Welcome to lesson 2. The journey continues with Chromatic Aberration, a slightly more troublesome issue than vignetting, although it can usually be corrected quite easily.
Have you ever seen the effect of a prism on white light? Light in all lenses are bent at an angle which depends partly on the type of glass, partly on the angle at which the ray enters the glass and partly on the colour of the light. White light contains light of all colours from violet to red and each if refracted (bent) at a slightly different angle, which creates the rainbow effect.
Photo lenses use individual elements with complementary characteristics to cancel out this tendency to create rainbows but rarely succeed completely. Most lenses exhibit some level of chromatic aberration, that is either they are (1) unable to bring all wavelengths of color to the same focal plane, or (2) they focus different colors at different positions in the focal plane, or both. In the first case, the aberration is called longitudinal, in the second it is lateral.
APO lenses (aprochromatic) are lenses in which in-focus areas of the image are corrected for this aberrations for 3 colours (typically Red, Green and Blue). Very high quality lenses extend this correction to the out of focus areas and further out towards violet and infra-red colours (important because our camera sensors are sensitive to a broader variety of wavelengths than our eyes). Very few lenses, whatever their price and design, extend this perfectly to the out of focus areas.
In others, chromatic aberrations manifests in two distinct flavours:
What it does to the image
This is the most commonly seen form of chromatic aberration and creates purple halos along high contrast edges. You can see it on my reflection on this shiny motorbike. In benign cases such as this, the effect is barely visible unless you enlarge the image (click any picture for a 100% view). But in more acute cases, the photographs loses its clean aspect. Colours look a bit muddy and the image seems blurred.
What it does to the image
Here’s a 100% portion at the center of a high contrast scene. You can see the blue smearing that surrounds the white trellis.Blue is out of focus. What you’re seeing is a sharp image with blue that is out of focus (therefore fuzzy).
In the background, you see what is referred to as sperochromaticism (see further below).
A second form of Loca is visible in out of focus areas. Even in very high quality lenses, such as this high-end portrait lens, below, you can find tell-tale red and green edge lining along high-contrast edges. Red is typically inside focus (closer to the camera than the plane of focus) and green outside focus.
Below is another example with an older lens, also at 100%.
When LoCa is combined with spherical aberration (discussed in a future episode), you obtain spherochromaticism that produces green/ping out of focus blobs, seen above.
Below is the complete photograph, which shows the real-life effect of this hard to correct aberration. You can decide for yourself whether it is an issue for you or not.
The point here is not to dig into the physics of these aberrations but to understand that chromatic aberrations come in several flavours and create variously damaging effects. When you test your lens, what is importat is to be able to distinguish between spherochromaticism, which is hard to correct, and other forms of chroatic aberrations, which aren’t in most cases.
Find a high-contrast scene such as the edge of a sunlit window, tree branches against the sky or snow, reflexions in chrome, the sun shining through tree leaves … Make a series of photographs at various apertures with the contrasty edge successively in center and off-center.
Then review your photographs and look for purple and red / green fringes as well as pink / green blotches along the high contrast transitions, in or out of focus.
If your lens has a lot of chromatic aberration, you can try to avoid it by not shooting very high contrast scenes at wide apertures. Closing down usually helps prevent chromatic aberration significantly. Aperture has more effect on LoCa than on purple fringing.
If your photographs show signs of this aberration, the best way to correct it is, as for vignetting, to use lens profiles.
If profiles aren’t readily available, most editing software will let you use dedicated sliders that gradually correct chroma problems. Set these at the minimum setting that cures the problem satisfyingly. This type of correction is somewhat destructive and severe cases can lead to some loss of sharpness. Most software is set to correct purple or green, by default. See these sliders in Adobe LightRoom, below:
If moving the Amount sliders doesn’t cure the issue completely, the colour settings of the correction slider may not correspond precisely to the fringing in your photograph. Adjust the colour sliders to so as to include your hue into the range of frigne colours to be detected and corrected.
I generally shy away from lenses with really strong chromatic aberration, even for black & white work because it produces fuzzy edges that I don’t find visually pleasing and even post-processing correction can degrade image quality.
However, some soft-focus lenses such as the old Boyer Opale rely on this aberration for their specific look.And don’t think that because a lens has some chromatic aberration, it is a poor lens. In the examples below, one lens is a 40 year old medium format lens which I used with a tilt-shift adapter and the oher is a range-topping 35mm lens from one of the world’s most prestigious brands. That lens is incredibly good and I can’t see myself ever selling it. Yet it does exhibit almost as much lateral colour as the much older one.
So, as usual, test and judge for yourself what works for your tastes. And have fun !
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