In an optical system, light often passes through one or more lenses and when it does so, several types of optical aberrations can occur.
In this article, we will discuss a type of aberration that is common in photography and astrophotography, namely Chromatic Aberration.
What Is Chromatic Aberration?
Chromatic Aberration, also known as purple fringe or dispersion, is an optical aberration common in photographic lenses and refractor telescopes.
Chromatic Aberration (CA) is visible as colored fringes around highly contrasted edges, such as branches and buildings edges against a bright sky.
What Causes Chromatic Aberration
You may all be familiar with the simple experiment of using a prism to decompose white light into its different components, also known as colors.
What happens when white light passes through a prism is that the different wavelengths (colors) that compose interact differently with the glass of the prism and get deflected in slightly different directions.
When the light comes out from the prism, the colors do not “superpose” anymore, and you can see them individually, forming a rainbow.
In photography, lenses are made so that light decomposition (dispersion) is greatly reduced or avoided altogether.
But no lens is perfect and light dispersion can still occur, particularly with entry-level gear.
When a lens cannot focus all the different colors in the same point, we have CA.
Different Types Of Chromatic Aberration
There are two different types of CA: longitudinal, or axial, and lateral, or transverse.
The job of a lens is to focus light on a plane, the focus plane, and all the colors on a point.
The difference between the two types of CA is in the way the lens fails to focus on the different colors in a single point.
Longitudinal (Axial)
With longitudinal CA, red, blue and green colors are focused on different planes: if you focus on red, you will have blue halos, if you focus on blue, you will have purple fringing.
Here is a real life example of longitudinal CA: note how the color of the fringe changes with the focus.
True Longitudinal CA is not easy to correct during editing, but it can be reduced with accurate focus and, as shown in the image above, but stepping down your lens.
Lateral (Transverse)
Lateral, or Transverse CA, occurs when the red, green, and blue colors are focused on different positions on the same focal plane, as shown in the scheme below.
This type of CA affects targets that are off-axis, as it is more evident with light coming into the lens with an angle. As the right image above shows, the central part of the frame is CA free.
I took the image below at dawn with an ultra wide-angle lens, and lateral CA is evident around the dock poles.
Unfortunately, while it is easier than longitudinal CA to eliminate in post processing, lateral CA cannot be reduced in-camera by stepping down the lens.
How Does Chromatic Aberration Affect Images In Astrophotography?
Astrophotography is very demanding not only for the photographers but also for lenses and telescopes.
A lens taking wonderful images in daylight can give average or subpar results when used for astrophotography.
The reason why astrophotography is so demanding is the need to get both the stars shape and their size right.
Optical aberrations such as coma and field curvature will create weird and elongate stars, particularly towards the edges of the frame.
And then there is the CA, which is shown in the image as blue halos around bright stars.
Purple fringe is even more common and is visible around many stars, like in the image below.
Finally, CA can often be seen around the Moon too.
The use of low dispersion glasses in a lens or telescope strongly reduces or completely eliminates CA.
Tips To Avoid Chromatic Aberration
There are a few tips you can follow in order to deal with CA when you shoot.
It should be noted here that lenses and telescopes do suffer from a certain amount of both longitudinal and axial CA.
Tip 1: Use High Quality Gear
High quality lenses and telescopes have extra lenses made of Extra Low Dispersion (ED) glass to reduce or eliminate CA.
Photographic lenses with this type of glass often have “ED” in their name. Refractor telescopes using ED lenses are called Apochromatic, and the terms doublet, triplet, and quadruplet refers to the number of ED lenses these telescopes use.
Reflectors are CA free, as the light does not pass through a lens, but bounce off mirrors. Instead, fast reflectors almost always suffer from coma (bad star shapes) and need a coma corrector.
While photographic lenses often have a good correction for field curvature, telescopes need field flatteners when used for astrophotography.
Tip 2: Step Down The Lens
Longitudinal CA in photographic lenses is often stronger when the lens is used at its widest aperture (smaller f-number).
Stepping down the lens by one or two f-stops can significantly reduce the CA.
