First, before we begin editing the photo with adjustment layers, I'll revert my image back to its original unedited state by going up to the File menu at the top of the screen and choosing Revert. My photo is now back to looking dark and underexposed, and I'm back to having just one layer - the Background layer - in the Layers palette:

If you remember, the first thing we did to brighten the image was duplicate the Background layer and change its blend mode to Screen. Problem was, by duplicating the Background layer, we doubled the file size of our Photoshop document. Let's see what happens when I use an adjustment layer instead. I'm going to click on the New Adjustment Layer icon at the bottom of the Layers palette, which brings up a list of all the adjustment layers I have to choose from. As we saw in the Non-Destructive Photo Editing with Adjustment Layers tutorial, most of Photoshop's standard image adjustments, found by going up to the Image menu and choosing Adjustments, are also available as adjustment layers. I'm going to select a Levels adjustment layer from the list:

When the Levels dialog box appears, I'm just going to click OK in the top right corner to exit out of it, since there's absolutely nothing I need to do with it. I'm simply adding the Levels adjustment layer instead of duplicating the Background layer. Photoshop adds the Levels adjustment layer directly above the Background layer and names it "Levels 1". Then, just as I did before, I'll go up to the Blend Mode option at the top of the Layers palette and change the blend mode of the adjustment layer from "Normal" to Screen:

The photo now appears much brighter. In fact, it appears just as bright as when we duplicated the Background layer and changed its blend mode to Screen. The results are exactly the same:

If the results are the same, what's so great about using the adjustment layer instead of a copy of the Background layer? What's great about it becomes clear when we check out the new file size of our document. If you recall, when we duplicated the Background layer, it doubled the size of our Photoshop document from 22.8 MB up to 45.6 MB. This time, by using a Levels adjustment layer instead of duplicating the Background layer, our file size remains unchanged:

We just saved ourselves 22.8 MB of additional file size simply by using an adjustment layer in place of a normal pixel-based layer! Whenever we add an adjustment layer to our document, Photoshop stores all the information about the image within the adjustment layer itself, but it does it without the large file size increases that we get from adding pixel-based layers. In fact, in my case here, the file size increase was so small that it didn't even register. That won't always be the case, but the increase will always be much less than if you had used a pixel-based layer.

Why did I choose a Levels adjustment layer as opposed to a Curves or Hue/Saturation adjustment layer? I chose Levels only because it's one of the most commonly used adjustment layers, but since I wasn't planning on doing anything with the adjustment layer (other than changing its blend mode) and only added it in place of a pixel-based layer, I could just as easily have chosen any other type of adjustment layer from the list. Photoshop would have stored all the information about my photo within the adjustment layer no matter which type I chose.

Let's continue on following the same steps we used previously to edit the image. Since my image still needs a little more brightening, I'm going to duplicate my Levels adjustment layer using the same keyboard shortcut we use the duplicate a pixel-based layer, Ctrl+J (Win) / Command+J (Mac). Photoshop adds a copy of the Levels adjustment layer directly above the original and names it "Levels 1 copy". Normally, we'd want to rename layers and give them more meaningful names, but to save us some time, I'll just carry on. I'll leave the blend mode of the new adjustment layer set to Screen and fine-tune the brightness of the image by lowering the Opacity of the adjustment layer to 50%, just as I did with the pixel-based layer. Notice how Photoshop allows us to do many of the same things with adjustment layers that we can do with pixel-based layers, like duplicating them, changing their blend mode and changing their opacity settings:

Again, we can see by looking in our document window that the results are exactly the same as if when we were using pixel-based layers. I've corrected the exposure of the photo using nothing more than a couple of adjustment layers, the Screen blend mode and the Opacity setting:

When we were using pixel-based layers, our file size at this point had tripled from what it was originally, going from 22.8 MB up to 68.3 MB. Yet even though we've now added two Levels adjustment layers, our file size still hasn't changed:

We've now managed to save ourselves over 45 MB in file size simply by replacing pixel-based layers with adjustment layers! Again, it won't always be the case where you see no file size increase at all, but the increase will always be much smaller than if you had used a pixel-based layer. Let's finish things off by boosting the color saturation of the photo. Rather than merging all three layers onto a fourth layer like I did last time and then using a standard Hue/Saturation adjustment from the Image menu, I'll simply click on the New Adjustment Layer icon at the bottom of the Layers palette and choose a Hue/Saturation adjustment layer from the list:

This brings up the exact same Hue/Saturation dialog box that we saw when I chose the standard Hue/Saturation command from the Image menu earlier. And just as I did earlier, I'll first select Reds from the Edit option at the top of the dialog box so I'm boosting the colors in the flower itself, not the green leaves behind it. Then I'll click on the Saturation slider and drag it towards the right to a value of +45:

I'll click OK to exit out of the dialog box, and my photo now looks identical to the result we achieved using pixel-based layers and a standard Hue/Saturation adjustment:

When we performed the same steps using pixel-based layers, the file size of our final result was all the way up to 91.1 MB, a considerable jump from the photo's original size of 22.8 MB. This time, we've achieved the exact same final result using adjustment layers in place of the pixel-based layers, yet the file size of our Photoshop document still hasn't increased beyond the original:

Again, this was just a quick example of how adjustment layers can keep the file size of our Photoshop documents down to a minimum, yet even in this simple example, I still managed to shave around 68 MB off the size of my file simply by using adjustment layers in place of pixel-based layers! Of course, digital images are made up of pixels and you can't replace every pixel-based layer in your document with an adjustment layer. But the next time you find yourself duplicating a pixel-based layer so you can apply one of Photoshop's standard image adjustments to it like Levels, Curves or Hue/Saturation, try adding an adjustment layer instead. Photoshop and your computer will thank you for it!

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