How to Use the Image Histogram in iPhoto

Allow me to introduce you to your friend and mine – the Histogram. You may have seen the little feller lurking around not only within iPhoto, but in Aperture, Photoshop, and just about every other image editing apps on the Mac. He's also been spotted hanging out in several photo editing apps on iOS devices.

Let's address the histogram found in iPhoto. We'll examine what a histogram actually is, and look at how it can help you add some life to your photos. Understanding the histogram can also assist you in "fixing" some of the bad ones.

The Histogram

An image histogram is nothing but a dense bar chart. It's a simple graphical representation of the range of brightness levels (luminosity) present in your image – from darkest black to brightest white.

A typical image histogram.

The three main sections of a histogram define the overall image tonalities.

To make this concept easier to understand, you can also consider a histogram of a color image to represent a graphical view of the distribution of colors that are present in the image. Nevertheless, it’s best to try thinking of the luminosity values and where they fall on the histogram.

These values span across the bottom horizontal X-axis of the graph from left (darkest black) to right (brightest white). The vertical Y-axis represents the relative number of pixels, for each luminosity/color value, that are present in the image. This axis represents how much of the image is found at any particular luminosity level.

There are no numbers assigned to the histogram's axes because numbers are irrelevant. All the values for each level are relative to each other. When analyzing an image, you concentrate on the shape, size, and position of the histogram's "curve" over its entire range.

Incidentally, now you know why you had to study all this stuff in high school. And, you thought you'd never need to use it...

Let's look at some examples. When examining a histogram with the curve skewed to the left, this means that most of the image pixels are in the darker region.This may generally indicate an underexposed image, but not always. If you photograph a black cat sitting on a black sofa, the histogram will be completely skewed towards the left-most side of the histogram, yet the image is exposed properly.

Three histograms side-by-side, depicting an under-exposed image, a

Usually – but not always – the position of a histogram curve is indicative of exposure quality

One more example: you frequently see a very high spike on the right edge of the histogram for an image that has a large bright overcast sky. The spike is high because there is a relatively greater number of pixels in that high (bright) range. This is where the Y axis comes in, giving you an indication of the frequency of those particular pixels in the image.

The rule-of-thumb when evaluating histograms is that a well exposed image of a typical outdoor sunlit scene with even lighting and normal contrast should have a nicely distributed histogram across the entire range of luminosity values.

An image open in the Edit module of iPhoto and showing the histogram.

This histogram shows a good distribution of tonalities from black to white

This carefully crafted photo of an Alitalia air sickness bag has an evenly distributed histogram. There are tonalities from total black to white giving it a nice, even contrast. A nicely exposed photo like this always looks swell when framed and proudly displayed in one's home or office.

So, how does all this fit in with working on our pictures in iPhoto? Image editors, such as iPhoto, allow us to redistribute the luminosity values in an attempt to "normalize" otherwise problematic images. You know, like the ones that are a bit flat, underexposed or overexposed. JPEG images can be "fixed" up to a certain point, but you would be amazed at how you are able to save a photo from the bit bucket with a little tweak of the histogram.

It's interesting to note that histogram adjustments (known as levels adjustments) were not featured in iPhoto until only a couple of versions ago. For those who understood the benefits of evaluating and working with an image histogram, it's introduction was instrumental in pulling iPhoto out of the lame-photo-editor category.

Getting to an image histogram in iPhoto is straightforward. Select an image to evaluate and tweak. Enter editing mode by clicking on the Edit button at the bottom of iPhoto's window.

A photo selected in iPhoto. Clicking on the Edit button sends the image to the Edit module.

In iPhoto's image browser, select a photo, then click the Edit button to send the image to the Edit module

Once inside iPhoto's Edit module, you see three tabs at the top. By default you are placed into Quick Fixes. Click on the Adjust tab. And you are taken where all the "pros" hang out: advanced photo editing. This is where you do heavy-duty photo enhancing and "fixing." Forget that Enhance button back in the Quick Fixes section. Sheesh, that's for amateurs!

The histogram for the selected image is prominently displayed at the top. For the purposes of this tutorial, I will cover setting histogram levels only. However, make note of the other controls available to you in this module. All are quite important tools to assist you in photo editing. Feel free to experiment – and you should. You won't hurt anything as iPhoto is very good at preserving your original image intact. Your escape is the Revert to Original button at the bottom.

Let's edit a lamentable iPhone photo captured during my departure from the Palermo, Italy airport. It's dull, flat – the lack of contrast makes this an unappealing photo. Yet, it's an interesting photo, and I want to at least make an attempt to "fix" it.

I think it's about time we examine its histogram in iPhoto.

iPhoto's Adjust panel in the editor. This is where the histogram is located.

This photo's low-contrast appearance is evident in its histogram

As you can see, the darkest and lightest parts of this image – represented by the left and right edges of the histogram curve – are "pulled-in" indicating that all the pixels of this image fall within the mid-tones area of the curve. The lack of true blacks (shadows) and true whites (highlights) gives this image its dull, uninteresting appearance.

iPhoto's histogram levels adjustments to the rescue! By simply clicking and dragging the "handles" under the histogram, an infinite number of adjustments can be made to the image's tonalities.

The histogram is normalized by adjusting the levels control handles under the histogram.

Redistributing the histogram data using the levels control handles results in considerable image improvement

By dragging the the left "black-point" handle to the right and the right "white-point" handle to the left until each touches the outer limits of histogram data, we see a dramatic improvement to the photo. A little tweak of the center "mid-tones" handle may also serve to improve. To reiterate, these are guidelines; ultimately, you adjust to the point where you are satisfied with the results.

These levels adjustments actually served to redistribute, or stretch, that histogram curve and shifting tonalities in order to provide the much-needed shadow and highlight areas resulting in a boost in contrast. Contrast gives any photo that extra "oomph." Even photos that don't suffer from exposure problems – like mine, humbly speaking – can stand a slight boost in contrast. I do this for almost all of my keepers.

As alluded to above, the beauty of iPhoto is that, like any good image editor, it allows "non-destructive" editing. That is, your original image remains untouched. As soon as you begin editing a selected image, iPhoto makes a copy for you to work on. You can always revert to the original photo if things get really messed up. This also means that if you change your mind later about any editing you did to a photo – perhaps you want to back off that contrast boost or add a touch more, for example – you can go back in and modify your previous levels adjustments. This is true of any kind of image adjustment iPhoto provides.

Finally, you may have noticed that when I talk about "fixing" a photo, I always place the word "fix" in quotes. As I tell my students, never dismiss poor photography technique with the excuse that you're going to "fix" an image in iPhoto. You should always strive to get the best composition and exposure in-camera as opposed to relying on a "fix" in software at a later time. The levels and other adjustments actually reduce image quality to a certain degree, because a number pixels are destroyed in the process of "fixing" them. This is particularly true with JPEG images, as opposed to RAW camera images. The reduction in image quality will become evident when making and printing enlargements.

With an understanding of what an image histogram actually shows you in iPhoto and other editors, as well as how levels adjustments work, you are well on your way to having many of your otherwise run-of-the-mill photos really stand out above all the rest!