will be x per cent thinner/thicker.

Companies such as Norville, Essilor and Nikon produce dispensing mats which illustrate lens thickness and help explain thicknesses to patients.

To calculate the thickness of a lens, you first need to calculate the sag, of both surfaces, and take in to account the centre thickness of a minus lens or the centre thickness of a plus one.

Patients generally want thinner and lighter lenses, and expect a knowledgeable practitioner to help them through the maze of indices and lens forms. It's important that you dispense what the patient wants and needs, rather that what you decide they are going to have. Always dispense a lens for a reason, and 'because that's what I always do' is not one of those reasons.

You will need to check with the manufacturer to find the centre thickness of a specific material, but as we could be dealing with fractions of a millimetre between manufacturers, it is a useful guide to assume that glass and higher index resins were 1mm thick and CR39 2mm. Any thinner than this and the lens could be a potential danger for your patient.

The details captured by security cameras varies widely from standard definition 4CIF through ultra-high resolution 4K cameras.

We saw in the last part of this series (Optician, January 12, 2007) the importance of dispensing the correct index, very relevant when talking about thickness of lenses. Later in this series we shall look at some other aspects of thinner high index lenses, such as chromatic aberration. ?

View PDF    Sometimes a patient may enquire about the different lens options and how thick they may be. This is especially for a myope interested in paying more for higher index materials. The myopic patient needs to know how much thinner the lens will be and be able to relate this to the cost of the lens. Hyperopes tend to be less bothered by the 'value' for the thickness of their lens, and base their thinking on how thick they 'feel' the lens is. Telling a hyperope that the lens is actually 0.5mm thinner may be meaningless, if the lens still appears thick.To calculate the thickness of a lens, you first need to calculate the sag, of both surfaces, and take in to account the centre thickness of a minus lens or the centre thickness of a plus one.Meniscus lens From Figure 1 (above)t + s2 = e + s1sot = e + s1 - s2There are two approximate sag formulae, the first from when crown glass was the norm in the optical world s = y2 F1000 Where y = half the diameter of lensToday we are more likely to use it in the following format as this allows the index of the lens to be taken in to accounts = y2 F2000 (n-1)These approximate formulae are best used on the 'power' of the lens rather than each surface as the accuracy achieved by looking at both surfaces is negated by the inaccuracies of the approximate methods. Strangely the accurate sag calculations are just as easy as the approximations.Exact sag formula is:s = r -vr2 - y2Where you will need to calculate the radius of curvature using:n-1 = rFExample: Calculate the sag of a -11.00 curve, diameter 50mm in crown glass.Using the accurate methodn-1 = 1-1.523 = 0.0475 mF 11.00r = 47.5mm s = r - vr2 - y2s = 47.5 - v47.52 - 252s = 47.5 - v2256.2 - 625s = 47.5 - v1631.2s = 47.5 - 40.38s = 7.1mmUsing an approximate formulas = y F 2000 x n-1s = 25 x 25 x111046s = 6.57mmWhen calculating the thickness it is essential that you remember to include the edge thickness in plus lenses and the centre thickness in minus ones.You will need to check with the manufacturer to find the centre thickness of a specific material, but as we could be dealing with fractions of a millimetre between manufacturers, it is a useful guide to assume that glass and higher index resins were 1mm thick and CR39 2mm. Any thinner than this and the lens could be a potential danger for your patient.For plus lenses you need to be aware of the minimum edge 'thinness'. A lens any thinner than 0.6mm will be dangerously thin, but in a plastics frame the edge should ideally be about 1mm and in metal a little thicker at 1.8mm. Exceptions to this are supras and rimless. A supra needs to be 2-3mm thick to take the supra cord. So care needs to be taken in dispensing a plus lens in to a supra, as the lens will need to be thicker than in a full rim equivalent. Be aware that if you dispense a 'thinner' aspheric lens into a supra frame this would also need to be made a little thicker. This may defeat the object of a thinner lens, but it would still be 'flatter', and consequently cosmetically better. In a rimless frame the edge needs to satisfy a minimum thickness, which may vary with the method of attachment. A useful guide would be to keep it at 2-3mm.More often that not, it is easier to compare the thickness of a new lens to that of the old. If you know what the patient is wearing you can tell them it will be x per cent thinner/thicker. The curve variation factor (CVF) can be used to compare thicknesses of lenses to that of Crown glass (1.523)n CVF 1 498 1.05 (5% thicker than crown)1.523 1.00 1.7 0.75 (25% thinner than crown)1.8 0.65 (35% thinner than crown)1.9 0.58 (42% thinner than crown)Companies such as Norville, Essilor and Nikon produce dispensing mats which illustrate lens thickness and help explain thicknesses to patients.Most big lens manufacturers now have their own lens ordering softwear, and these often contain a lens comparison module, such as Essilor's 'Opsys' system. Figure 2 shows its use for a comparison between a -8.00 lens in CR39 and 1.8 glass. The Essilor Opsys system also enables you to digitally rotate a lens (to scale) so that the patient can view it from all angles (Figure 3).Patients generally want thinner and lighter lenses, and expect a knowledgeable practitioner to help them through the maze of indices and lens forms. It's important that you dispense what the patient wants and needs, rather that what you decide they are going to have. Always dispense a lens for a reason, and 'because that's what I always do' is not one of those reasons. We saw in the last part of this series (Optician, January 12, 2007) the importance of dispensing the correct index, very relevant when talking about thickness of lenses. Later in this series we shall look at some other aspects of thinner high index lenses, such as chromatic aberration. ?? Janet Carlton is dispensing manager at the City University Optometry Clinic

Many factors can impact the video that is viewed and recorded, however when all things are equal, what a security professional expects to capture for review following an incident varies across camera resolutions. Selection of the perfect resolution for your application depends on several factors:

Our team set up series of cameras: 720p, 1080p, 5 megapixel, 4K (8MP equivalent), and two panoramic 12 megapixel cameras, to see what differences can be expected by owners and end users of video surveillance systems. Nick, Tom, and Matt give us an inside look at security camera resolution comparisons in this video.

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