Recently someone was asking if it mattered whether you use the Exposure control or Levels or both to adjust an image and whether there was any difference in the results. So this week I’m going to review some basic adjustment information.
The fact is that although you can set the white point using the Exposure slider or Levels, the results you obtain will be slightly different. Let’s look at a low contrast abstract image of color reflecting in a river. Here’s the original image.
You can see on the histogram that the image is very low contrast since the pixels are bunched together. Additionally there’s a gap to the right of the data on the histogram indicating that the image isn’t using the lightest 20% of tonalities. There’s also a large gap to the left indicating it isn’t using roughly the darkest 40% of the tonal range.
If you first adjust the Exposure slider to try to use most of the lightest tonalities, and then use Levels to set the black point, you end up with this image. (I’m not going to adjust the middle slider or the quarter tone controls for this comparison.)
Compare that to this version where I left the Exposure slider alone and only used Levels to set the white and black points. The image is, as you would expect, very similar but it’s slightly darker. If you think about it that makes sense. You use the Exposure controls to set the overall brightness for the image and then fine tune the white and black points using Levels.
You will see a similar difference between adjusting the midpoint in Levels versus adjusting the Brightness slider. The Brightness slider will increase or decrease the brightness of most of the pixel values, whereas the Levels control will affect a narrower range.
There’s one other thing to consider. The above illustrations were done with the histogram and Levels set to display RGB histograms and avoiding any clipping. Here’s what happens if you use the Luminosity histogram instead and use only Levels.
Usually I use RGB histograms so I am aware of when I’m introducing clipping into any of the individual channels.
Obviously you could get very similar results using either type histogram and if you spend enough time and adjust all the parameters in Levels. There’s not one “right” way for every image, but as a general rule I use the Exposure slider to determine the overall “exposure” of the image and then I control the contrast using the Levels endpoint sliders to set the white and black points. If the image is too contrasty, then I use the Shadow/Highlight controls to reveal more detail in those areas. In that case I use the Advanced controls and limit the tonal range to just what’s needed to restore visible detail. Then I adjust the midtone brightness, when necessary, followed by the quarter tone controls to set midtone contrast. I personally find that the midtone contrast setting is one of the most important adjustments because it gives that extra pop to many images, without making any areas too light or too dark. Access the quarter tone controls by clicking the little box to the upper right of the levels histogram. Moving the bottom of the quarter tone adjustments towards the middle increases midtone contrast. You can hold down the Command key and drag each of the quarter tone controls to move their overall placement to the right or left. Doing so will increase or decrease the range of midtone values that are affected.
If any of you have developed certain approaches to tonal adjustments that you regularly use in your workflow, please feel free to suggest them.