Nov 9, 2009

Mind Your Histogram…

Bad Histograms
Hard Work Blunted…
Recent review of many Google and Bing images created a technical quandary. When I found a Magic Hour landscape image I sort of liked, it often had 'unbalanced' histograms with 'blown out' shadows or highlights – or both.
How do you know?
If the histogram touches or runs up either vertical axis, you’ve blown your histogram and may loose valuable data.
The shooter might take several shots of the same composition; thoughtfully evaluating each histogram. Several shots are a heuristic way of assuring captured data fit within the histogram - creating a well-balanced image.
We won’t touch on Expose to the Right – use Goolge to find valuable tutorials to enhance your digital growth.

Magic Hour is that time around dawn and dusk when light is soft and lustrous...
Now, if you consider I like deep wilderness shots, the shooter really took some extraordinary pains to get to that hard-to-find place where a particular image was captured. Yet, finding a balanced histogram from my image search was an exception - rather than the rule. So, why not apply the right technical steps in camera to assure a 'well-balanced image'?

Histogram Fundamentals
Back in the digital studio…
When histograms violate black and white points, the shooter doesn't always apply the first rule of capture - don't blow out shadows or highlights. Your primary step in color balance is to choose that fitting black and/or white point.
Little spikes at each end of the histogram indicate clipping – rather important markers. If spikes are black, you're okay. If not, you need to practice eliminating blown out areas in your histogram.

Reading Histograms
The underexposed histogram tells you 2 essential things: somewhere on the image, the red channel is blown, and, on the linear sensor distribution you're missing about half your highlights.
In the overexposed histogram, red, green, and blue channels are blown and you're missing about half your shadows. If you see yellow, cyan, or magenta, you have a combination of 2 blown channels.
Needless to say, when you get into high dynamic range photography, histogram interpretation gets much more complicated...

If you're shooting raw and you've blown a highlight or shadow (or heaven forbid, both), when you open the file in Lightroom or Photoshop, Adobe Camera Raw lets you make immediate corrections to black-and-white points. Simply move Exposure or Blacks sliders until vertical triangles go black.
If you want to learn more about histogram fundamentals, get a hold of one of Bruce Fraser's books Real World Adobe Camera Raw. Details may vary from book to book, but what you need is the well-written overview and how-to examples.

DSLR, Linear Sensor, nonlinear Eye, balanced histogram

Sensor Technacrati…
Bear in mind - digital cameras have a linear sensor. If you think of your eye as being a sensor, it's not linear; it's nonlinear! In other words, you and your camera clearly do not see the same way. So, it may help to learn to think like a camera - you get better images.
Here’s a diagram comparing important camera and human qualities. The big deal; where are most pixels in an image from a linear sensor? Upper 50% of the histogram…

Although there are other sequences of advanced steps which color balance and tone map your images to fine art perfection, our discussion is a pretty basic start!
So I urge - think like a camera, learn and use its remarkable capacity to your advantage, and you'll simply get better pictures...

No comments: