Here's the image I'll be using for this tutorial:
The original image.
Important: One thing to note up front here is that not all images actually have any areas in them that are supposed to be neutral gray, and obviously, you can't find what's not there, so if that's the case with your photo, no technique for finding midtone gray will work. In cases where it's unlikely that anything in the photo should be gray, your best bet is to color correct the highlights and shadows and leave the midtones alone. If there is an area that's supposed to be neutral gray, though, this technique will find it.
And now with that little disclaimer out of the way, let's get started! I'll be using Photoshop CS5 throughout this tutorial, but any recent version will work.
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Step 1: Add A New Layer
With our photo open inside Photoshop, the first thing we need to do is create a new blank layer, so click the New Layer icon at the bottom of the Layers panel to create a new blank layer above the Background layer:
Click on the New Layer icon.
Nothing will happen in the document window, but we can see in the Layers panel that Photoshop has added a new blank layer named "Layer 1":
Photoshop adds the new layer and names it "Layer 1".
Step 2: Fill The New Layer With 50% Gray
Next, we need to fill our new layer with 50% gray, the same gray we're looking for in our image. To do that, go up to the Edit menu at the top of the screen and choose Fill:
Go to Edit > Fill.
This opens Photoshop's Fill dialog box. Select 50% Gray from the drop-down list at the top of the dialog box:
Set the Use option to 50% Gray.
Click OK when you're done to close out of the dialog box. Photoshop fills the new layer with gray, temporarily hiding the photo below it from view in the document window:
The new layer is now filled with 50% gray.
Step 3: Change The New Layer's Blend Mode To 'Difference'
Go up to the Blend Mode option in the top left of the Layers panel and change the blend mode for Layer 1 from Normal toDifference:
Change the blend mode for Layer 1 to Difference.
The Difference blend mode isn't one that's used too often since it tends to make images look, well, different (some may even say frightening), as we see with my image after changing the blend mode to Difference:
The Difference blend mode certainly makes photos look different.
Believe it or not though, there is a method to the madness. The way the Difference blend mode works is that it compares the layer with the layer(s) below it and looks for differences between them (hence the name). Any areas in the layer which are different from the layer(s) below it show up as strange colors in the image, which is where the purples, blues and yellows are coming from, but any areas which are identical between the layers show up as black, or at least almost black if they're not 100% identical but still pretty darn close to it. In other words, with the Difference blend mode, any areas between the layers which are identical become the darkest parts of the image, and we can now use that to locate any areas that are supposed to be neutral gray!
How? Simple! We're now comparing our image on the Background layer with the layer above it that's filled with 50% neutral gray, which means that any areas in our image which are closest to that midtone gray color will now appear as the darkest parts of our image. So now, finding a midtone gray area is as easy as finding the darkest part of the image!
Step 4: Add A Theshold Adjustment Layer
Click on the New Adjustment Layer icon at the bottom of the Layers panel:
Click on the New Adjustment Layer icon.
Choose Threshold from down near the bottom of the list of adjustment layers:
Choose "Threshold".
If you're using Photoshop CS5 as I am, the Threshold controls will appear in the Adjustments Panel, new to CS5. In Photoshop CS4 or earlier, the Threshold dialog box will appear. Click on the little white triangle at the bottom and drag it all the way to the left:
Drag the white slider all the way to the left.
This will turn the image in the document window completely white:
The document window now appears filled with white.
Now slowly drag the triangle towards the right until you begin to see black areas appearing in your image. The first black areas that appear are the neutral gray areas you're looking for (assuming, as I mentioned at the beginning of the tutorial, that there actually areareas of gray in the photo). Continue dragging the triangle until an area of black becomes just big enough that you'll be able to click on it to select it, which we'll be doing in a moment. This is the exact same way you would normally find the darkest parts of the image when you're removing any color casts from the shadow areas, but now, thanks to the Difference blend mode, these darkest areas represent the areas closest to midtone gray. Click OK when you're done to exit out of the Threshold dialog box (Photoshop CS5 users can simply leave the Adjustments Panel open):
Drag the white slider towards the right until a small area of black appear
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