What is chromatic aberration?

Chromatic aberration is an optical phenomenon in which the camera lens is unable to focus the different wavelengths of light on the same plane (in this case the image sensor) in order to produce a correct image, resulting in a halo or fringe around objects.

This is especially noticeable in high contrast situations and when shooting at wide apertures. Such a situation is presented below, where the hands of a statue were shot against a bright sky, resulting in a bad case of purple fringing.

Image taken by Mike Baird

It should be noted that color fringes may also be caused by other factors such as lens flare or the camera's sensitivity to the different wavelengths of light but since chromatic aberration is the most common cause it will be the main subject of this article.

The (somewhat) simple physics

As you might imagine, DSLR lenses are a complex piece of equipment but for the sake of simplicity, let's consider them as a single piece of glass.
When light hits the lens it is refracted, ie, it changes direction. As most people probably know just by looking at the rainbow, visible light is comprised of different wavelengths which are perceived as different colors. What happens in this situation is that the different wavelengths are bent differently by the lens, causing them to diverge (a phenomenon known as dispersion) and hit different planes, as shown in the following diagram:

This will cause the sensor to pick up "misplaced" colors at certain spots, resulting in the familiar chromatic aberrations.

Camera lenses and chromatic aberrations

In order to correct this problem, lens makers came up with ways to minimize the divergence of light waves. The cheapest solution is usually used in consumer level optics like the common 18-55mm kit lenses and is known as an achromatic lens design. It uses a second lens that has a different dispersion than the main lens and tries to converge the light rays in order to try to correct both spherical and chromatic aberrations that light suffers when passing through the first lens. However, this isn't a perfect process and small imperfections are still present.

A simplified diagram demonstrating how achromatic lenses work

What is used in higher end lenses is an apochromatic lens design, in which a third element is added, correcting even further the aberrations.

A simplified diagram demonstrating how apochromatic lenses work

In addition to this design, lenses like the Canon EF 70-200mm F2.8L IS (and all other L lenses, for that matter) contain super-low dispersion glass, which as the name implies, doesn't disperse the different wavelengths as much, minimizing this problem. Because low dispersion materials like fluorite are usually used, the final lens cost is driven up quite a bit, which partly explains why they cost so much in the first place.

How to avoid it on the field

Avoid high contrast situations

This might seem like a no-brainer but it really helps to downplay the optic design imperfections, especially if using consumer level lenses like most of us do. Common suspects include fences, tree branches and shooting a subject against a bright sunny sky.

Stop down your aperture

While it is very tempting to use a wide aperture to get nice bokeh, if you get some fringing try to stop down the aperture at least one stop. This will greatly help minimize the visible aberrations.

Avoid the extremes of your zoom lens

Not only in life but also in photographic lenses it is said that virtue is in the middle. Zoom lenses usually perform better when using its middle focal lengths. For example, when using a 70-200mm zoom, shooting at 135mm will usually wield better results than at 200mm.

Avoid super zoom lenses

Unless you don't want to carry extra weight, like when going on vacations, avoid super zooms at all costs. They have terrible chromatic aberrations on the wide end. Alternatively, get some prime lenses, as they are generally optimized to reduce these optical artifacts and are relatively cheaper and lighter.

How to correct it in Photoshop

After opening your image in Photoshop, go to Filter -> Distort -> Lens Correction…

On the right side panel you should find a set of controls named Chromatic Aberration.

Depending on your image, you should try to correct the fringing in the best way you can by adjusting the sliders. To do this in an optimal way make sure you are viewing the image at a magnification of 100% or more.

You might have to move only one or both sliders: it all depends on the image you’re dealing with. In this case, I had to move both.

Moving the Fix Red/Cyan Fringe slider to the left will correct red fringes, while sliding to the right will conversely correct the cyan fringes. The Fix Blue/Yellow Fringe slider works in a similar fashion.

The Before and After Using Photoshop

Color Models

Depending on your background and your purpose, you will view primary colors differently. There are three generally accepted models: RGB (red, green, blue), CMY (cyan, magenta, and yellow), and RYB (red, blue, yellow).

Primary color models can be divided into two categories: additive and subtractive.

The additive primary colors are obtained by light: red, green and blue (RGB). When combined, these colors create white. Scientists in the late 19th century established that color perception is best described in terms of the primary additive colors RGB because it is based on the human perception of colors.

The subtractive primary colors are obtained by the subtraction of light: cyan, magenta, and yellow (CMY). They form the basis of printing colors and combine to form black, the K in CMYK. The second set of subtractive primary colors is: red, yellow and blue (RYB). This model is primarily used in art and design education, particularly painting. According to Johannes Itten, a Swiss color and art theorist, the primary use of this color wheel is for mixing pigments.