Because photographic lenses have variable aperture, you can take advantage of the Chromatic Aberration Index, CAI.
For a lens with no ED glasses, like old legacy lenses, the CAI is an index to estimate the amount of CA: if it is equal or larger than 3, then the image is basically CA free.
The index is calculated as the ratio of diameter, d, of the front lens in inches and the f-number, f:
CAI = f / d
Let’s consider the image above: that was taken with an Olympus Zuiko OM 300 f/4.5, 72mm in diameter, equal to 2.8 inches.
To have no CA, the CAI should be 3 or higher: let’s consider a value of 3.
The f-number that will give a CAI of about 3 is: 3 * 2.8 = 8.4
As you can see from the image above, at f/8 the CA is negligible, while it is well visible for faster apertures.
If you are interested in using legacy lenses, favor smaller diameter lenses, as they allow you to use wider apertures.
Tip 3: Nail The Focus
The more you are out of focus, the stronger the CA. Particularly if your lens exhibits strong CA, take your time to accurately focus on the stars or the Moon.
Tip 4: Use A Fringe Killer Filter
There are filters for astronomic uses, such as the Baader Semi APO or the Baader Killer Fringe, which are designed to reduce the CA.
The Baader Semi APO filter in size M48.
These filters do so by removing the light that is not focused properly, at the cost of introducing a fair amount of color cast.
The topic is debated: some say these filters work and are worth using, others say that is money you could spend better.
Do keep in mind that these are filters for astronomy, so they come in 1.25” and 2” sizes, which correspond to an M48 photographic filter, so they are usable with small diameters lenses only.
Tips 5: Avoid Low Quality Teleconverter, Barlow Lenses and Eyepieces
Teleconverter, barlows, and eyepieces of low quality can introduce CA on their own, but even good quality ones will magnify the CA from your lens or telescope.
And this is true both for photographing and observing.
How To Fix Chromatic Aberration: Two Simple Techniques
Nowadays, CA can be easily corrected and reduced with any post-processing software.
I mostly use Adobe Lightroom or Adobe Photoshop for my final editing and image tweaks, but you can reduce CA in Affinity Photo, Gimp, and all the other Adobe alternatives.
CA can be corrected by using and combining different techniques, and here are two of them I regularly use.
For this, let’s consider the following image, where some mild purple fringe is visible around the stars.
Use The Eyedropper Tool To Sample Color Fringes
In Photoshop, duplicate the level containing the image and go to Filter->Camera Raw Filter to load the current image into Adobe Camera Raw.
In Adobe Camera RAW, magnify the image 100% for accurate sampling.
From the sidebar on the right, pull down the Optics Menu, containing the settings to remove the CA. Here, you can use the eyedropper to sample the color of the CA you want removed, and the software will do the rest.
You can use the sliders to manually fine-tune the settings the program has used on the base of your sampling.
You can further tweak the strength of the correction by reducing the opacity of the level.
Blur The Colors
For this technique to work, you need to use Photoshop and Layers.
The idea is to blur the colors of the image so that CA gets “smeared” away. The procedure is fairly quick:
- Duplicate your image in a new level.
- With the new level selected, apply a gaussian blur using a radius large enough that CA is removed.
- Blend the new level using the color mode.
- With the level of the original image selected and the blurred image visibility toggled off, choose the Select->Color Range function to select the CA around the star with the eyedropper. With Select->Modify->Expand expand the selection of few pixels and Select->Modify->Feather to smooth the selection.
- Use the mask icon to create a mask and drag the mask from the current level to that of the blurred image and toggle on its visibility.
- Finally, you can further tweak the strength of the correction by reducing the opacity of the blurred level.
Both methods are quite simple and fast and give good results.
Conclusion
Chromatic aberration is a very common issue in photography and astrophotography. Using high quality equipment is the best way to keep CA under control, but carefully focusing and proper camera setup will go a long way in reducing CA with low budget and even legacy equipment.
Finally, residual CA can be further reduced or totally removed in post-processing with the simple techniques we discussed in this article.