Local Contrast Correction in Photoshop

Curves (Curves) – an incredibly powerful tool. They provide the speed and efficiency of the post-processing process like nothing else. However, for a beginner, they may even be too powerful. Do not be afraid! Curves are not as complex or stunning as they may seem.

In this lesson we will go through the main functions. Curves Correction Layer (Curves Adjustment Layer), find out where its functionality comes from, and consider how to effectively use this tool for local photo correction.

Image TO:

Picture AFTER:

Before proceeding to the study of technology, we touch on a little theory. Here is what will be in this lesson:

  1. How Tone, Contrast and Curves work
  2. How to read tone curves and histograms
  3. How to use control points
  4. Order of operations
  5. How to make local contrast corrections using Curves

Ready? Let’s start.

Part 1. Dynamic range

When you create Curves Correction Layer (Curves Adjustment Layer) in Adobe Photoshop, you get a tone curve, a histogram, as well as many other settings and buttons. What is all this?

All essence in the Dynamic range

First of all, remember one thing: each digital image is a grid of tiny pixels.

Further, all pixels have value (value).

Dynamic range (dynamic range) of the image shows how many shades of gray, from black to white, are in the picture. In 8-bit monochrome photography, which we will discuss below, each pixel can have one (and only one) of 256 discrete intensity values, or, more simply, shades of gray. James Thomas recently reviewed color models, and I recommend reading more deeply about how digital photos work.

bar chart (histogram) – this is a convenient visualization. In fact, it represents the image of your image. For any given intensity, it shows how many pixels of a certain shade of gray are in the image. This is a quick way to get an idea of ​​the dynamic range of a photo without looking at it. This separation of photographic values ​​from visual information is very useful. Below we learn how to use it.

Curve (curve) – a way to manipulate the distribution of these tones using graphics, as well as the key point of our lesson. Go ahead to learn more!

Part 2. We read curves of tones and histograms

As Harry Guinness explained: “Tool Curves (Curves Tool) is a graph. On the X axis we have Input (Input) level, Y axis – Output (Output) level. Each axis has values ​​from 0 to 255. “From left to right and from bottom to top:

  1. At the zero point, which is located in the lower left corner of the chart, is located the black (black) color: pixels that have no intensity.
  2. Up and to the right of black go the shadows (shadows) photographers often call them “darks”.
  3. In the middle of the chart medium gray (middle gray), on both sides surrounded by “semitones”.
  4. Even higher and to the right are the light zones, they are also “highlights”.
  5. At the very top right, with a value of 255, is white (white): pixels with full intensity.

The upper part of the curve controls the light, the middle – in semitones, and the lower – the shadows.

The slope of the curve controls the contrast.

Contrast (contrast) expresses the difference between the tonal values ​​of the image. We can designate two types of it: global for the entire image and local for a specific area.

Each new curve is initially a straight line with a slope of 45 degrees. This means that the ratio between Typing (Input) and The conclusion (Output) one to one, the filter does not affect the image.

Changing the slope of the line changes the ratio. If the slope is more than 45 degrees, the contrast increases, if less – decreases.

Moving the curve also controls the levels. Shifting the whole curve down reduces the output value: the photo becomes darker. If you move the entire line up, the output value increases, and the picture brightens.

Looking at the histogram of our example, you can see that most of the pixels are in the middle of the tonal range, almost equally divided between shadows and highlights with a depression in the halftone area. There are also many pixels in dark and light areas. This shows that we have an image with a relatively equal distribution, as well as a good comprehensive exposure. Here is the snapshot itself so you can appreciate the original look:

Extremely flat snapshot directly from the camera – quite normal and the desired result. To protect glare and shadows, digital camera manufacturers cautiously approach the interpretation of raw, linear information from sensors into the photo itself. It’s better to start with a slightly flat result than to lose important information for the sake of excessive contrast!

Strength of contrast

Our gaze always clings to contrasting areas. Think of a polar bear against a background of snow. It’s hard to notice, isn’t it? This is a low contrast. Now imagine a panther in the snow; see it is much easier: it is a high contrast. The whole trick with getting the right amount of contrast (and this is quite subjective) is to make white not too white, and black not too black.

Simply put, contrast is the difference between the light and dark parts of a photo. If you increase this difference, the picture will be larger, and if you reduce it, the picture will become flatter or dimmer. Global contrast corrections are generally helpful, but they are not as powerful when you need to make certain photos perfect. For this we need local corrections.

