11/4/2022 0 Comments Chromatic aberrationRefractor telescopes using ED lenses are called Apochromatic, and the terms doublet, triplet, and quadruplet refers to the number of ED lenses these telescopes use. Photographic lenses with this type of glass often have “ED” in their name. High quality lenses and telescopes have extra lenses made of Extra Low Dispersion (ED) glass to reduce or eliminate CA. It should be noted here that lenses and telescopes do suffer from a certain amount of both longitudinal and axial CA. There are a few tips you can follow in order to deal with CA when you shoot. The use of low dispersion glasses in a lens or telescope strongly reduces or completely eliminates CA. Purple fringe around the brightest stars.įinally, CA can often be seen around the Moon too. Purple fringe is even more common and is visible around many stars, like in the image below. Other times CA is visible as purple fringes, like purple rings around the bright stars. Chromatic aberration is visible around the brightest star in the image as a blue halo. Optical aberrations such as coma and field curvature will create weird and elongate stars, particularly towards the edges of the frame.Īnd then there is the CA, which is shown in the image as blue halos around bright stars. The reason why astrophotography is so demanding is the need to get both the stars shape and their size right. My Olympus Zuiko OM 300 f/4.5 is a decent legacy lens for daylight and even Moon photography, but the star shape is pretty awful, even stepping the lens down to f/8. How Does Chromatic Aberration Affect Images In Astrophotography?Īstrophotography is very demanding not only for the photographers but also for lenses and telescopes.Ī lens taking wonderful images in daylight can give average or subpar results when used for astrophotography. 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. Lateral CA is visible around the dock poles in this crop of an image I took with an ultra wide-angle lens. I took the image below at dawn with an ultra wide-angle lens, and lateral CA is evident around the dock poles. As the right image above shows, the central part of the frame is CA free. This type of CA affects targets that are off-axis, as it is more evident with light coming into the lens with an angle. The Origin and manifestation of Lateral, or Transverse, CA. 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. 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. Here is a real life example of longitudinal CA: note how the color of the fringe changes with the focus. On the right, how this kind of CA affects the image. The scheme illustrated the origin of Longitudinal Chromatic Aberration. 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. 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. The job of a lens is to focus light on a plane, the focus plane, and all the colors on a point. There are two different types of CA: longitudinal, or axial, and lateral, or transverse. When a lens cannot focus all the different colors in the same point, we have CA. In photography, lenses are made so that light decomposition (dispersion) is greatly reduced or avoided altogether.īut no lens is perfect and light dispersion can still occur, particularly with entry-level gear. When the light comes out from the prism, the colors do not “superpose” anymore, and you can see them individually, forming a rainbow. 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. A prism used to decompose white light in its different components (colors). 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. 7) Conclusion What Is Chromatic Aberration?Ĭhromatic Aberration, also known as purple fringe or dispersion, is an optical aberration common in photographic lenses and refractor telescopes.Ĭhromatic Aberration (CA) is visible as colored fringes around highly contrasted edges, such as branches and buildings edges against a bright sky.
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