Two of the most common vision defects experienced by Australians include myopia and hyperopia. Eyeglasses and contact lenses, featuring concave or convex lenses in the right prescription, are the primary method of correcting these refractive errors. From complex contact lenses to the a wide range of myopia treatment options, Abernethy Owens is here to help with a complete range of professional optical services. Read on to learn more about concave and convex lenses, their differences and how they can help correct particular refractive problems.

What is a Lens?

A curved and transparent piece of plastic (originally made in glass), a lens refracts light rays in a certain manner to provide focus. Based on its shape, a lens can be classified as convex or concave.

Concave Lens - A concave lens has surface that curves inwards. It diverges or disperses a straight beam of light to form an image which is upright, virtual, and smaller in size than the object. A concave lens is used in applications where light needs to diverge like torches, laser devices, telescopes, etc.

Convex Lens - A convex lens has a spherical surface that curve outwards. A convex lens brings together a parallel beam of light forming an image which is inverted, real and smaller than the object when the object is placed at focus. Convex lenses are used in applications where faraway objects need to be focused such as microscopes, cameras, magnifying glasses etc.

Key Differences

Shape:

  • A concave lens is thinner in the middle than at the edges – it ‘hollows out’ inwards.
  • A convex lens thicker in the centre than at the edges - it bulges outward.

Effect on parallel rays:

  • A concave lens diffuses the parallel rays of light passing through it.
  • A convex lens focuses the parallel rays of light that pass through it.

Image generation:

Concave lenses always create virtual, reduced, and upright images, no matter where the object is stored. The type of image produced by convex lenses depends on where the object is located.

The Use of Lenses to Treat Vision Defects

Vision defects occur due to issues with the lens within the eye, or where the eyeball is not spherical. The most common vision defects are:

Myopia (Near-Sightedness or Short-Sightedness)

Those with myopia can see up close but faraway objects appear distorted. This occurs because the eye is more elongated, and the light focuses in front of and not on the retina. Myopia is corrected with a concave lens as this allows light entering the lens to diverge so that the light will focus on the retina.

Hyperopia (Farsightedness or Long-Sightedness)

Those with hyperopia can see far away but near objects seem distorted. This can occur due to a decrease in the eyeball diameter or a decrease in the convexity of the eye lens surface which results in the light focusing on an imaginary point behind the retina instead of on the retina. Hyperopia is corrected with a convex lens as this allows the light entering the lens to converge so that it will correctly focus on the retina.

The power of a prescription spectacle lens is determined by the combination of curvatures on the front and back surfaces. A lens made for a myopic prescription will generally be flatter on the front surface and thicker at the edges, while a lens made for a hyperopic prescription will have a thicker centre and curve outwards on the front surface.

Looking for an Optometrist? Perth Relies on the Team at Abernethy Owens

If you are experiencing any vision or eye health problems, book in for a thorough eye examination today. Using the latest and most innovative equipment, our Optometrists perform a non-invasive, comprehensive eye test to fully assess the vision, health, and function of your eyes. It is recommended that every adult has their eyes checked every one to two years. Book an appointment online with us today and experience our friendly and professional service.