SFredrickPhoto.com Blog

In Part 1 we learnt that increasing the overall contrast in an image by setting a white and black point can add to the overall appearance of an image. And the procedure for setting those points is the same with either Levels or Curves. This time we’ll take a look at adjusting the overall brightness of an image with both Levels and Curves. And then we’ll introduce some of the more advance adjustments you can do with Curves that are not possible with Levels.

In the Levels and Curves dialogs shown below the black and white points have already been set, as discussed in Part 1. Added,  highlighted in the red circles, are similar edits to increase the overall brightness of an image.

Adjusting the overall brightness of an image with Levels is done by moving the middle gray triangle, the Gamma slider. This allow the midtones to be adjusted without affecting the highlights or shadows. Moving the Gamma slider causes Photoshop to reassign the midtone value of 128 to a new tone. Moving the slider left will map a darker tone to 128 and moving the slider right will map a lighter tone to 128. The net result of this is moving the slider to the left, mapping a darker tone to 128, will lighten the overall image. So to lighten the image drag the Gamma slider to the left and to darken the image drag the Gamma slider to the right.

Dragging toward the blacks to lighten and toward the whites to darken is a little counter intuitive. However,  when dragging the Gamma slider the image display is “live” providing immediate feedback as to what is happening with the image.

To make the same adjustment with the curves dialog position the cursor so that the Input value reads 128. Now drag up to lighten the image and down to darken the image.  The adjustment shown on the left will map midtones, 128, to a brighter value, 165 in this case.  

Comparing the mapping that is taking place we see that Levels and Curves “get to the same place” but operate differently. In the Levels dialog above the midtone value of 128 was assigned to what was before a darker value (higher number). And in the Curves dialog above the midtones, values of 128 were reassigned to lighter value of 165. The affect on the image however, was the same increase in overall brightness.

Now we have exhausted the image enhancement capability of Levels (at least when working on the RGB channel): Black Point, White Point, and overall brightness. Hopefully the 2 articles in this series has shown how easy it is to implement the same enhancements with Curves. But with these three adjustments we have just scratched the surface of what is possible with Curves. 

In effect, with Levels we are limited to adjusting just 3 points on the curve: white, black, and midtone; but Curves gives us access to the rest of the points on the curve. The major advantage of curves is the ability to affect tones in a range with out affected tones outside that range. For example we can open up the shadows without any affect on the midtones and highlights. That’s just not possible with Levels. You can apply very sophisticated adjustments with Curves because you are able to add anchor points that lock certain tonal values on the curve, and then lighten or darken other tonal values on an individual basis.
However, a more common use of Curves is to enhance midtone contrast without clipping shadow or highlight detail. (Note: Midtone contrast is different form of contrast than the overall contrast created by setting black and white points.) Midtone contrast will had snap to a photo. In the Curves dialog on the left, after the white and black point were set, the lights were raised and the darks were lowered. This steepened the curve between these two new points (and flattened the curve outside that range). Making a steeper curve, lighten lights and darkening darks, increases contrast. In this case the midtone contrast.

Setting the black and white points sets how much overall contast there is in the image. But by steepening part of the curve we increase contrast in the tones for that part of the curve. So here we are increasing midtone contrast as the expense of highlight and shadow contrast. However, visually, this type of curve adds a lot of snap to an image.

In short, Curves provides much greater control over your tonal adjustments than any other tool available to you in Photoshop. So I encourage you to start using it. And to get you started using Curves, hopefully, this two part series has shown how easy it is to implement Levels types of adjustments using the Curves dialog. And has wet you whistle to go further with curves.