We use local contrast corrections to add emphasis to the image or its parts, increasing or reducing the difference. We adjust the distribution of tones in the image to maximize the use of the dynamic range and, more importantly, to direct the viewer’s gaze. How you do this is entirely dependent on the content of each photo.

Part 3. How to use control points

You might think that this is all like a tool. Levels (Levels) and be right. However, there is a key difference: Curves (Curves) allow you to use control points to obtain fine-tuning instead of coarse corrections of black, white and gray ranges.

Control points (Control points) are coordinates on a curve. They can be pulled up or down to change the relationship between Typing (Input) and The conclusion (Output). The location of the point affects which part of the tonal spectrum you want to change.

To demonstrate this, I made three control points in our demo photo: one near the bottom, another in the middle, and a third near the top. I adjusted them to get an S-shaped curve. You can see how it affected the image:

The shadows became darker, the glare brighter, and the half tones remained almost intact, but received more contrast. The S-shaped curve adds contrast to the semitones, reducing it for highlights and shadows. (It also enhances the saturation of the color image.) Such a curve is a frequent step for global correction.

Moving control points may not seem intuitive, but you just need to click on them and then drag them to the right place.

When a point is highlighted, it can also be moved using the arrows on the keyboard. This helps with subtle corrections. Click Tab, to switch between points.

Drag a point behind the chart if you want to remove it.

Part 4. Order of operations

Now that we have gone through the basics of curves and control points, let’s consider the process of adjusting local contrast.

Step 1. Perform all global corrections and adjustments.

I know you can’t wait to start improving your photos right now, but make sure every shot goes through global corrections before moving on to local work. If you start local adjustments on one photo before the general work is completed on other pictures, your group of images will not be harmonious, and the process of their correction will become more complicated. Always place photos from the group at the same post-processing stage.

In the case of our example, as well as in most of your photos, a smooth S-shaped curve will suffice for global improvements.

Step 2. Make the necessary local corrections.

Some photos need additional processing. Before you do local adjustments, make local corrections. This includes vignetting, barrel distortion, fringing, chromatic aberration, etc. Adjusting these things after local adjustments made will be much more difficult, so fix them now.

Step 3. Rate this picture and make a plan.

Good. Global corrections and adjustments are ready, local corrections are made. Now you can clearly see your photo. Look at her. What is she talking about? What is special about her? What sensations does it cause? Write a note about these things in your workbook.

What does this photo need? What elements do you want to emphasize? Are there specific areas of the image that need more contrast? Maybe there are areas whose contrast should be reduced! You should ask yourself these questions when deciding how the picture should look. Write the answers to these questions in your workbook.

The adjustment process can become intuitive when one change leads to another, but it must always start from your assessment of the picture, your imagination about it, and also how far you want to go.

Step 4. Make local adjustments

For each large area that needs to be improved, create a new adjustment layer. More about this process below.

Step 5. Re-evaluate and compare

After a few adjustments, stop and look at your photo again. Did the image you created when creating the plan come out? Perhaps the understanding of the picture has changed. This is normal! If this happens, write a note in the workbook.

Also compare the image with the rest of the photos in the group. Do they still match? If not, try to correct some corrections.

Part 5. How to make local contrast correction using Curves

As was seen in the previous steps, the S-shaped curve gave a good result to our image at the global level. However, there are several parts that need local adjustments.

The sky is still rather flat, I would like to make it more dramatic. I really like the diverse and contrasting textures of the image — the hardness of the tree, the opulence of the clouds, and the way they create an atmospheric depth feeling. I want to visually emphasize all this. How do we achieve what we want? My goals are to make the sky darker, the clouds are more voluminous, and also to highlight the darker shadows around the boat and the beach. Each of the plots must be adjusted separately.

Step 1. Add a Curves Adjustment Layer

Create Curves Correction Layer (Curves Adjustment Layer), you will find it in the menu Newadjustment layer (New Adjustment Layer) on the panel Layers (Layers Panel). Add a control point on a curve in the tonal range you want to adjust.

Using the selection “in the image” (hand with index finger), I highlighted the sky:

This tool shows me where the tonal range of the sky is on the curve. Clicking and dragging the point down, I darkened it. Be careful – the changes affect other areas with the same tonal range, in my case they touched some parts of the rocky beach. Creating new corrections, you will need to compensate for this change.

