Nowadays in the digital world, we can fortunately tell immediately after taking a photo – and, with modern cameras, even before taking the photo – whether the exposure is correct or not. This is because camera manufacturers have provided us with a (mostly) foolproof tool for assessing the exposure. And no, we’re not talking about the camera display!
The Histogram
The histogram is the graphical display of the number of pixels in an image that have a certain brightness (phew). Common digital photos can display 256 brightness levels (for simplicity, let’s think only in terms of light and dark here and neglect the color). Black corresponds to the numerical value “0”, and white to the numerical value “255” (see figure below).
The higher a bar, the more pixels correspond to this brightness value. How many pixels this might actually be is completely irrelevant.
• Average brightness distribution
When you photograph an average subject, the brightness levels between black and white are evenly distributed. There are only a few completely black or completely white pixels (image below).
• Low-Key Image
The term “low-key” refers to a subject that consists predominantly of dark image areas. Accordingly, the histogram shows a lot of high bars in the dark tonal values on the left side (on the left side of the histogram, not the image).
• High-Key Image
The term “high-key” refers to a photo that consists predominantly of bright image areas. Therefore, in this case, the “mountain” of the histogram is shifted to the right where the bright tonal values are located.
• Underexposure
At first glance, you might think the following photo is a low-key image. However, the histogram shows us that the photo is simply underexposed. This can be seen from the high bar on the far left of the diagram, but also because there are no pixels on the right of the histogram.
When the clipping warning is activated, the completely black pixels are displayed in blue.
What this means is that in these areas, there is no longer any texture in the costume visible. Instead, you can only see pure black areas, and even in postprocessing you won’t get any texture back.
Compare this hsitogram to the one of the low key image. Though there are also many dark pixels in the histogram of the low-key image, there is no pronounced high bar at the extreme left edge of the graph.
• Overexposure
Overexposure is indicated in the histogram by a high bar at the extreme right edge of the diagram and missing dark tonal values in the left area. The highlight areas, i.e. the bright areas of the image, are without texture. Again, you won’t be able to restore the textures in the bright areas of the image in post-processing.
When the clipping warning is activated for the highlights, the white pixels are displayed in red.
• Contrast of the subject is too high
Each sensor can only record a certain brightness range (contrast) between very bright and very dark parts of the image. If the contrast of your subject is greater than what the sensor can record. An average exposure will result in both featureless shadows (blue) and blown out highlights (red).
In this case, we have reached a technical limit, and you as the photographer must now decide which areas of the image are more important to you – the shadows or the highlights.
I hope this post has helped you. Please feel free to leave a comment.
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