Histogram is perhaps the most neglected aspect amongst photographers. I’ve come across photographers who’re not even aware of what is exactly an histogram. Those who are aware of it just know as to how it looks. They have no clue of what the peaks & canons suggest in an histogram. However to become a photographer of a substance, one must know how to read histograms. Its not rocket since & its fairly easy.
A histogram can be found either at the back of your camera or under photo softwares like GIMP etc. Further there are two types of histogram – the RGB(Red,Green & Blue) histogram & the unified histogram. For now, we will have a look at the unified histogram(The RGB histogram works similar to the unified one & most of the following applies to it too). Upon pressing ‘INFO'(when in image preview mode) a couple times on a Canon DSLR one can see a unified histogram. A histogram tells us about the gradation of tones in a photograph. The histogram is divided into multiple sections by some vertical lines. The extreme left end of the histogram denote a ‘0’ while the extreme right denote ‘255’. All histogram have the same range of 0-255. These are not magical numbers but a combination of tones(RGB). A zero denotes absolutely no light, whereas 255 denotes perfect white light.
The peaks & canons in an histogram tell us as to how dominant a particular shade is in the photograph(scene). The left side deals with the shadows, the middle part with the midtones & the right side with the highlights. If your histogram peaks mostly towards the left side, you can safely tell even without looking at the picture that it is underexposed. However this is not a safe conclusion as we’ll get to know soon. Whereas a right inclined histogram suggest very bright(over-exposed) photograph. A histogram peaking the centre is a good indication of a balanced exposure. Again, do not yet jump into conclusions as histograms can paint a wrong picture at times.
So now that you know, an underexposure is shown by a left side dominated histogram don’t get fooled into increasing the exposure every-time you get to see such an histogram. For e.g. If you’re shooting the moon against a pitch dark sky, your histogram will most certainly tilt towards the left as most parts of the scene(sky) are in pure black i.e. value ‘0’. Hence you will get a peak at extreme left & then the histogram will recede down & give a slight peak somewhat near middle. This tiny peak near middle is the presence of light areas of the moon(considering you got the exposure right). In such a case if you were to increase the exposure just by looking up an histogram, you’ll certainly get a overexposed moon(a bright white spot sans any details) & not so dark sky. Hence, I say earlier that histogram are not always to be taken seriously.
Moving along if you happen to see any of the histogram touching to the extreme sides(either left or right), this indicates you’re losing details in either shadows or highlights(or even both). Remember, the left side having a ‘0’ value? These are your shadows. The middle portion of ‘128’ are your midtones whereas the extreme right with the value ‘255’ are your highlights. If you happen to touch the ‘255’ range, your photograph will lack detail in highlights. Given the latitude(ability to capture highlights & shadows perfectly without any loss) of digital sensors most of the times it means you’re hunting for the right middle ground exposure so your shadows & highlights both remain preserved. However again, if your histogram touches either the left or right side does not mean you should always try to rectify it. For e.g. if you’re shooting a portrait outdoors with the sun in the frame, then you can be assured your histogram will clip(touch) at the right end losing details in the highlights. Here the person may be exposed well but the loss the histogram indicates in highlights is that from the sun. In this situation there is no way you can prevent the histogram from clipping as the sun is very bright & the dynamic range of the scene is beyond the latitude of your digital camera sensor. So in this scenario, it is better you move on happily without worrying about the loss of details in the highlights.
Yes, you should worry if the highlights being clipped fall on an important part of your frame. A beautiful bride in her lovely wedding white attire will surely strangle you if you happen to lose highlights(details) in her meticulously designed dress. In such a situation, you definitely cannot afford to lose the highlights. In such case, you should take care to keep the histogram from touching the extreme right. The same case can be reversed for preserving shadows. If the shadow details are important in your photograph, then only you should take care to preserve them.
At times, you might notice a histogram having many blank spaces in between at regular intervals. Such spaces occur not in camera but when on editing the photograph carelessly. When edits are made to a photograph heavily, there are loss in details which cannot be recovered. Such edits mean the values of a particular area are uprooted completely & hence you will notice blank spaces in the histogram. So it becomes crucial you keep an eye on histogram not only during shooting but also while editing a photograph.
So now you may conclude that for a photograph to have proper gradation of tones, it must be centre dominated, right? There is no definite answer to this. As we saw with the moon case above, a centre dominated histogram is not possible at all. Similarly, high key photographs cannot be centre dominated. At the end of the day, the histogram is just a guideline to tell you how the tones are spread across your photograph. In no way it is designed to convey whether your exposure is correct or not. A middle dominated histogram may at times get your a properly exposed & finely toned photograph but aiming for a middle dominated photograph in most of the situation is highly optimistic approach prone to failure. A photographer must remember, a histogram is not a tool to set exposure. A good photographer cares in getting a evenly spread histogram but is not obsessed with getting it so every single time.
Picture Courtesy : Digital Camera World