The specificity is that with the help of curves you can do much more than using levels.

Step 2. Use layer mask to limit corrections in certain areas.

As with all adjustment layers, we are not working directly on the pixels. The curves layer is white. Mask (Layer Mask). It can be painted over to remove the effect of the layer on certain parts of the photo.

Now remove unwanted corrections in the image with Maskslayer (Layer Mask) and Brushes (Brush).

Using a soft brush with a low opacity (opacity), as shown below, paint over areas that do not need adjustments. In the case of our example, you can see that the wheel in the foreground and the shadows on the boat have become too dark.

This is the layer mask shown in red (click with the key pressed Alt on the thumbnail of the mask), so you can see which zones I painted over. I processed some areas more strongly, with a brush with about 50% opacity, to get a gradual effect without rough transitions.

In the upper image, the places where the filter is blocked by a mask are marked in red. Everything else is amenable to effect. Here is the result:

After corrections

Now the sky looks much better. The next step is to make the boat, clouds, beach and glare on the water stand out. Therefore, we will create a new Curves Correction Layer (Curves Adjustment Layer).

Step 3. Repeat!

Following the same method, I used the selection “in the image” to find the point of glare on the boat (circled) and dragged it up, lightening the area.

This correction affected the tone-like parts of the entire image. The clouds, as well as the bright areas of the beach and sky, are now brighter. This is not what I wanted, so we will need masks again.

Since this time we are adjusting smaller areas, it’s easier to invert the layer mask (Ctrl + I), and then work, painting the necessary areas, rather than hiding unnecessary ones.

In the image above, the places where the changes are not applied are painted in red, and the image is lightened in the lighter pink areas; I focused mainly on the boat, and also slightly increased the stones, the glare on the water and part of the clouds.

After applying the correction

Potential problems

As with all editing methods, there must be a trade-off. With curves, you stretch or shrink tones. If you go too far, you get weird results; this is often called posterization. It happens if the output tones are stretched:

If you try to lighten and darken areas with a similar tonality, get posterization.

The fact that the screenshot above is a gross exaggeration (you will never use a similar curve), but demonstrates the effect that can be obtained if you overdo it with a certain part of one layer of curves.

When in doubt, work from big to small.

If you can’t decide everything on one layer, don’t worry: just add a new one. It is much easier to work with one layer for each “problem”, treating large areas first and then smaller ones.

Use different layers and focus each on a specific task. This will help you stay organized (do not forget to name the layers!), And also allow you to make edits in the course of building the image.

Almost done

As with the majority of such corrections, it is often better to go a little far, knowing that you can later reduce the opacity of the layer if you need to weaken its effect. Try to merge your edits into a group. Just click on the folder icon at the bottom of the Layers panel, and then drag the desired layers into it:

Then you can cut opacity (opacity) group – it will affect all layers that are in it. Alternatively, each individual Curve can be adjusted. I reduced the opacity of all my layers to 75%.

Is done

Good, exhale. Let’s take a look at our shot again.

The original photo looked a bit flat; everything on the histogram was grouped around halftones. There was not much difference between the light and dark areas. Adding a little global contrast helped, but that was just the beginning! Sometimes we don’t need changes to the whole photo. If you have a lot of tonality in a similar range, as is the case with our example, local adjustments will be needed to attract the viewer’s eye.

The image I started with had little difference between boat, bus, beach, or sky. Each of these elements had its own distinctive feature, so it was reasonable to use local corrections to emphasize them. Since the clouds were already fairly light, the darkened sky helped them to stand out. Also, by concentrating on the glare of the boat, the beach and the water with the help of curves and layer masks, we were able to create brighter zones to attract attention without overdoing places that were already light. Now I like the contrasting textures of these areas.

Now you can subtly own curves

Curves (Curves)! What a great tool. They get better with each version of Photoshop. I highly recommend taking on this feature and familiarizing yourself with the settings and effects that appear for different types of photos.

Starting with black and white pictures is easier, since you do not need to worry about excessive saturation, and it is also easier to see the changes. Also, monochrome images are great for high contrast, so they are comfortable to practice. Of course, not all images need additional contrast, and the curves can be no less efficiently used to reduce it!

First, try working with presets and watch what effect each of them has, then use this knowledge to more accurately correct your shots. Remember that you need to keep the correction light and realistic. Otherwise you will get anomalies in the image. All adjustments can be corrected by using the layer mask and removing all errors.

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