Saturday, October 22, 2011

Adobe Photoshop Lightroom Tutorial - Part IX - Reading and Interpreting the Histogram - Basic Adjustments

Part I - Index and Introduction
Part X - White Balance

The Image Histogram

The image histogram is a graphical representation of the number of pixels of an image with a specific luminance value. The histogram is shown in the Histogram panel that is available both in the Library and Develop modules.

Lightroom - Histogram Panel

The Lightroom histogram is the superposition of the histogram of the three RGB channels using the following color codes:

  • The histogram is a primary RGB color when the channel histogram of such color is not overlapping any other channel histogram.
  • The histogram is gray when all of the three RGB channels histograms are overlapping.
  • The histogram is a secondary RGB color when the two channel histograms corresponding to the primary colors whose mix is the specified secondary color are overlapping:
    • The histogram will be yellow when the red and green channel histograms are overlapping.
    • The histogram will be cyan when the green and blue channel histograms are overlapping.
    • The histogram will be magenta when the red and blue channel histograms are overlapping.
The histogram is an useful tool to inspect the tones used in a photo and it can provide useful insights about the quality of photo. Remember that there's not a good histogram and a bad one. The histogram is just an analytically computed representation of the colors that are present in a photo. Depending on the result you want to achieve, you can use the image histogram to evaluate whether the tones that are present in the photo are the ones you were looking for or not.

The bottom line is: the histogram is a tool and you should learn how to use it. However, use your eyes and your feelings to judge your photo, not the histogram.

The Luminosity Scale is Logarithmic
One of the keys to correctly understanding the histogram is this: the luminosity scale (the horizontal axis) is logarithmic. This may sound confusing at first but it needs not be: photographers are using logarithmic scales so often that many don't even realize they're doing so.

As we've seen in another post, the basic assumption is that our own eyes behave as logarithmic sensors on most of the spectrum range we're dealing with. If fact, you soon realize that other scales we often use are logarithmic as well: the zone system, shutter speeds and aperture values.

How to Read and Interpret an Histogram
Instead of thinking about a logarithmic scale, let's make it simple and just think about f-stops. The f-stop scale is logarithmic as well with respect to the light quantity we're letting in into our camera sensor): an f-stop increment (resp: decrement) doubles (resp: halves) the quantity of light that will hit our sensor. If you're used to think about f-stops, you can conveniently use them when reading and interpreting a luminosity logarithmic axis such as the one you find in histogram, levels and curves graphs. This way, you're life will be easy.

When looking at a graphic with the luminosity in a logarithmic scale (most of which you're using: histograms, levels, curves), just think: equal distances in the logarithmic scale correspond to an equal difference in terms of f-stops. This is also the reason why, usually, such graphics are often divided into a number of squares: to let you think about f-stops and zones. If you look at the screenshot of the Lightroom Histogram panel, you'll notice that it's horizontally divided into 4 segments of equal width. If you consider the histogram to be 8-stops wide (which it often a good approximation, but you should really be aware of your camera dynamic range), every segment is 2-stop wider. Half of a segment will be 1-stop wide. You don't need any more technicality to proceed and use Lightroom proficiently. However, the web is full of detailed information related to dynamic range, gamma correction, tone mapping and so forth.

In the following posts, when interpreting histogram to understand the effect of a develop setting, we'll always use this trick to make things easier.


Basic Adjustments

The editing workflow is where your real Lightroom artist comes out, so that there's not such a thing as a "right way" to proceed. However, the odds that you'll start using the editing controls in the Basic panel are very high, so that's the place we'll start from. Afterwards, we'll describe every panel in detail in the same order it appears in the Lightroom user interface.

The Basic panel contains the most basic image adjustments. Namely, they are:
  • Treatment: colorblack & white.
  • White balance: temperature and tint.
  • Tone: exposurerecoveryfill lightblacksbrightness and contrast.
  • Presence: clarityvibrancesaturation.
Don't be deceived by their name, though. Basic doesn't mean powerless. On the contrary, they're pretty powerful and they will be fundamental adjustments for your image more often than not.

I also found that their own meaning is deceiving and many people find it difficult to understand what their purpose exactly is. To make sense of them and unleash all of their power, we're going through a detailed explanation of what's their effect and what they're meant for.


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3 comments:

Anonymous said...

What an awesome guide, thanks for the info! It's nice to see a guide that explains that each of the 5 sections in a histogram don't represent one f-stop, which is confusing considering a DSLR has 8+ F-Stops in DR, represented in a histogram.

Enrico Maria Crisostomo said...

I'm really glad it helped.

web development India said...

Thanks for sharing the photoshop color info here. Keep up the good work. All the best.