In this section we’ll very briefly consider three image attributes that occasionally require some correction in post-process.  The first is the issue of tilt.
    When shooting birds that are either perching on the ground or are wading in the water, it is usually important to make sure that your camera is level to the horizon—that is, that it’s not tilted slightly.  For birds in flight or perched on branches it’s usually less critical, since the viewer usually can’t tell how the horizon lines up with the image anyway.  But for birds on land or in water, any more than the slightest tilt can be obvious to the discerning viewer.  In some cases, such a tilt may convey an artistic slant to the image (no pun intended).  In others, you’ll want to correct the tilt.
    The good news is that correcting tilt in Adobe Camera Raw (ACR) is quick and easy.  Simply select the Straighten Tool from the tool palette, and then drag a line (with the mouse) along any horizontal feature that you can find in the image.  The figure below illustrates this.



In the figure above, we’ve dragged a dotted line (using the Straighten tool) along the shallow in-focus region in the water.  This indicates to ACR that the image should be rotated so that the dotted line becomes perfectly horizontal in the resulting image.  In this case, choosing a feature to serve as a guide for the tilt line was simple, because of the shallow depth-of-field (DOF).  In other cases it can be more difficult.  For example, when photographing waterbirds in a small body of water, you may be tempted to use the line of the far shore as the guide; this may or may not be a reliable guide, however, depending on whether the far shoreline is perpendicular to your line of sight.  Likewise, for birds on land you need to be careful not to simply use the horizon as the guide, since the horizon itself may not be level (e.g., in the case of a hill).  A general rule-of-thumb is to try to select a tilt line that traces features of the image that are all the same distance from the camera.  In the case of a shallow DOF (as in the figure above), this will be simple; for a wider DOF, you may need to look more carefully at the image and try to discern the topology of the landscape.
    The figure below shows what happens in ACR when you release the mouse cursor after drawing the tilt line.  ACR clips the corners of the image so as to form a rectangle that is parallel/normal to the tilt line.  And so, here we have the bad news: while correcting tilt is quick and easy in ACR, it results in a reduction in image size, because some corners need to be clipped in order to perform the rotation and still end up with a properly oriented rectangular image.


     In cases of only slight tilt, the resulting reduction in image dimensions may be acceptable.  Larger corrections require larger reductions in image size.  Depending on where the subject is located in the original image, it may or may not end up too close (aesthetically) to the edge of the photo in the resulting, corrected image.  For this reason, when shooting in the field it’s sometimes worthwhile to keep the subject closer to the center of the frame than you might otherwise like.  Though your artistic instincts may tell you to position the bird away from the center of the image (for compositional purposes), if you know that you have a tendency to tilt your camera away from the horizontal and that the scene’s natural features will betray that tilt to your viewer, you may want to err on the side of having the bird closer toward the center of the image so that after tilt correction in ACR you won’t end up with the bird appearing unnaturally close to the (clipped) edge of the frame.
    The image below shows the result of using the Straighten tool on our working image.  Note that the shallow in-focus region now appears level, rather than sloped.


Note that you can also correct tilt in Photoshop proper (rather than ACR) via the Image > Rotate Canvas > Arbitrary menu option.  This is useful if you realize you have a tilt problem only after you've already converted from RAW and invested time in various other postprocessing operations.  The downside is that rotation via Rotate Canvas in Photoshop is a tedious, trial-and-error process, because you have to guess at the angle of rotation.  An alternative is to duplicate the image to a new layer (via Cmd-J / Ctrl-J), activate the Move Tool (by pressing the V key), and then grab the corner of the image with the mouse cursor and rotate free-hand.  The image should immediately rotate in real-time, so you can continue rotating until it appears level to you.
    The next issue we’d like to briefly discuss is color correction.  When using high-fidelity cameras and lenses, color correction is rarely needed for outdoor photography, in my opinion, since color casts in nature photos are often a reflection of the natural quality of light that was present at the time and place that the photo was taken.  Late in the day, for example, light often takes a reddish hue, due to the physics of light in the context of a setting sun.  Attempts to “correct” this reddish hue not only run counter to the nature of the actual scene photographed, but also tend to introduce artifacts that to some viewers may appear unnatural.
    Nevertheless, there are cases in which nature photographs can benefit from some very subtle color correction—and in special cases (primarily sunsets) even extreme correction.  There are several tools in Photoshop and Adobe Camera Raw that can aid in this.  In ACR, there are the Temperature, Tint, Saturation, and Vibrance sliders in the main control panel.  The figure below shows the controls in their default settings, for a rather drab sunset shot of two herons at the famed Venice Rookery in Florida.


