Controlling Contrast in Monochrome Infrared Images

PhotoShop CS2 - Adobe Bridge CS2 – Canon D400 IR-Only

The best monochrome infrared images are those that boldly show off the special characteristics of infrared. Skies are black, grasses and leaves are intensely white, and the image has an other-worldly glow. Not all infrared images come out that way, nor do they need to, but it’s nice when you can get it to happen. This tutorial will explain the techniques that I’ve been exploring to attain that noble end.

Of course you are right to say that those characteristics are required of any monochrome image. The thing is, getting there with a digital infrared image takes some work. The RAW image doesn’t usually come out of the camera looking that way. In fact, by the time you correct for the frequently overexposed highlights, the initial image is usually rather flat.

The whiteness of chlorophyll-filled plants is one of the coolest things about black and white infrared photograph – but it is also one of the greatest challenges. Plants often emit so much infrared radiation that, with a normal exposure, they lose detail in the leaves. The leaves glow so much that the veins and textures are overwhelmed. This is part of the reason that using just Levels and Curves Adjustment Layers does not give you the ultimate infrared look. If you raise the right end of the Curve, the highlights get blown out and lose detail very quickly, because (1) so much of the highlights are at a very similar level and (2) much of the highlights don’t have much detail to start with.

The tools I am working with are a Canon 400D (Digital Rebel XTi) that was converted to shoot infrared-only; Adobe Bridge CS2 with its Camera Raw 3.6 plug-in; and PhotoShop CS2. The methods that I shall explain involve using PhotoShop’s Blending Mode for Adjustment Layers as a way to emphasis the zones that you want to work on. I create Adjustment Layers with the Blending mode set to Multiply to darken the dark parts of an image; and I create Adjustment Layers with the Blending mode set to Screen to bring up the highlights. But it’s not quite so simple – often I’ll Invert the Adjustment Layers and then paint on them to reveal or conceal. I also use the Gradient tool on Adjustment Layers to darken edges. Setting the correct Opacity for these Adjustment Layers is also important.

Just to give you a feel for what I’m talking about, here are the "before and after" shots of the image on which this tutorial is based.

Figure 1. Image after initial conversion to monochrome

Figure 2. Final image

Camera RAW

When you bring up your RAW infrared image Bridge, you’ll often see that the highlights appear to be blown out. Never mind that the histogram on the back of the camera looked fine, with very little information showing at the right (the highlight) end. From what I’ve read it seems that the histogram shown on the back of the camera is reporting more of the green sensors’ input than the red. That’s fine for regular color work because it matches our vision. But in this case there is no real color information reaching the sensor at all, because infrared radiation has a longer wavelength than the light we see as color. But something is hitting the sensor! And it has more of an impact on the red sensors than on the green, so what you see doesn’t reflect the actual amount by which the sensors were affected by the infrared radiation. You just have to get used to it. What you would consider to be a very nice histogram for an ordinary color picture would probably be an overexposure for infrared. I like to start out with images whose histograms show that most of the data is at the left side, with very little to the right.

Here’s how our sample image looks with the Adjust tab selected in Camera Raw, before making any changes to it.

The red areas in the image are overexposed in the Adobe RGB (1998) color space; and the blue areas are shadows that have gone to black (the lamp posts, shadows to the left).

We fix the overexposure by moving the Exposure slider on the Adjust tab to the left until the red spots disappear, and then maybe move the slider just a bit more. When you’ve done that you have "recovered the highlights," meaning that you should be able to end up with an image that has detail well into the highlights with no big areas that don’t have detail. The Exposure value was -0.80 after this operation.

Regain detail in the shadows by dragging the Shadows slider to the left. In this case the final value was 2.

I left the Brightness, Contrast and Saturation alone because they looked OK.

I left the Curve (from the Curve tab) in a straight line, as I couldn’t see that there was much to do to improve the image.

