Confessions of a Keytarist

This post is a continuation of the first histogram post entitled “Getting Toned”.

In the last article, we discussed the basics of what a histogram is, what it represents and how it is generated. What we didn’t cover is why they’re important to us digital photographers. That’s where this post comes in!

Most digital cameras will have next to no trouble reproducing images that have a midtone-centric histogram (a histogram where the majority of the tonal range is within the midtone section). This is because they have an “automatic” imaging mode which uses a clever set of algorithms that attempt to estimate how bright an image should be, and these algorithms frequently result in an image who’s average brightness is placed in the midtones.
(it’s all down to the fact digital cameras have to measure reflected, rather than incident light, but we’re not going to discuss that here.)

This is usually acceptable, however images where the majority of the tones occur in the shadow range (low key) or highlight range (high key) are often unacceptable, as the algorithms try to adjust the brightness of the scene too much (moving the tonal range back into the midtones, rather than leaving them in the shadows or highlights).

To give you an example of this in action, below is an image taken by manually setting the white balance and exposure:

As you can see, the histogram shows this as a high key image, with the tonal range right up in the highlights – exactly as wanted.
However, the next image is the same scene but taken using the automatic mode within the camera:

As you can see, the camera’s algorithms have got involved and tried to shift the overall brightness down so that the tonal range is nearer the midtone section, which has resulted in an effectively underexposed image.

Whilst this isn’t great, it is a relatively easy process to fix this in Photoshop (and we will cover that later on). However, if we were taking a low key image, we could hit a big problem.

Take the following image as an example. This scene was very shadowy and dark, but the camera’s automatic mode has increased the overall brightness:

The problem however, is that it’s lost some of the highlights from the original image; In the histogram, you can see that a large number of pixels are pure white (or very close to).  This is known as having “blown” or “clipped” the highlights.
Unlike an underexposed image, it is impossible to recover any detail from the “blown” highlights.

This is where being able to understand histograms comes into play!

Most digital SLR cameras will allow you to view the histogram of an image on the LCD back panel, giving you instant feedback that can help improve your image, without having to wait till you get the images back to Photoshop.
That quick glance is enough to tell you if the image you’ve just taken is likely to be a decently lit image.

For example, if you took the image of the gate shown above, you would quickly be able to see that you’ve got blown highlights, and as such adjust the exposure on the camera to compensate, giving you something much nicer:

In the final post about histograms, I’ll be covering contrast…
http://www.blog.ianmellor.co.uk/2009/07/30/contrasting-views/