Notice in the above image that the red, blue, green, and yellow components of the image’s histogram are largely coincident (i.e., piled on top of each other).  Now look at the next figure (below) and its histogram.  In this case we’ve moved the Temperature slider to its maximum value, which has resulted in a drastic separation of the histogram components, with the larger red component now far to the right and the blue components now further to the left.  (The Contrast slider has also been adjusted to increase brightness in the sky while leaving the silhouettes black).  The scene now looks more like a sunset. 


Futher manipulations of the brightness, saturation, and contrast in Photoshop proper (via the Levels tool and the Saturation tool) then sufficed to create a more striking final image, as shown in the figure below.  (Note that the Straighten tool in ACR was again used to straighten the image, this time based on the relative orientations of the bird’s upper bodies, rather than on the substrate). 

    Note that while saturation can be adjusted directly via the Saturation or Vibrance sliders in ACR, or via the Hue/Saturation tool in Photoshop, there are other ways to adjust saturation which sometimes yield more desirable effects—whether more subtle or bold than is possible with the direct Saturation tools, or simply more controllable in the context of the image being processed.  In particular, any operation which adjusts the brightness or contrast of the image can affect the perceived saturation.  In the figure below, we use the Levels tool to adjust brightness and contrast by moving the black point to the right and the white point to the left.  Note that the resulting image (the bottom pane in the figure) has richer colors, suggestion greater saturation, even though the Saturation tool was not touched.


As already mentioned in section 10.4, this effect of brightness parameters on the perceived saturation can sometimes be detrimental when you want to adjust brightness or contrast without changing the overall saturation.  In a great many cases you’ll find that after adjusting the brightness and/or contast of an image, you’ll then need to use the Saturation tool to restore the original saturation level.  After adjusting the overall exposure of an image in Photoshop I’ll often open the Saturation tool and decrease the saturation by -5 or so.
    A separate issue from saturation is that of hue, or tint.  Whereas saturation has to do with the intensity of color in an image, the hue or tint has to due with the identity of colors.  Adjustments to hue/tint are only required when you perceive an unnatural color cast in your image.  The figure below illustrates this: the bird on the left has had its hue adjusted by -5, which moves the overall color profile toward the red/purple end of the spectrum, while the right image has had its hue adjusted by +5, shifting the color profile toward green/blue.  Notice that the background in the left image has more red than in the right image, whereas in the left image the bird’s cere (the fleshy area anterior to the eye) has a slight greenish cast as compared to the image on the left.


The hue/tint can be adjusted either in ACR or in Photoshop proper (or both).  I rarely find a need to do either.  As mentioned previously, you should be cautious about using these controls to adjust color casts that are natural (e.g., due to environmental lighting conditions and/or color temperature of natural sunlight, such as at dusk, etc.).  A better justification for adjusting the hue/tint is if your equipment imparts an unnatural color cast to your images.  Some third-party lenses, for example, impart a slight yellow cast to images, due to their antireflective coatings (see section 3.6 for an extended example demonstrating such a yellow cast from an 800mm Sigma lens).  Correcting these types of subtle issues can usually be done with a very, very slight adjustment to the hue slider in Photoshop or the tint or temperature sliders in ACR.  Another case for making such adjustments is if you find that the color of your photographic prints doesn’t match the color you see on-screen—an indication of a calibration error in your monitor’s color profile (see section 14.1).