RGB histogram

An RGB histogram shows the distribution of brightness levels for each of the three color channels— Red, Green, and Blue. Each color channel is shown in a separate graph so you can evaluate the color channel's saturation, gradation, and color bias. The horizontal axis shows how many pixels exist for each color brightness level, while the vertical axis shows how many pixels exist at that level. More pixels to the left indicate that the color is darker and more prominent, while more pixels to the right indicate that the color is brighter and less dense. If pixels are spiked on the left or right side, then color information is either lacking or oversaturated with no detail, respectively.

Choosing the type of histogram to display depends on the shooting situation and your priority in that situation. For a fashion shoot where color reproduction is critical, the RGB histogram is likely most useful. For a wedding, outdoor shooting, and nature shooting, the Brightness histogram can be most useful for evaluating critical highlight exposure.

The histogram is a very accurate tool to use in evaluating JPEG captures because the histogram is based on the JPEG image. If,

2.7 This is a combined histogram from a RAW capture image in Adobe's Camera Raw conversion program. The graph reflects the dark shadow tones in the image but, with the pixels crowded to the left edge, also shows that shadows are blocked.

400d Histogram

2.8 This is a Brightness histogram from a RAW capture image in Adobe's Camera Raw conversion program. This histogram reflects the white seamless background with the highlight pixels spiking on the right side of the graph showing that those pixels have no detail in them.

however, you shoot RAW or sRAW, the histogram you see is based on the JPEG conversion of the image. Due to the linear nature of RAW data, it isn't feasible to display a RAW histogram. So if you shoot RAW, remember that the histogram is showing a less robust version of the image than you'll get during image conversion. For example, there will be more data in the highlights than the JPEG-based histogram indicates. Despite the JPEG-based rendering, the histogram is still a very useful tool to gauge exposure in the field.

Tip If you're shooting RAW capture, you can set the Picture Style setting to a lower contrast and get a better overall sense of the RAW histogram and reduce the likelihood of clipping (or discarding image pixels). Picture Styles are detailed in Chapter 3.

Histograms on the 40D are displayed during image playback. Once you set the type of histogram you prefer, it is displayed until you change it. You can set the type of histogram in all shooting modes.

To set the type of histogram displayed during image playback, follow these steps:

1. Press the Menu button, and then press the Jump button to display the Playback 2 (blue) menu.

2. Turn the Quick Control dial to highlight Histogram, and then press the Set button. The camera displays the Brightness and RGB histogram options.

3. Turn the Quick Control dial to highlight the histogram option you want, and then press the Set button.

To display the histogram for a captured image, follow these steps:

1. Press the Playback button. The camera displays the most recently captured image. If you want to check the histogram of a different image, turn the Quick Control dial to select the image.

2. Press the Info button twice if the playback display is set to singleimage display to display the histogram. The camera displays the image thumbnail, the type of histogram you've chosen, and detailed exposure and file information for the image. You can press Info a third time to display both the Brightness and RGB histograms.

Differences in Exposing JPEG and RAW Images

For those new to digital photography or to RAW capture, it's worthwhile to point out important differences in exposure techniques for JPEG and RAW capture.

When you shoot JPEG images, you expose images as you would for slide film, by metering for the highlights. Just as you can't recover detail that isn't captured in a film image, you also can't recover highlight detail that's not recorded during a JPEG capture. To ensure that images retain detail in the brightest highlights, you can use one of the exposure techniques described later in this chapter such as Auto Exposure Lock or Exposure Compensation.

Conversely, for RAW capture, exposure is more akin to positive film in that you have a fair amount of latitude to overexpose the image and then pull back the highlight details during conversion of the RAW images in Canon's Digital Photo Professional or Adobe Camera Raw. In fact, because CMOS sensors are linear devices in which each f-stop records half the light of the previous f-stop, the lion's share of image data is recorded in the first f-stop of light. To be exact, fully half of the total image data is contained in the first f-stop in RAW capture. This fact alone underscores the importance of taking full advantage of the first f-stop by not underexposing the image. In the everyday world, that means weighting the exposure slightly toward the right side of the histogram, resulting in a histogram that has highlight pixels just touching, but not crowded against, the right edge of the histogram.

With this type of exposure, the image preview on the LCD may look a bit light, but in a RAW conversion program, you can bring the exposure back slightly So for RAW exposure, be sure to expose with a slight bias toward the right side of the histogram to get the full first f-stop of image data.

In addition to evaluating the histogram, you can also display localized areas of overexposure, or where the highlights are blown (they have no detail). The camera displays the blown highlights with a flashing display when you play back images. This is a way to quickly determine if you need to reshoot a picture because of overexposed highlights.

To turn on Highlight alert that displays overexposed areas during image playback, follow these steps:

1. Press the Menu button, and then press the Jump button to display the Playback 2 (blue) menu.

2. Turn the Quick Control dial to highlight Highlight Alert, and then press the Set button. The camera displays the Disabled and Enabled options.

3. Turn the Quick Control dial to highlight the option you want, and then press the Set button. If you select Enable, overexposed areas are displayed in flashing black and white on the image preview during playback. If you find the flashing display distracting, you can repeat these steps to turn off the display.

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Responses

  • JONATAN
    How do you set the histogram on a canon 40d?
    7 years ago
  • Guido
    How do you turn off the histogram on the canon eos?
    7 years ago
  • Daniel
    Where you turn it and a different image shows?
    7 years ago
  • ville
    How to increase brightness in 40 d pictures?
    7 years ago

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