Chromatic Aberation in Optics

What is it? And How to Fix it?

Chromatic aberration example — _{Photographic example showing high quality lens (top) compared to a lower quality lens (bottom)}

When you look through binoculars, a spotting scope, or a refractor telescope, you expect a crisp, true-to-life image. But sometimes you’ll notice coloured fringes – purple, green, or blue outlines – around bright objects like the Moon, Jupiter, or even a bird in daylight. This optical effect is called chromatic aberration, and it’s one of the most common issues in optics.

In this article, we’ll explore what chromatic aberration is, why it occurs, and the techniques optical designers use to correct it across binoculars, spotting scopes, and refractor telescopes.

What Is Chromatic Aberration?

Chromatic aberration (CA) is a type of distortion in optics caused by the fact that different wavelengths (colours) of light bend at different angles when passing through a lens. This is known as dispersion.

Shorter wavelengths (blue/violet) bend more strongly.
Longer wavelengths (red/orange) bend less strongly.

Because of this, not all colours focus at the same point, leading to colour fringing and reduced sharpness.

Chromatic Aberration diagram — _{Chromatic Aberration; Wikipedia}

Types of Chromatic Aberration

There are two main types:

Longitudinal (Axial) Chromatic Aberration
Different colours focus at various distances along the optical axis. Common in fast (low f-ratio) refractors.
Lateral (Transverse) Chromatic Aberration
Colours spread out sideways at the edge of the image. More visible in wide-angle optics like binoculars.

_{The two types of Chromatic Aberration – Image from Nikon Consumer}

Chromatic Aberration in Different Instruments

1. Binoculars

In binoculars, chromatic aberration often shows up as purple or green edges around tree branches against a bright sky. Lower-end binoculars, especially those with single lenses, are more prone to it. High-quality models use ED (Extra-Low Dispersion) glass to reduce fringing.

_{Left: Without ED Glass | Right: Using ED Glass | Photo by: Nikon}

2. Spotting Scopes

Spotting scopes are often used in bright daylight, which makes chromatic aberration more noticeable. ED or fluorite glass elements dramatically improve clarity, especially for birdwatchers or digiscopers attaching a camera.

_{Fluorite has a very low dispersion, so lenses made from it exhibit less chromatic aberration than those made of ordinary glass – Image by: Geologyin.com}

3. Refractor Telescopes

Refractors, especially achromatic doublets, are notorious for showing colour fringing on bright stars and planets. This is why astrophotographers often prefer apochromatic triplets (APOs), which bring three wavelengths of light into focus simultaneously. These produce sharper, colour-corrected images ideal for astrophotography.

_{Thors Helmet taken with a high quality APO refractor telescope – Photo by: BBC Sky}

How Do We Fix Chromatic Aberration?

Optical designers use several techniques to minimise CA:

1. Special Glass Types

ED Glass (Extra-Low Dispersion): Reduces dispersion and aligns colours more accurately.
Fluorite Elements: Extremely effective at eliminating CA, often found in premium spotting scopes and telescopes.

2. Lens Combinations

Achromatic Doublets: Combine two types of glass to bring two colours into focus, reducing fringing.
Apochromatic Triplets (APOs): Use three or more elements to correct for three wavelengths, offering near-perfect colour correction.

3. Aperture Masks & Filters

Reducing the aperture (masking) or using colour filters can suppress CA in telescopes, though at the cost of brightness. For astrophotography, software correction tools also help.

4. Digital Correction

Modern cameras and smartphone adapters can remove chromatic aberration in post-processing, but starting with good optics is always best.

Why Chromatic Aberration Matters

_{Blurring viewing can ruin your observations}

For Birdwatchers: Chromatic Aberration can blur fine feather details and spoil photographs.
For Stargazers: Planetary features and bright stars lose definition when fringed by purple or green halos.
For Photographers: High-resolution sensors reveal Chromatic Aberration that the human eye may not notice.

Correcting chromatic aberration means sharper images, truer colours, and a more enjoyable experience—whether you’re observing Saturn’s rings or an eagle in flight.

Conclusion

_{ED glass, apochromatic lenses and reflector telescopes deliver incredible clarity}

Chromatic aberration is a natural consequence of light dispersion in lenses, but modern optical design has come a long way. With ED glass, apochromatic lens systems, and advanced coatings, today’s binoculars, spotting scopes, and refractor telescopes deliver breathtaking clarity with minimal colour fringing.

When choosing your next optic, keep an eye out for terms like ED, HD, fluorite, achromatic, or apochromatic—these are your best defence against chromatic aberration.

Optics Central have a wide range of telescopes, spotting scopes and high end binoculars that use ED glass.
https://www.opticscentral.com.au/catalogsearch/result/?q=ED+Glass