Satisfied that I had all the shadow and highlight detail available, and that the overall contrast was good, I opened the file into PhotoShop CS2.

Convert Image to Monochrome

I recorded a PhotoShop Action that I call "Infrared Setup." It opens a new Channel Mixer Adjustment layer in which Red is set to 75%, Green is set to 25%, Blue is set to 0%, and the Monochrome checkbox is checked. Next it creates a new Levels Adjustment Layer; and last it creates a new Curves adjustment layer. Running that Action is the first thing I do when the image opens up in PhotoShop. See Figure 1 above.

Initial Levels and Curves Adjustments

Levels

Double-click the half-black, half-white circle in the for the Levels Adjustment Layer in the Layers palette to open the Layers dialog. In this case we can see that the large reduction in the Exposure value for the RAW image has reduced the tonal scale. We want to close up the gaps at both ends so that the end has a complete scale, so we move the sliders under the histogram in from left and right until they are under the places where the data begin.

If you look closely at the highlights end of the histogram you can see that the slider position will result in a loss of some data because there are pixels to the right of it. But they are few in proportion to the overall image, and I want the more of the image’s information to be at the brighter end. If the slider were left all the way over to the right, it would be more difficult or impossible to have the brightness I want for the picture elements – too much of the image would be too far down the scale.

Curves

I don’t usually make any changes to the Curves adjustment layer at this point. With ordinary color or monochrome images I usually make a simple S-curve to increase the highlights and darken the shadows. But that doesn’t work very well with infrared images because, despite what we’ve done, there is very little "head room" for the highlights. Raising the Curve at the highlights end even just a bit can blow them out. That’s where using the Blending modes as described below can save the day.

Just the changes to the Levels Adjustment Layer have made an improvement in the image. But too much of the information is in the same narrow tonal range, around Zone VI and Zone VII. Although the image is brighter, it still appears flat.

Figure 3. Image after Levels Adjustment Layer changes

"Darken" Adjustment Layer Using the Multiply Blend Mode

Create a new Levels Adjustment Layer. Name it "Darken," but do not make any changes. Just click OK to get the new layer into the Layers palette. By default when you have clicked OK to create the new Adjustment Layer, its Blend mode will be "Normal."

Figure 4. The Layers Blend pop-up menu showing "Normal" before the change to Multiply

Click on the pop-up menu and change the Blend mode from Normal to Multiply. The image will get much darker. But notice that most of the darkening takes place on the darkest parts of the image – the lighter an area, the less it is affected by this.

Figure 5. Image after creation of the "Darken" Adjustment Layer

We’re going to leave that as it is for now. Let’s create another Adjustment Layer.

"Lighten" Adjustment Layer Using the Screen Multiply Mode

Create a new Levels Adjustment Layer. Name it "Lighten," but do not make any changes. Just click OK to get the new layer into the Layers palette. Click on the pop-up menu and change the Blend mode from Normal to Screen. Now the image is much brighter. You can see that the effect is a mirror to that of the Multiply mode. With the Screen mode the lighter parts of the image are lightened, and the darker elements are less affected.

Figure 6. Image after addition of "Lighten" Adjustment Layer has been added

Invert the "Lighten" Adjustment Layer

That is an improvement, but it has lightened up elements of the image that I want to keep dark. What is needed is a way to selectively brighten the areas I want brighter but leave the rest alone.

Hold down the Command key and press the letter "I." The image returns to the dark appearance it had before you created the "Lighten" adjustment layer. What you have done is "Invert the Adjustment Layer." The brightness change is still there, just hidden.

Paint in the Highlights

Now what we want to do is reveal those highlights and leave the dark areas dark. Press the letter "b" to get a Brush tool. Set the Brush’s hardness to zero. Set the Opacity in a middle low area, say around 30%, and set the Flow to around 50%. Those settings will vary from image to image. In this case I was mostly working pretty fast in broad strokes, just to show how this works.

