Before we move on to the middle tone adjustments, allow me to restate and review just a little of the hist-related issues we’ve covered so far. I know for some this will be an unnecessary rehash, but for some that have just joined, it will help them get acclimated.
Here’s some image-editing terminology I’d like you to familiarize yourself with. There are three definable Tone Range Points and three Internal Contrast Regions to be recognized. The Shadow point, the three-quarter tones, the middle tones, the Midtone point, the quarter tones, and Highlight point. Dividing the tone range like this will help you understand how to control the ranges using two powerful tool dialogs found both in Photoshop and Lightroom. The two main tone adjustment dialogs in Photoshop are Levels and Shadows/Highlights. I’ll present the tonal range as these two adjustment dialogs address it.
The Levels Panel offers controls over specific individual Range Points; the Black slider affects the Shadow Point placement, the Gamma slider affects the Midtone point, and the White slider affects the Highlight Point placement.
Both the Black and the White sliders influence the value of the Shadow and Highlight Points respectively. Note that these triangle adjustment sliders are not the Shadow, Midtone, and Highlight points, but they are the tools you will use to adjust and set those points. This is a very important distinction to be remembered. Only if either of the sliders intrude into the graph do they actually set those points to Black or White.
The difference a midtone move can make is obvious here. Scene averaging metering in the camera resulted in a serious underexposure of the mid and three-quarter tones. The picture on the right was the result of midtone adjustment. Notice that the shadow point and the highlight point were undisturbed with this midtone adjustment.
The Gamma “pointer” points to the desired placement of the 50% value point in the image. It determines where the 50% value will fall in an image- regardless of where the Highlight and Shadow points are located. If the Gamma slider is moved to the left, the middle tones will get shifted toward the right side, resulting in a lightening of the middle tones. Conversely, if the Gamma slider is moved to the right, the middle tones will be shifted toward the three-quarter tones, resulting in the middle tones getting darker. In effect, the midtone point (50%) will be moved within the tonal range. The original value of the Gamma point (1.00) will change when the slider is moved, though the number that appears in the Gamma field is (for most non-mathematicians) of little relevance. What is important to remember is that the new location of this slider will become an absolute 50% value. Note that this value is not the middle of the tonal range, but the location of the 50% value. The next time the Levels Adjustment panel is opened, the Gamma value will once again be 1.00.
The Expanded Middle Tone Range.
Obviously, an image’s middle tones are located in the “middle” of the image, not the geometric middle, but the middle of the tonal range. Tonal controls (in the Levels dialog box) provide only highlight, midtone, and shadow sliders. In order to fully control tonality, three internal contrast regions should be added.
I’ll divide the histogram information into three tone range points and three internal contrast regions. These three tonal regions (quarter-tones, midtones, and three-quarter tones) make up what I call “the expanded middle tone range,” and includes everything between the Shadow Point and Highlight Point. This range plays a huge role in the overall tonal balance and the clarity of every digital image. There are several tools in Adobe Photoshop that address and adjust this expanded middle region.
Measuring Light and Managing the Middle
The image sensors in a digital camera capture light in what is referred to as “linear” fashion. This means that light values are recorded in a purely mathematical fashion; as twice the light passes onto the sensor, twice the voltage is applied to each CMOS/CCD photo cell element. Image sensor devices convert light energy to electrical signals whose numerical values are stored on the camera’s memory card.
The human eye interprets light logarithmically. We humans see better in the dark than most cameras. Most of the light-capturing capability of a digital camera sensor is geared toward the bright side of the tonal scale, recording a lot of detail in the lightest areas of the picture. Unfortunately, the middle and three-quarter tones in a scene don’t get recorded with as much accuracy. There just isn’t enough light in those areas to make as much impact on the image sensor. What this means is that pictures taken in medium-to-low lighting situations often appear dark and lacking in detail. This phenomenon is not widely known, or understood when folks print their pictures. Many people simply print their pictures as they appear, right out of the camera, assuming the camera has taken care of that all this technical stuff. O, were it only that simple!
Here’s the simple truth. Unless your photographs are taken under controlled lighting, the darker parts of your pictures will not record the detail that was evident to your eye when you took the picture. To compensate for this problem you must make some simple adjustments to your images before publishing them either in print or display.
To be fair, when digital cameras save files in JPEG format they force images to adapt to a pre-determined adjustment curve in the process. Incidentally, even when files are captured in raw format the camera records the current JPEG-based file settings which get displayed as a “first guess” display in the raw interpreter software. This JPEG setting applies a curve that attempts to boost the middle tones and take a guess as to what the picture really ought to look like.
When a picture was taken under sufficient light this usually works out well. But when the picture was taken in either low light or very strong light (creating a high contrast photograph), these JPEG curves don’t correct the problem. The JPEG solution to lighting correction works about as consistently as buying one-size-fits-all clothing; occasionally both behaviors work out acceptably, but neither should really become a habit.
The next post in this series will address the all-important internal contrast areas of the Three-quarter and Quarter tones in your image. Stay tuned.
This series is a small excerpt from my soon to be released book titled The Digital Image: From Capture to Presentation and Everywhere In-between. If you find this series helpful, imagine what the book will do for you.
That’s the way eye sees it. Feel free to leave a comment and keep the conversation going. If you saw this post listed on a LI group page, add a comment to the listing in that group! Thanks for joining me. If you like this blog, let me know and tell your friends.
This is the fourth entry of the spooky series called Image Tonality and the Histogram. This series is open to the public but I highly recommend that you join the followers of this blog to keep the flow intact. Sign in now as a follower (top right of this page). This does not insinuate that you are a sheep in need of a shepherd! Follow is just a term.
See you next time, Herb
Click the Follow button at the top of the page so you don’t miss any future posts.
PS. if you have an iPad and are interested in learning about more about the fundamentals of digital photography, I suggest that you take a look at my Accurate Color iBook in the iTunes Store