In the next tutorial, we'll learn how to use the histogram, along with Photoshop's Levels command, to correct overall tonal problems in an image. But before we jump in and start fixing things, let's take a quick look at a few examples of images where the histogram may look "wrong" but the image itself looks "right". In other words, images that are properly exposed not based on the shape of the histogram but on their specific subject matter and the photographer's or image editor's creative ideas. This will help us learn not just how to read a histogram but also how to evaluate the image itself and understand its tonal character, giving us a better sense of why, ultimately, it's not the histogram that shapes your image; it's the image that shapes the histogram.
If you're not familiar with how image histograms work, you'll want to read through the previous tutorial, How To Read And Understand Image Histograms, before you continue.
Let's begin by looking at an image that benefits from the "typical" exposure we talked about earlier, with a full range of brightness values from black to white (woman in orchard photo from Fotolia):
An image with a "typical" exposure.
We'll use Photoshop's Levels dialog box once again to help us read
and evaluate the histogram. To open Levels, I'll go up to the Image menu in the Menu Bar along the top of the screen, then I'll choose Adjustments, then Levels. I could also open Levels using the faster keyboard shortcut, Ctrl+L (Win) / Command+L (Mac). Either way works:
Going to Image > Adjustments > Levels.
This opens the Levels dialog box with the histogram in the center, along with the handy black-to-white gradient bar below it:
The Levels dialog box showing the image histogram.
I'll zoom in closer to the histogram so we can get a better view, and
here we see that this image contains a full range of tonal values. The
histogram starts at the far left, directly above black in the gradient
bar below it, so we know that a few of the pixels in the image are
already as dark as they can be. It extends all the way to the far right,
directly above white in the gradient bar below it, telling us that some
of the pixels are already as light as they can be. And, the histogram
shows lots of pixels at each of the tonal values in between. If there were such a thing as an ideal shape for a histogram, this might be it:
A histogram for a "typical" image showing plenty of detail throughout the tonal range.
Low-Key Images
The truth is, even though the histogram we just looked at may strike you as ideal with its full range of tonal values, in reality, it's ideal only for that specific image. Every image is different, and the tonal character of an image is greatly affected by its subject matter as well as the type and amount of light that was captured. That's why, before we start correcting images, we first need to remove any notion from our mind that there is such a thing as a standard, typical or ideal histogram. After all, you wouldn't expect a photo captured outside at sunset to share the same tonal character as a photo shot in the middle of the day.For example, let's take a look at this second image (lighthouse at sunset photo from Fotolia):
A naturally low-key image.
This is an example of what's known as a low-key image,
meaning that unlike the first image we looked at which showed an
evenly-distributed range of brightness values from dark to light, most
of the tonal range of this image naturally falls within the shadows.
I'll press Ctrl+L (Win) / Command+L
(Mac) on my keyboard to quickly bring up the Levels dialog box, and if I
zoom in on the histogram, we can easily see that while there is some
detail in the highlights and a bit more in the midtones, most of it is
found in the shadows. If you had the idea in your mind that every
histogram should look like the previous one, it could lead you to think
that this photo is too dark and needs to be corrected, when in reality,
it looks great just the way it is:
The histogram for a low-key image showing the majority of detail in the shadows.
Notice that tall spike at the far left of the histogram? That's
telling us that some of the darkest areas in the photo are so dark,
they're being clipped to pure black, and that means we've lost detail in
those areas. Normally, that would be a warning sign that the image is
either underexposed or that we've darkened those areas too much in the
editing process. But with this photo, it's not a problem. In fact,
darkening areas to pure black helps enhance the effect of the lighthouse
and the grass silhouetted against the setting sun. We don't need (or
even want) detail in those areas, so darkening them to pure black was
not a "mistake" but rather a good creative choice:
The spike on the far left means we have shadow clipping, but is it a problem? Not with this image.
Here's another example of an image that was purposely shot low-key (low-key portrait photo from Fotolia):
A low-key portrait photo.
Just looking at this image, you would expect to see its histogram
bunched up in the shadows, and if I bring up the Levels dialog box, sure
enough, we see exactly that. There's even less detail in the highlights
and midtones than what we saw in the lighthouse photo, and while we're
not seeing any shadow clipping this time, it's still extremely dark
overall. Again, you may be tempted to "correct" this image if you
thought that every histogram should look like that first one, but would
it really make sense to try and force this photo to look like it was
shot outdoors on a sunny afternoon? Of course not. Just as with the
lighthouse image, the overly dark tone is well-suited for this type of
photo. Remember, it's the image that shapes the histogram, not the other
way around, and knowing how to recognize the natural tonal character of
an image is every bit as important as knowing how to use Photoshop's
editing tools:
Another example of a histogram for an image with naturally dark tonal characteristics.
High-Key Images
The opposite of a low-key image is a high-key image, where the tonal range consists mostly of lighter highlight tones. While low-key images are often dramatic and intense with striking contrast, high-key images are softer and more subtle. Here's an example of a high-key image (winter mist photo from Fotolia):
A high-key landscape photo.
With its predominantly lighter tones and lack of any real contrast,
this high-key image conveys a sense of peacefulness and tranquility.
What do you think the histogram for this image will look like? Well,
most of the photo is made up of very light tones, so right away, we know
that we should expect to see the histogram bunched up into the
highlights. There's a few darker areas in the trees, the dock and along
the ground, but with so much of the image in the highlights, it's
possible that our eyes are being tricked into seeing those areas as
darker than they really are. In fact, that's exactly what's happening.
The histogram, which doesn't suffer from the limitations of our
eyesight, shows that we have virtually no shadow detail at all. This
image is made up almost entirely of midtones and highlights. Yet, just
as with the low-key images we looked at, the tonal characteristics of
this image match its subject matter. We could, if we wanted to,
push those darker areas down into the shadows just to make the
histogram look more impressive, but the image itself would suffer.
We'd be making it worse, not better:
The histogram for the high-key images shows the tonal range mostly in the highlights.
High-key techniques are often used in beauty and fashion photography
as a way to smooth out the model's skin by reducing or removing unwanted
details. Our eyes see differences between light and dark pixels as
detail, so as we push the entire tonal range up into the highlights,
more and more of the detail disappears (high-key beauty photo from Fotolia):
A high-key portrait photo.
Since the background in this photo is pure white, it's no surprise
that when we look at the histogram, we see a tall spike on the far right
telling us we have clipping in the highlights. In a more "typical"
image, that spike could mean that the photo was overexposed or that
we've brightened the highlights too much in the editing process. In this
case though, with an image that was purposely shot high-key, it's a
technical and creative choice, not a problem that needs to be corrected:
The histogram indicates highlight clipping, but that's often the case with high-key images.
The goal of this tutorial was simply to show that there really is no
such thing as a typical or ideal histogram. Every image is different, so
every histogram will be different. Some photos will naturally favor the
shadows, others the highlights, and while the histogram may be
shouting, "Something's not right here!", the image itself may be saying,
"No, don't listen to him, everything's good." An essential skill for us
as retouchers is knowing how to tell the difference between a problem
that needs correcting and what is simply the natural tonal character of
the image.Now that we've looked at some examples of different types of images, in the next tutorial, we'll take our first big step into the world of professional-level image correction by learning how to improve the tonal range of an image using a Levels image adjustment in Photoshop!
0 Response to "Understanding Histograms – Low-Key And High-Key Images "
Post a Comment