Deval
10-Jan-2015, 12:57
Hi everyone. Happy new year. I've learned so much from this forum since I've joined, I thought I'd share something if people are interested. I ran into a foolproof quick technique to remove color casts from any image in Photoshop using a fill gray layer and thresholds. I know a lot of tutorials are out there, but this is a mathematical foolproof way without the need for a real life grey card. I've tested it with Kodak Ektar 100, Portra, Fuji Velvia 100, digital. It should theoretically work with any type of file with a color cast. I've tested this technique versus "eyeballing it" in curves or mathematical curve adjustment...
I would like to put a video together to as a video is worth 10000 words, but I thought I'll start with this.
Step 1)Open the color image file.
Step 2)Step 2, Create a new layer(Shift-Command-N)
Step 3)Fill the layer with 50% gray(Shift-F5). Select 50% gray.
Step 4)Click on the gray layer and change the overlay method from normal to difference
Step 5)Select the gray layer(if not already selected) and create an adjustment layer for threshold. The image should look black and white.
Step 6)Drag the threshold slider all the way to the left until the image is completely white. Then slowly drag the slider to the right until the first black pixels start showing up on the image. These pixels should technically be the first 50% gray pixels. Click the zoom button and zoom until you can find individual black pictures. Click on the eye dropper tool and hover over any one of those black pixels and Shift-Click on the pixel. This will identify the 50% gray pixel of choice with a number(starting with one)
Step 7)Turn the Visibility of the gray and threshold adjustment layer off(eyeball button to the left of the image).
Step 8)Zoom into the saved image point of the 50% gray pixel you created in step 6. Select the image from layer and make a curves adjustment layer. On the left side of the curves are three eye droppers(Black point, gray point, and white point from top to bottom). Select the gray point eye dropper and click on the pixel that you put a target on step 6. That will get rid of the major color cast. If you are happy with the image, you can stop here. If you want, try step 10.
For those that already know a lot more about this than me, feel free to correct or comment. If this was helpful let me know as well. Thanks again to the forum as a whole for teaching me so much about the art of large format. If you have localized color casts you can use lumosity masks with this technique. You can also use threshold to find the black and white point from the original image to add points to select to from curves correction.
I would like to put a video together to as a video is worth 10000 words, but I thought I'll start with this.
Step 1)Open the color image file.
Step 2)Step 2, Create a new layer(Shift-Command-N)
Step 3)Fill the layer with 50% gray(Shift-F5). Select 50% gray.
Step 4)Click on the gray layer and change the overlay method from normal to difference
Step 5)Select the gray layer(if not already selected) and create an adjustment layer for threshold. The image should look black and white.
Step 6)Drag the threshold slider all the way to the left until the image is completely white. Then slowly drag the slider to the right until the first black pixels start showing up on the image. These pixels should technically be the first 50% gray pixels. Click the zoom button and zoom until you can find individual black pictures. Click on the eye dropper tool and hover over any one of those black pixels and Shift-Click on the pixel. This will identify the 50% gray pixel of choice with a number(starting with one)
Step 7)Turn the Visibility of the gray and threshold adjustment layer off(eyeball button to the left of the image).
Step 8)Zoom into the saved image point of the 50% gray pixel you created in step 6. Select the image from layer and make a curves adjustment layer. On the left side of the curves are three eye droppers(Black point, gray point, and white point from top to bottom). Select the gray point eye dropper and click on the pixel that you put a target on step 6. That will get rid of the major color cast. If you are happy with the image, you can stop here. If you want, try step 10.
For those that already know a lot more about this than me, feel free to correct or comment. If this was helpful let me know as well. Thanks again to the forum as a whole for teaching me so much about the art of large format. If you have localized color casts you can use lumosity masks with this technique. You can also use threshold to find the black and white point from the original image to add points to select to from curves correction.