Lesson 4 - Understanding Histograms

Want to know why you should be taking note of what your camera's Histogram is telling you? Andrew James explains all...

By Andrew James

This FotoSkool lesson is going to deal with the very, very useful facility that your digital camera gives you and that is the Histogram. I know from working with lots of FotoBuzzers this last 12 months that plenty of you are already harnessing the information that a Histogram gives but an equal number of you aren’t. In fact, I’ve shot with quite a number of people who have never bothered to look at the Histogram at all. To a certain degree I can understand this. After all, your average Histogram really isn’t very glamorous. Not only is it unglamorous but you might think that, thanks to the ability of the LCD preview on the back of your camera to show us what we have taken, there is no real need to bother with it at all. I confess, in the early days of shooting digital I had this attitude. Why look at a Histogram when there is an instant ‘polaroid’ on the back of the camera? Accuracy, that’s the simple reason and I’ll explain this more as this article goes on. There is a lot potential for waffling on about the science of the Histogram but as you know, we’re not keen on science here – rather what is can do for you in the real world. So let’s get the technical stuff out of the way first.

What is a Histogram?
In essence it is simply a graphical representation of the pixels contained within an image. To be truthful, it is actually representative of the JPEG preview so still isn’t 100% accurate. But because it is a graph it is more accurate than just looking at the LCD panel. If you just look at the LCD you can easily be fooled by a lot of different factors. For example:

* The brightness the LCD is set at
* Anything potentially reflecting off the surface of the panel
* Simply how good/bad your own eyesight is

I’m not saying DON’T use the LCD panel as a guide, I’m saying use it as a guide in conjunction with the Histogram for a more accurate assessment of the exposure. Because it is a graphical representation of the exposure, it’s easier to make an assessment on the fine detail of under and overexposure. When you take a look at the Brightness Histogram on the back of the camera the graph is representative of 256 RGB (Red, Green, Blue) tones. So you start at 0 and that is Black. This then goes through the range of tones right up to 255, which is white.

When you look at the graph, you can roughly divide it into thirds on the horizontal axis. The first third is the Shadows, the second third the Midtones and the final third the Highlights (look at the diagram right with apologies for my terrible design skills). The vertical axis is simply representative of the number of pixels at each value. So if your ‘peak’ runs off at the top it really doesn’t matter. When you are starting out it is sometimes easy to forget which side of the vertical axis is the shadows and which is the highlights. I always tell people to remember it as Black & White. Black comes first on the left and white last on the right. Simple! If we were reading the Histogram above we can see that it is neither under or overexposed, although it leans towards the midtones and shadows. We'll come back to what we might do about this later. With many modern cameras you also get a Histogram that will show the tonal range within the different Red, Green and Blue Channels. It works in the same way but separates the different colour channels. This next diagram is simply the RGB Histograms for the same image as the Brightness Histogram above.

By the way, if you are a total beginner and are wondering how you even access the Histogram, then look on the back of your camera for an info-button. In most cases this is what you press but of course, you must already have a photo you've taken being previewed on the LCD. If you don't have one, then consult the manual for your model. Okay, so in essence you now know what a Histogram is and what it represents. I could continue to waffle on about various technical aspects but frankly that's boring and you don't really need to know it to be a good photographer and get the most from the Histogram. Instead, let's take a look at a couple of example images to show you how it works in practice. The relevant information for each image is captioned beneath it...

Yes, this is a dull photo but it makes a good example for what I want to show you. The camera, set to evaluative/matrix metering, gave me an exposure of 1/4sec at f/8 using ISO 100. The slow shutter speed didn't matter in this case because the camera was on a tripod and the object (a spice rack) is static. It is lit by even natural window light. Viewed on the back of the LCD the image looked okay and I might have accepted it as okay if I hadn't looked at the Histogram (below).

So, this is the Histogram or rather both the Histograms visible on the back of my 5D MkIII for the image at the top. Obviously, depending on your camera's make and model, yours may look a little different but the principles are the same. I can see at a glance that the Histogram has some room to the right for the tones to move into. In this case, because the scene is evenly lit and relatively evenly toned, I want to try and get a good spread of tones throughout the range. So what do I do? Well, I change the exposure to compensate. My camera is set to change exposure by 1/3rd of a stop increments and this is the default setting for most cameras. I'd recommend you have this set as it allows you to make fine adjustments to your exposure when required. Anybody guess how much plus extra exposure the shot needed to get the image correct? Well have a look at the side-by-side comparison in the next image to find out.


The image on the right has the exposure added by changing the shutter speed and is clearly much lighter than the first image. You can see from the inset Histograms that the tones have moved to the right but are not overexposing. It may not be exactly how I  want the finished image to look but by dialling in the exposure compensation I can make the most of the tonal range without overexposing it, leaving me to be able to finish off the image in post-production knowing I have all the necessary tones to work with. To go from the first shot, as the camera metered it - 1/4sec at f/8 if you recall - to the 'correct' exposure needed +11/3rd stop of exposure as you can see clearly marked next to the +/- symbol in the image below.

Okay, so now's let's try this same process with a more colourful photo. Please go to the next page...


 

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