The graph gives you an idea of image tones and tells you whether it's too bright or too dark (Window > Histogram). The histogram is a graph that shows different shades side by side. The left side of the graph shows you the darkest tones (a group of darker shaded pixels), while the right side shows the lightest tones (a group of lighter shaded pixels), and the middle shows the midtones. If your peak cuts high into the side of the box, this means that you have a number of pixels in your image that are either completely black or white, and therefore ‘clipped’. If your histogram looks like a comb with lots of gaps, this may be because you're added too much contrast. If the gaps are small then the loss in quality may not be noticeable.
Each pixel in your image is given a brightness value between 0 and 255. Pixels rated at 0 will be pure black, those rated at 255 will be pure white. In between, you have every pixel represented on the scale as a certain shade of grey. (Understand histograms)
Understand histograms. (n.d.). A photographer's guide to Photoshop, 22-23.
Most images will appear at their best when they display the broadest range of tones and utilize every shade from light to dark. View the histogram in the Levels dialog and check whether the edges of the histogram reach the ends of the box. If not, then drag the black point and white point sliders inwards until they meet the edges of the histogram (expanding the range fully from light to dark). If the tonal range doesn't include any areas of very light or very dark pixels, the image looks flat. We recommend that you always apply tonal tweaks on Adjustment Layers.
Histogram: This displays the 256 shades from dark to light along the X-axis of the graph. Each pixel is given a tonal value on this scale. The more pixels of a certain shade, the higher the peak will appear on its corresponding graph.
If you cancel adjustments and start again, there's no need to exit the box completely. Hold Alt and the Cancel button changes to Reset. This applies to all dialog boxes in Photoshop. Preview: Check or uncheck the box to see the results of your tonal tweak. This can be very useful for reference and allows you to judge whether you deem the changes to be an improvement. (Introducing levels)
Always use an Adjustment Layer: You can lower layer Opacity to reduce the effect, change the Blending Mode to control the colours, and crucially, you can add a mask to target certain areas.
Control the tonal range: It's almost always best to improve the tonal range of an image so that your pixels represent every tone from pure black to pure white. To do so, drag the white and black point sliders to the edges of the histogram.
The eyedroppers in the Levels dialog give you interactive control over your tonal adjustments. If there is an area in your image you know you want to set to pure black, select the Black Point Eyedropper and click on that area. Everything darker than this point will turn to pure black. The White Point Eyedropper works in a similar way – set a point and everything lighter than that point will go to pure white.
Ideally, you'll want to clip the shadows slightly to have a few small areas of black, and leave the highlights completely unclipped. (Levels 101: Killer tips)
Introducing levels. (n.d.). A photographer's guide to Photoshop, 24-25.
Levels 101: Killer tips. (n.d.). A photographer's guide to Photoshop, 26-27.
Like Levels, we recommend that you always apply Curves on an Adjustment Layer for the extra control this gives you. Click anywhere along the diagonal line to add a control point. Drag this point upwards to lighten the image and down to darken it. Add more points along the line if you like and drag these up or down to alter different tonal values. To get rid of one, drag it outside the box. If the bottom or top of the curve flattens against the box, then the tones at this corresponding point in the histogram will be clipped to pure black or pure white.
Choose from a drop-down list of Curves presets to quickly adjust tones. Presets can be useful for a quick fix and as a starting point for further tweaks. Just like Levels, Curves displays a histogram that represents the tonal range of the image as different shades grouped together from dark (0) to light (255). The diagonal line is used to remap tones in an image to different values. The horizontal axis displays the original tones and the vertical axis specifies how the tones are changed. The steeper the line of the curve, the more contrast there will be along that part of the tonal scale.
Highlight the Hand icon then click within the image to select a certain shade of pixel. Drag up or down to interactively lighten or darken all similar shades. The edit points option is checked by default and allows you to click on control points and move them to adjust the curve. (Introducing Curves)
The most intuitive way to give your images more punch is to add contrast with Curves. To do so, click on the curve and drag up in the highlights to lighten them, then make a second point and drag down in the shadows to darken them. This results in an S-shaped curve that squeezes the lightest and darkest tones and adds contrast. The more pronounced the S, the greater the resulting contrast. A subtle-shaped S will usually do the job.
Curves can be applied to individual colour channels. This can be very useful if there's a need to correct colour casts in specific tonal areas. For example, you may have an image with a noticeable red cast in the shadow areas. If you make a universal adjustment to the colours, you risk disrupting the other tones. Curves solves the problem by allowing you to specifically target a certain tonal area. To correct a red cast in the shadows, you'd need to select the red channel and 'pin' the highlight and midtone areas by adding a couple of points to the curve, then add another point towards the lower end of the curve and drag down to reduce the red. (Curves 101: Killer tips)
Introducing Curves. (n.d.). A photographer's guide to Photoshop, 28-29.
Curves 101: Killer tips. (n.d.). A photographer's guide to Photoshop, 32-33.
Step-by-step: Mask the tones
Fill, Blur, & Sharpen
This new feature intuitively fills in a selection or brushstroke based on what's in the surrounding area. Simple distractions like power lines can quickly be erased, but even large, complex subjects like people and vehicles can be removed instantly and seamlessly. The Content-Aware Fill can be used by making a selection (A) and choosing Edit > Fill > Content-Aware.
Photoshop CS5: New features in CS5. (n.d.). Quick Study, 1-2.
Blur Softens hard edges and reduces contrast. Sharpen Hardens soft edges and increases contrast.
Photoshop CS5: Retouching tools. (n.d.). Quick Study, 3-4.
DOF - describes range of acceptable focus in an image; 3 factors control depth of field:
f/stop f/2 - 1 thing in focus; f/22 - everything in focus.
subject to camera distance the closer you are to your subject, the shallower the DOF will be for any given f/stop choice; the farther away you are from your subject, the greater the DOF will be for any given f/stop choice.
focal length wide-angle lenses (great for landscapes) - increases DOF of any given f/stop, keeping more things in focus; telephoto lenses (great for portraits and sports) - decreases DOF of any given f/stop, making backgrounds blurry, which helps isolate subjects better.
Good choice for everyday outdoor shooting - ISO 100; good choice for everyday indoor shooting - ISO 400.
Photography (Digital Essentials): Camera Use. (n.d.). Quick Study, 3-5.