Color Model Application:

RGB: Photography, video/film and design.

CMYK: Print design.

RYB: Art, painting and design.

Now that we’ve covered the various color models and their uses, we can proceed with confidence. We will be using the RGB color model.

RGB Color Model

Primary, Secondary, and Tertiary Colors

In the RGB (additive) color model, the Primary colors are: red, green, blue.

The Secondary colors: cyan, magenta and yellow, are formed by the sum of two primary colors of equal intensity.

Lastly we have (six) Tertiary colors, which are comprised of mixing half primary color and half secondary color: orange, yellow-green (lime), cyan-green, cyan-blue, blue-magenta (purple), red-magenta (pink).

The additive and subtractive sets of tertiary colors RGB and CMY are exactly the same, just on different points of the wheel.

“The color wheel not only helps understand the relationship of different colors but also the classification of colors. It also provides a quick reference to the primary, secondary and tertiary hues.” – Mark Boulton

Color Harmonies

Monochromatic

Monochrome selections are simply one color from the color wheel.

Complementary

Complementary color schemes use colors that are opposite each other on the color wheel.

Split-complementary

The split-complementary color scheme is a variation of the complementary color scheme. In addition to the base color, it uses two colors adjacent to its complement.

Analogous

Analogous color schemes use colors that are next to each other on the color wheel. Analogous schemes are often found in nature and are harmonious and pleasing to the eye.

Accented Analogic

Accented analogic color schemes are combinations of analogous and complementary color schemes. In addition to colors that are adjacent to each other, it uses a complementary accent at its opposite.

Triad

A triadic color scheme uses colors that are evenly spaced around the color wheel.

Tetrad (double complementary)

The tetradic or double complementary color scheme uses four colors arranged into two complementary pairs.

Square

The square color scheme is similar to the tetradic, but with all four colors evenly spaced around the color wheel.

Warm and Cool Colors

The color wheel can be divided into warm and cool colors. Warm colors are vivid and energetic, and tend to advance in space. Cool colors give an impression of calm, and create a soothing impression. White, black and grey are considered to be neutral.

Term Definitions

Let’s take a look at the meanings of a few color-related words. These words can often confuse and be erroneously substituted for one another. This following list of definitions will help us proceed unambiguously. All definitions are taken, of course, from The Merriam-Webster Dictionary.

Hue

The attribute of colors that permits them to be classed as red, yellow, green, blue, or any intermediate between any contiguous pair of these colors.

Saturation

A chromatic purity; freedom from dilution with white.

Brightness

The attribute of light-source colors by which emitted light is ordered continuously from light to dark in correlation with its intensity.

Tint

A variation of color produced by adding white to it and characterized by low saturation with relatively high lightness.

Tone

The color quality or value; a tint or shade of color; the color that appreciably modifies a hue or white or black.

Shade

A color produced by a pigment or dye mixture having some black in it.

Chroma

A quality of color combining hue and saturation.

Hue, Saturation and Brightness

Hue

In practical terms, hue refers to a specific tone of color. It is not another name for color as color can have saturation and brightness as well. For instance, colors with the same hue are distinguished with adjectives referring to their lightness and/or chroma, such as with “light green” or “pastel blue”.

Saturation

Saturation is the intensity of a hue from grey. At maximum saturation a color would contain no grey at all. At minimum saturation, a color would contain mostly grey.

Brightness

Brightness refers to how much white or black is contained within a color.

The image below (left) of Photoshop’s Color Picker shows the saturation and brightness of the cyan hue. The saturation is increased by dragging the picker horizontally towards the right of the box and decreased towards the left. Brightness is increased by dragging vertically towards the top of the box and decreased towards the bottom. The hundred percent mark of a hue’s saturation and brightness is the upper-right corner.

Examples for hue, saturation, and brightness are best shown in the hue/saturation adjustment layer. Each of the three is affected separately and the display sliders intuitively show the principles referenced above. See the example below (right).

Conclusion

Cognizance of colors and/or combinations that are pleasing, and savvy use of post-production tools are just two of the ways that comprehension of color theory, relations and harmonies is useful to us. We have already put this knowledge to use to improve and stylize images in How to Use Curves in Photoshop and Color Correction Basics in Photoshop.

We will continue to use color theory insofar as every photograph can be improved with the post-production tools within Photoshop, Lightroom, or other image-editing software. Specifically, a few adjustment tools where color theory knowledge helps include: Curves, Hue/Saturation, Color Balance, Channel Mixer, Selective Color—and the list goes on. The mastery of color theory, relations and harmonies is one of the primary steps to uncovering the full beauty and potential of your images.

