I believe most of us know what histograms are. A histogram is an important tool that is built into your camera. Even the cheapest of the digital cameras have the histogram feature built in these days. Not only does it help you while shooting, but it also helps you during post processing. A histogram tells you whether the photograph has been properly exposed or not. Now, you might find thousands of posts which would teach you what histograms are, but like I have always said, I would try to explain the mathematics behind histograms in the simplest language and the least number of lines possible.
In the last post, I had talked about Photoshop Levels. To understand the mathematics behind Levels, it is important that you should be able to read histograms.
Image Courtesy- www.nikonusa.com
The spiky diagram that you see on the right in this picture is the histogram of the photograph of this flower. Last post also talked about the Shadows(Blacks), Midtones(Greys) and Highlights(Whites) regions of the histogram. Now, coming to the point- how do we relate a histogram to its photograph?
A histogram maps the shadows,midtones and highlights present in the photograph in the form of a graph. A histogram expresses the tonal range of the photograph. The very basic and the most layman oriented explanation of a histogram can be summarized as follows-
1) An under-exposed photograph (I hope now you’ve started using under-exposed instead of the word ‘dark’ ) will generally have most of the spikes in the left (shadows) region of the histogram.
2)An over-exposed photograph will generally have most of the spikes in the right (highlights) region of the histogram.
3) A properly exposed photograph will have the spikes spread evenly, all over the graph.
Now let us go through a few examples.
Now you can see for yourself. Here, since the photograph is quite under-exposed, all the spikes are present in the left region of the histogram.
Since this photograph is over-exposed, its corresponding histogram has majority of the spikes in the right region of the histogram.
This photograph has been exposed properly for everything present in the scene and thus its histogram has spikes spread evenly, all over the graph! (The whitish thing in the top centre of this photograph is the smoke that was coming out of the preparation.)
It must have happened with you, that you clicked a photograph and really liked it on the camera LCD but when you transferred them to your computer, you found out that the exposure that you saw on the LCD was quite different. This is a common experience because when you click a photograph, the camera’s computer takes a few seconds in processing the shot and then it gives it to you on the LCD. Things aren’t generally quite clear on the LCD (Specially when you are shooting in the RAW format and not JPEG). Histogram can always tell you whether you’ve properly exposed your photograph or not, so that once you transfer your photos to the computer, you don’t have to regret. Whenever you have an opportunity, do take a look at the histogram, so as to make sure that you’ve exposed the photograph properly. Whenever I feel that the lighting is tricky, I always make sure that I check the histogram after having clicked a shot, find out what is wrong in my exposure, dial in the new settings accordingly and re-take the shot so that I am sure that I’ve nailed it!
Now, let me tell you one thing. Photography has many rules associated with it but sometimes it is good to break those rules. You must have observed that in the photographs above, the third one has the best exposure but yet the second one looks better. Yes, this is because this shot is such that this high-key effect is suiting it. Although the second one is over-exposed, yet it stands out of all the three because this ‘over-exposed effect’ suits it. Let me take one more example, just one more.
Dussehra-2011
You can see that the photograph is quite under-exposed (There wouldn’t have been any spike in the right region had the sun not been there in the photo, everything would have been in the left region), but still it looks pleasing. The whole purpose of making this photograph had been defeated had I exposed the photograph as per the rules. Had I done so, the details in the effigy would have been visible which wouldn’t have created this effect. So, what I want to say is that it is a good habit to check the histogram always, but if the situation demands, it is okay to break the rules!
There are quite a lot of other things that can be discussed in the histogram. I’ve talked only about the horizontal spread of spikes in the histograms and not the vertical spread. The vertical spread has to do with the contrast and luminosity. But I wouldn’t discuss about them because I feel that this knowledge is not important at all. The understanding of horizontal spread is quite sufficient to assist you while shooting and post-processing.
Ok! I just hope that this post gave you an idea about reading histograms to some extent if not completely. Next time, whenever you use your camera, keep an eye on the histogram. I am sure you’ll feel happy when you’ll be able to relate it with your photographs.
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Thanks. Happy reading!
