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Adjusting Color & Exposure
In Adobe Photoshop
Solving Exposure Problems
The exposure in digital photography, is best visuallized in a statistical graph called a "histogram," which plots the amount of information in the tonal values ranging from pure black, to middle gray, to pure white. Where to find histograms.
For most images, perfect exposure is represented by the typical "bell curve," shown in red. As shown in the graphic below, the bulk of the image's values (shown in black) reside in the midtones. This image may not have a perfect bell curve, but it is correctly exposed, so it comes close.
The image represented by this histogram has a pleasing exposure, as the majority of the image is in the midtone range, and it tapers off to subtle highlights and shadows.
In digital imaging, exposure mistakes can result in overexposure:
Image values reside only in the midtone-to-white range, "hitting" the white-point "wall"
Or under exposure:
Image values reside only in the midtone-to-black range, "hitting" the black-point "wall"|
Exposure mistakes in digital imaging only favor one direction: under exposure.
If a file is underexposed, the values and information containing the detail of the image are mostly there, it is just compressed. We like to refer to such an image as being only "mostly dead."
If, however, the file is overexposed, and especially has "hit the white-point wall," we affectionately refer to that as "all Dead!" There is not much that can be done with it. The detail and information beyond the white point has all been converted to pure white, and is therefore lacking in any detail - for good.
There is one way to save your back, however, but must be done before the shoot: Shoot in Raw format, or in what Kodak claims is just as good, ERI JPEG (only supported by Kodak, and a format we have never rewiewed). RAW captures the images exactly as the chip records the scene, and still contains highlight detail and information.
Most Photographers daily debate if they will use the RAW format, it does provide all power, but in inconvenient. Click here for more info on the pro's and con's of the RAW format, and all formats.
JPEG is often the preferred format for busy photographers. It is compact, writes and reads faster, and if you shot correctly, it captures an almost lossless image at a quarter to one eighth the size!
The problem is that it sets in stone the exposure, white balance, and colorspace of the image. This tutorial assumes that you must have an image in either TIFF, JPEG, or PSD; and therefore need to correct for exposure, white balance, and colorspace.
I have other pages that will discuss how to correct for exposure, the Color Balance section reviews correcting white balance, and the Color Management Workflow Tutorial covers the choices in colorspaces.
These are the other sections of this module:
- Using Levels to Correct Exposure - Covers the power of levels as an adjustment layer to correct underexposed and slightly over exposed images. This is my favorite method.
Using Curves to Correct Exposure - This method is favored by many who like te freedom to move the adjustment in all directions. (I like the controlled, linear levels, but then again I'm not that artsy). It is also most powerful in lightning/darkening only certain ranges (highlights, midtones, or shadows).
- Using Contrast/Brightness to Fix Exposure Corrections - Having used one of the above methods to correct exposure, the image may become flat and without contrast. This will fix it!
- Using Curves to Add Contrast - A better, more controlled approach to adding contrast. It "pinches" shadows and highlights rather than clipping off information.
- Dodging and Burning - There are many known ways to do this in photoshop, including using the tool of the same name. However, we won't discuss that tool, we'll be using a more powerful method that involves layers. This allows it to be turned up or down and on or off!
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