I will be using Photoshop CS4 for this tutorial, but all of the features I will use are available in previous versions of Photoshop. Or in any decent photo-editing software.

Primary Color Correction

In this tutorial, our work will be primary color correction. “Primary color correction affects the whole image, utilizing control over intensities of red, green, blue, gamma (mid tones), shadows (blacks) and highlights (whites).”

Overview

Before we begin, you’ll want to be sure that you have at least a novice understanding of the following tools in your photo-editing program: Curves, Hue/Saturation, Photo Filter, and Black & White adjustment layer.

Monochrome and Sepia

A monochromatic image is one whose range of colors consists of shades of a single color or hue.

Step 1: First, let’s make our image grayscale. There are various ways to do this, but we’ll look at just two:

I think the best way is to use the Black & White adjustment layer. (This can be found at the bottom of the Layers toolbox.) What is so great about this adjustment layer is the ability to control the luminance of the 6 primary and secondary colors – red, green, blue, cyan, magenta and yellow with the sliders. This is a very useful tool for fine-tuning the the tonal range of your image.

However, the Black & White adjustment layer is only in Photoshop CS3 and newer. So, if you’re using an older version, skip to the next step.

Step 2: Apply the Hue & Saturation adjustment layer. Click the Colorize checkbox and this will automatically desaturate and tint your image. However, the Hue is wrong. So set the Hue to 35. Now it’s starting to look right.

Step 3: Lastly, perfect color brightness of your image with the sliders in the Black & White adjustment layer. Use the eyedropper tool that comes with the filter to select the part of the image you want to alter and it will hightlight which color is in range. But be careful not to drastically change the colors or you’ll be in danger of posterizing the image.

Usually changes solely in the Black & White filter are not enough to correct the image; so apply a Curves adjustment layer on top and get your image looking right. (It’s best to do these steps last – after your color adjustments – so that you’re seeing and effecting the color-corrected spectrum.)

And, for sepia, you’re done.

Rollover Image

In addition, using other monochromatic color schemes are done the same way. Instead of setting the Hue to 35, move the slider to the color of your choice. You also could up the Saturation a bit to make the colors pop. (For the image below, I set the Hue to 295 and the Saturation to 35.)

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Bleach Bypass

A popular image treatment is the bleach bypass. In actual film-developing, this means skipping the bleaching process. This leaves a silver tint on the photograph, as if the black-and-white and color image had been combined. So that’s exactly what we’re going to do.

Step 1: Apply the Black & White adjustment layer once again to your image. (Or desaturate with a Hue & Saturation adjustment layer.) Then, the only alteration we need to do is in the blending mode: change it from Normal to Overlay.

Step 2: Apply a Curves adjustment layer on top. A characteristic of most bleach-bypassed images is increased contrast. So add a contrast curve to your image. But be careful. Adjustments should be minor at this stage because the image has a lot of contrast. Make sure you’re not overexposing in the highlights or underexposing in the shadows.

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Quick Color Theory

Before we begin working on our next style, we’re going to talk quickly about color theory.

Take a look at the color wheels below. You’ll notice the three primary colors in our RGB color model: red, green and blue. Then, in between those you’ll notice the secondary colors of CMY: cyan, magenta, and yellow. Secondary colors are formed by the sum of two primary colors: cyan is green + blue, magenta is red + blue, and yellow is red + green. Simple enough.

The secondary colors are opposite on the color wheel to the primary colors. Hence:

Red – Cyan

Another six tertiary colors are created by mixing primary and secondary colors. But we don’t need to get into that in detail right now.

The primary and secondary colors, and their relation to each other, are important for us to know. In the Curves adjustment layer you can effect color in the red, green, and blue channels separately. So, for instance, any adjustments upward of the diagonal line in the red channel increase the red in the image. Lowering, below the diagonal line, increases the cyan. The other channels are the same: Upward in the green channel, green; lower, magenta. Upward in the blue channel, blue; lower yellow.

Blockbuster/Fashion Look

Ok, now that we got that covered, let’s begin. This style is used a lot in big summer movies, such as the Transformer films. It’s also popular in fashion photography.

First off, take a look at the picture above. In an effort to create this look, determine the color hue and tint of the shadows, midtones, and highlights. Most importantly, the shadows and highlights. You’ll notice that the shadows are very bluish and the highlights – mainly the part of the image containing the skin tones – are, well, the color of skin.

This is a complementary color scheme. Meaning the colors used, blue and orange, are opposite each other on the color wheel.

When working with humans in your compositions, you have to take the skin tone into account. You can’t have people turning green, magenta, or blue. That would look odd. Skin will always be between orange, orange-yellow (tertiary), and yellow. Therefore to complement the subject, generally use opposite colors: between cyan, blue or blue-violet (tertiary).

Now that we know this, let’s stylize our photo.

Step 1: Apply a Curves adjustment layer to your image.

Step 2: In the color dropdown menu, change from RGB – which affects the overall brightness and contrast of the image – to blue. We’ll work backwards up the list, starting with blue and ending with RGB.

What we’ll want to do is bring the lower end of the curves slider up. This will increase the blue in the shadows. Next, to compensate for our first adjustment, we’ll want to decrease the blue in the highlights – taking it towards yellow – so that the skin tones stay closer to being correct.

Step 3: You’ll notice that the image has quite a purple tint to it. This is because the green and red channels are at equal intensity. To get a more blue color you can either go to the green channel and raise the shadows, or go to the red and lower the shadows. But there’s a difference here and you would do well to note it: If you were to raise the green shadows, the image would get slightly lighter and loses contrast. If you lower the red shadows, the image gets darker and gains contrast. The first is subtracting color and the second is adding.

In most cases, you’ll want to go with the option that subtracts, not adds.

And that’s what we’ll do. Go to the green channel, and bring it up just enough to take the purplish-tint out of the image. This is where you can decide between how much blue-violet or cyan you’d like in your blue-tinted shadows.

Step 4: At this point, you may need to go back to the blue channel and make sure the skin tone looks good. Not too blue and not too yellow. Adjust the highlights and midtones until you’re satisfied.

Step 5: Now we can go to the RGB or master channel. Finalize the luminance and contrast of your image. All I had to do with this image was lower the shadows.

I’m sitting pretty with a great look.

Step 6: Lastly, you can complement your image with one more filter. Increasing the blue in the shadows flooded the image with a bit too much color. Add a Hue & Saturation adjustment layer and drop the saturation to around -10.

The change is ever so slight. But, it’s perfection. And now you’ve got a blockbuster or fashion-style image. Smashing.

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Bronze

Like in the film, 300, you can give your image quite a cool bronze look. This is similar to a sepia effect; however, the image shouldn’t be monochrome. Vibrant colors and those opposite to the bronze tonal region are still visible and actually stand out nicely, while the lights and the darks take on the bronze tint.

Let’s use a picture with more colors and a larger tonal range. It’ll be easier to see what’s happening.

Step 1: Start by applying a Photo Filter adjustment layer to your image. Change the filter color to Sepia and set the density between 90 to 100 percent. Make sure the preserve luminosity box is checked.

Step 2: Add a Hue & Saturation adjustment layer next. Desaturate the image anywhere from -10 to -50. Do what looks good to you.

Step 3: Add that trusty Curves adjustment layer. The bronze-style looks quite good with extra contrast. Make a nice contrast curve.

Step 4: An optional step, one which I think looks good, is to add a little diffusion. You can achieve this by duplicating your image layer (and keeping it under the effects). Next, apply a gaussian blur to the layer. Do not blur too much! If you put too much blur, your final image will look like it’s from a dream sequence – very soft.

For this picture, I’m using a 2.5 pixel blur.

Change the blending mode of the blurred image to Overlay. You’ll notice how contrasty your image gets. To keep it from getting too soft, change the fill anywhere from 25 to 50 percent. Then, compensate for the contrast accordingly with your curves layer.

That’s it for the big, bold, bronze look. This kid is vicious on his scooter. Watch out.

Rollover Image

Conclusion

Bear in mind that there are many ways to achieve these looks and styles. There is no correct process or proceedure. Different colorists use different tools. To each his own. Find out what works best for you.

Also, remember that each picture you add these effects to will react differently. You shouldn’t expect these settings – or one specific procedure – to work equally well on all your pictures. It will vary due to the levels or amounts of shadows, highlights, colors, and contrast, etc.

The tendency can be to go overboard with effects. Do your best to use them sparingly and appropriately. When in doubt, less is more.

Go out and take your knowledge further by emulating styles and effects you see in magazines, films, or on the web. That’s a great way to learn. Have fun.

Even if you’ve taken a good photo, chances are it can be improved or it needs to be adjusted to work in a collage or collection. Or even to just to intensify a mood. You can always make a good thing better – and curves is a one-stop-shop way to do that.

With curves you are able to:

• Adjust the over-all contrast or tonal range
• Adjust the local contrast or tonal range
• Adjust the color

Let’s jump in and find out how. It’s simpler than it looks.

