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Exposure Versus Brightness

When you move the Exposure slider to the right in Photoshop, the image you are editing gets brighter. That makes sense. When you do the same with the Brightness slider it also gets brighter, but not in the same way. And they aren't interchangeable. If you've ever puzzled over this, here's the full story.

To understand what's going on here, we need to start with what was going on back in early versions of Photoshop. The Brightness slider prior to Photoshop CS3 worked differently than it does today. Oh, you can still get Brightness/Contrast to do the same thing as it once did by turning on the "Use Legacy" check box, but you really don't want to. In Legacy mode, increasing Brightness added a set amount to every color value for the red, green and blue channels equally. It didn't matter what any given original value was; everything got shifted upward by the same amount. Increase brightness even more and every pixel got increased even more equally. Legacy Brightness thus had the effect of making bright areas brighter while it also made dark areas brighter. In so doing, it could push some bright areas bright enough to get burned out and lose detail. It also left you with an image lacking any pure black areas since even these areas got increased by the selected amount. And since increasing color values also increases saturation, what you ended up with as an oversaturated image with burned out highlights and poor contrast due to the lack of blacks. Given this, smart Photoshop users moved to using Levels to brighten up an image, abandoning the Brightness dialog entirely. Legacy Brightness serves only to ruin an image.

Finally addressing this problem, Adobe rewrote how Brightness/Contrast worked in the release of Photoshop CS3. Nowadays, the default (without "Use Legacy" checked) produces much better results. The new method functions more like the mid-tone slider in Levels. The black point and white point of an image remain relatively unchanged now when the Brightness slider is increased, with the effect targeted more appropriately to where it is needed in the mid-tones. Rather than pushing the entire histogram toward the right as with Legacy, Brightness now pushes over the middle "hump" of the histogram, keeping the bottom left and right corners of the histogram comparatively unchanged. If your histogram started out looking like a mythical evenly balanced bell curve, you will end up with a bell that appears to be leaning over somewhat to the right when you increase Brightness. The effect now has a far less devastating impact on saturation too since it operates mainly on luminance rather than directly on RGB color.

This brings me to Exposure. Basically, the algorithm for Exposure in Photoshop attempts to duplicate what increasing exposure in-camera does. Shadows get somewhat brighter, but bright areas get even brighter still. In a sense, the Exposure slider is a throwback to how Legacy Brightness works, but with one significant difference. Rather than increasing everything equally, Exposure increases values relative to how bright they are to begin with. The histogram won't end up leaning over, but it will shift to the right. But the highlight end will shift far more than the shadow end will. The Exposure control was first added to Photoshop to support the new High Dynamic Range capabilities of CS2 but has become a standard way to brighten up 8-bit and 16-bit images as well. If you shoot in raw mode, you're still better off using the Exposure slider in Adobe Camera Raw or Lightroom before ever opening an image in Photoshop. Raw images still retain the linear gamma color space where such edits can be made with less image degradation, but I do appreciate being able to adjust exposure a tad after raw conversion.

Histogram change from increasing Legacy Brightness
Histogram change from
increasing Legacy Brightness
Histogram change from increasing Brightness (non-Legacy)
Histogram change from
increasing Brightness (non-Legacy)
Histogram change from increasing Exposure
Histogram change from
increasing Exposure

So which method of brightening up an image should you use? Clearly, the Legacy mode of Brightness should never be used, but what of the other two methods I've detailed here? If you're needs are more complicated, you'll perhaps be better served by going straight to Levels or even Curves, but it is nice to have an easy way to fix common brightness problems. But not all common brightness problems are created equal.

If you're trying to compensate for underexposure but didn't get things quite right while still working in raw (or of course if you're shooting jpeg or even working with a scanned film image), the Exposure control should be your first choice. If the image you're working on isn't underexposed but still lacks sufficient detail in the mid-tones, the non-Legacy Brightness control will better fit the bill. These aren't interchangeable either. Although both Exposure and Brightness can make an image brighter, they do so in different ways geared towards different objectives. Don't make the mistake of thinking that just one of these tools can do both.


Date posted: February 5, 2012

 

Copyright © 2012 Bob Johnson, Earthbound Light - all rights reserved.
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Your Friend, the Histogram
Exposure Adjustment in Photoshop CS2
Finally, Brightness and Contrast that Actually Work!
 

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