Press the letter "d" to get the default settings for the "default colors for foreground and background" section of the Tools palette, namely black and white.

Figure 7. the "Default colors for foreground and background" area of the Toolbar

If the white square is not on top, press the letter "x," which causes the black and white values to flip-flop.

We want the Brush to be white because the white Brush reveals what is hidden – in this case it will reveal the whiteness of the leaves that you saw in the Lighten blending layer before you inverted it. Another way to think of it is that the white Brush acts like an eraser – it erases the inversion to let the Lighten blending layer show through.

A black Brush will do just the opposite. It will hide the parts of an Adjustment Layer that you paint over. More about that below.

As John Paul Caponigro wrote, "White reveals, Black conceals."

I made the Brush size fairly large, around 175 or 200 for this 3888 x 2592 pixel size image, and just began brushing over the leaves of the big tree at right, at left center, and the two trees thrusting up from the left rear. I wasn’t especially careful, but tried to avoid letting the brush get into the sky or other dark areas that I don’t want to be lighter.

After I had made the broad strokes I made the Brush size smaller and started getting into the smaller areas like the branches that stand out against the sky. I also used the brush on the lanterns on their black posts.

NOTE: The painting that you do with the Brush can be reversed, so if your Brush should stray into the sky, do the following. Press the letter "x" to reverse the colors and put black on top of the color picker. Then just paint over the places you want to fix. This reverses the process of concealing and puts that area of the Adjustment Layer back to its concealed mode, so it’s just as it was before you painted over it with the reveal mode (white on top of the color picker).

NOTE: By changing the Opacity setting for the brush you change the strength of the "eraser" effect. This can be useful when you are trying for subtle changes. Some parts of an image may require just a bit of lightening, while others may require much more. If you know that you want to make a rather extreme change, as I did here, then increase the Opacity setting. Otherwise you’ll find yourself painting the same spots repeatedly until you have made the degree of change that you want. Changing the Flow is something like changing the amount of paint on a brush – 0% is a dry brush and 100% is a fully loaded one. It seems to make the effects of your painting strokes more powerful as the Flow setting is increased.

Figure 8. Image after painting away some of the inverted Lighten Adjustment Layer on the trees

Paint Away Some Dark Areas

Some of the dark areas are too dark for me, in particular the tree trunks and the lampposts. More can be done to lighten the foliage of the trees, too.

The "Darken" Adjustment Layer that we created is where the darkening can be found and altered. So click on the Darken Adjustment Layer in the Layers palette.

Now press "b" to get your Brush tool, but press "x" to switch the "default colors for foreground and background" so that the black square is on top. Set the brush size appropriately for the dark areas in which you want to reveal more detail. Check the Opacity of the Brush – it should be around 50% or less, because this will have a more dramatic effect than you might think. Then go right ahead and paint over the areas you want to lighten. I painted over the items mentioned above.

How does this work? On the Lighten Adjustment Layer we set the brush to white to show what was underneath; but here the black shows what’s underneath!

You will recall that we Inverted the Lighten Adjustment Layer, but we did not Invert the Darken Adjustment Layer. So the effects of the Lighten Adjustment Layer were hidden, and only by painting over them with the white Brush were they revealed.

The Darken Adjustment Layer was not Inverted, so its effects were not hidden. Instead we took away that which is right there on the surface, namely the darkening effect of the Darken Adjustment Layer. The "base image" showed through in the places where we erased. In this case it was the base image layer plus the Levels Adjustment Layer that showed through.

Image 9. Image after painting away from the "Darken" Adjustment Layer in addition to the previous painting away of the "Lighten" Adjustment Layer

Adjust the Gamma of the Lighten Adjustment Layer

After a bit of that I thought that it still wasn’t lightened enough, so I made a new Levels Adjustment Layer and made it a Clipping Mask for the Lighten layer. I adjusted the gamma of that Levels Adjustment Layer to 105. That gave me the extra brightness I wanted.

See Figure 2 for the final image.