miércoles, 16 de enero de 2013

Types of Histograms

Versión en español
Well, from the previous post, we already know what the histogram is. Now, let's check what are the most frequent histogram types, and how can we use them in photography...

Types of histograms
Usually, the histogram of our pictures will look like one of the following:
Hist01 Histogram #1: uniform distribution on all tones, a slight maximum in the central ones. In this case, the image would have a rich variety of mid tones (or grays), and besides (in this example...) it also includes the extreme values for black and white. Opposite to what you may think, this kind of images might result too soft and lacking interest (except in the case of HDR, but that's another story...).

Hist02 Histogram #2: A values' peak is on the right side of the graphic (highlights). In this case, the majority of hich values corresponds to a bright image. This is the case of highly lit images (such as high key). In all cases, lowest values are also present, so we have to assume that exposition is right (it is not an over-exposed picture).
Hist03 Histogram #3: Opposite to #2, there is a majority of low values, corresponding to dark tones. In this example, there are also high values, so we would be facing a dark image (maybe dark objects in a shadowy environment... Limit case would be a low-key image, which still keeps some highlight detail.
Hist04 Histogram #4: double peak. A mix of the above, we may have an image with uniform tones and right exposition (both white and black values are present), but a secondary peak is also present.

In the example on the left, we would have a majority of dark tones on the left, but also an important set of clear values on the right: This is the case, for example, of night photos with triggered flash that may over-expose objects in the foreground. This can happen if the exposure is not well estimated by the camera (or user...), then background appears under-exposed - and dark.
Of course, we will also find the opposite situation, with a majority of high tones and a peak in the dark ones. As examples, pictures in the snow or at the beach will have this behaviour, being the dark peak related to the objects casting strong shadows.

Use of the histogram in photography...
Well, all the above... does it have any use? Yes, many people think so. In principle, digital cameras are limited by the sensor characteristics. Standard cameras will typically NOT capture the whole dynamic range of an image.
To reduce the impact of this limitation as much as possible, we need to try to have the "best" information possible at the sensor, so that input light values are between the valid thresholds - we do this by adjusting speed and aperture.
Ideally, a pircure will have information on all ranges of tones. If some tone is missing (i.e. high values), exposition was not right (in that case, under-exposed). If we missed to capture low values, the photo is likely to be over-exposed...
Histogram allows us to see all this at a glance. If the image was properly exposed, we will see values all allong the histogram. If underexposed, we may face a line as in histogram #3 above, but "compressed" to the right - this will mean that we must open the f value or slow down shutter speed.
If, on the contrary, we face a curve similar to #2 but compressed to the left, this may mean too much exposition. Then, we should close aperture or increase shutter speed...

How can I see this in my camera?
Modern cameras are capable of showing the histogram in two different ways: in shoot mode (advanced and professional cameras), histogram is shown live prior to taking the picture, so that minor adjustment can be done to improve exposition. In revision mode (most modern cameras), histogram for already stored pictures will be shown.

Typically, we will access the histogram (If the camera allows for it!) by changing the display setting, which is usually available in a dedicated key labelled as DISP...

You may also like...
The Histogram
Further posts on Photography
Originally posted on Jun 26th 2012 (Over-Blog)

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