Overview

The idea behind Curves is all about re–mapping values. A pixel starts out at a certain brightness, and you change it to be brighter or darker.

The curves box opens as a straight line because you haven’t made any changes yet. That means that the brightness values before and after are the same. You will effect a change by changing the shape of the curve.

The points from left (bottom) to right (top) affect: blacks, shadows, midtones, highlights, and whites. By altering the position in these regions will affect the corresponding tonal range of your image. Leaving the line in the center will leave the tones unchanged.

You begin altering the brightness values by clicking once somewhere on the line. This will establish a “point”; this point can now be dragged to a different place within the grid, which causes that tonal value to change, either lighter or darker depending on whether you drag it up or down. The reason it’s a curve is so that the change blends smoothly throughout the image. An abrupt change in value can be very noticeable. The increasingly gradual change of the brightness values on either side of the change permit a very smooth and believable adjustment.

It’s important to note, however, that you can’t increase contrast in one region without decreasing it in another. The curves tool redistributes contrast. Therefore think of the image having a contrast allocation or budget and you need to decide how to best spend it.

Also, the curves tool will preserve the tonal hierarchy (unless you use uncommon negative slopes). That means that the brighter parts of the image will stay brighter even after your conversion – just maybe not by the same amount.

S- and Inverted S-Curves

Rollover Image

The S-Curve and the Inverted S-Curve are two curves most commonly used. The S-Curve adds contrast to the midtones while subtracting from the shadows and highlights. The Inverted S-Curve does the opposite.

Often in photography, it’s difficult to expose your image perfectly. Brightness or darkness in tonal ranges can benefit from optimization. The S-Curve is often useful in these cases – not to mention, quick and simple.

Empty Tonal Range and Histograms

Rollover Image

One very useful and important function of curves is to correct empty tonal ranges – in the histogram edges (blacks and whites) or gaps in between (shadows, midtones, and highlights). An under exposed image can be helped by pulling in the black and white points to correct the exposure.

Or if there are gaps in between the tonal peaks you can decrease contrast in specific parts of your image – thereby freeing up the contrast to be used in the more visible areas of your image.

Clipped Highlights

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Images containing a bright light source, such as the sun, can often be harsh or posterized (also called color banding). Posterization of an image entails conversion of a continuous gradation of tone to several regions of fewer tones, with abrupt changes from one tone to another. This can create an unrealistic look, and often a smoother transition to white is preferred.

Correcting Color Balance

All curves thus far have been applied to RGB values of luminosity. But they can also be used on individual color channels to correct color casts in specific tonal regions. Often the color in an image is correctly balanced, but due to reflection or a light source with a varying temperature or color, you may see unwanted tints in a tonal region. Changing the white balance or adjusting the overall color would inadvertently harm the other tones. So we can selectively increase or decrease the amount of a color cast in the red, green, and blue channels to achieve perfect balance.

Any adjustments upward of the diagonal line in the red channel increase the red in the image. Lowering, below the diagonal line, increases the cyan. The other channels are the same: Upward in the green channel, green; lower, magenta. Upward in the blue channel, blue; lower yellow.

RGB color images should be thought of as being comprised of a composite channel and three grayscale channels containing the values of the three colors – red, green and blue. This is shown in the example above of the red on the left, green in the middle, and blue on the right.

Window > Channels to see this on your image.

Rollover Image

You can see in the image above that there is a slight blueish cast in the color tone. Not to mention, the image is slightly washed out (improperly exposed).

As you can see, the sky is already quite white, so we won’t want to effect the highlights and above. By lowering the curve in the midtones and shadows, without effecting the highlights, we solve the exposure problem. Then we’ll get rid of the blue color cast: By lowering the blue in the low end, we eliminate the problem and the gowns go to black – as they should be.

Blending Modes

Also, curves adjustment layers (Layer>New Adjustment Layer> Curves) can be set to make curves only apply to a channel – such as Color and/or Luminosity – which allows for further, varied control. Another benefit is that it can make your adjustments more subtle through use of the opacity controls for the layer.

Notes

Practice makes perfect. The more you use the tools and techniques available to you the better you’ll get, improve your photography, and have fun.

Here are some things to remember when using the curves tool:

• Minimize use of the curves tool, as anything which stretches the image histogram increases the possibility posterization.
• Avoid the use of the curves tool on an already altered image.
• Perform curves on 16-bit images when possible. (Image > Mode > 16 Bits/Channel)
• For extreme levels of color correction, consider applying curves using LAB mode.