Hope you have already read last three parts of this series on Reading Histogram @ HISTOGRAMS . If not, click on HISTOGRAM and come to this article.
By now, you must have some idea that why all histograms are different and what is the significance of a particular type of Historgram. During this, did you thought about following things -
1. Does a Histogram tell something about overall Contrast of a photograph?
2. What happens when most of the pixels in a photograph are on extreme?
3. If we have multiple photographs of same view, then which version is best for editing/processing in Photoshop, Lightroom or any other Image Editing Application.
4. Why some of the histograms got so much grey region and others not. Does it matter at all or not.
5. On similar lines, does it impact if a Photograph has least grey region and most of the regions are covered by other colors like Yellow, Cyan, Magenta or Red, Green, Blue ?
Before I get into more detailed description about all these questions, let me tell that reading Histogram is not pure science. It's subject and different Photographers use it differently. Idea here is to understand the significance of these colorful spikes, their patterns and placement on horizontal line. So let's go one by one on above mentioned questions -
#1 : Yes, a Histogram tells about Contrast. The First Histogram shows that it's more contrasty photograph than second one. But why? - Because in second one we have good distribution among dark and bright regions. While first one has much more dark regions than bright ones. Now here comes the subjectivity. If we get two histograms similar to second one, then we also need to look at the images to know the actual distribution of those pixels in a photographs. Hope all this makes sense. I know many folks ask lot of questions after this explanation. In class-rooms it's much more convenient to convey with variety of photographs. But, please feel free to put an comment with your questions and we would love to reply back with appropriate information around the same.
To clarify the things about contrast, let's also look at photographs corresponding to these Histograms.
#2 : This is something interesting. Vertical spikes on extremes tell us that corresponding photograph has most dark or bright pixels in the photographs. It means that if we are shooting a photograph where tree is coming as black, after editing nothing can be found in those region. Technically we call these no details zones. If you have ever use Adobe Camera Raw dialog in Photoshop or Basic Develop settings in Lightroom, you would be able to relate to it. If you want to have some details in all regions of your photograph, you need to ensure that you don't have anything on extremes. If you are able to adjust Camera settings in a such a way that you get nice histogram within the limits and nothing on extremes, then later you can make few things black by reducing exposure or brightness. Even there are ways to make some dark regions completely dark. Let's not start talking about all that here. So it's always recommended that we should avoid under-exposure or over-exposure. Some Cameras also highlight the regions where clipping happen. In case you see Blue highlights, it means under exposure and you may want to increase exposure by reducing f value or decreasing shutter-speed. ISO could be one of the alternative, but why I again started telling you basic things which you already know :) ... So let's move #3
With this, let me take some rest. Try it out and post your questions in comments. Next three points would be covered in next section on Histograms only.