how can I figure out what filter someone has used? I have searched it up on multiple websites and downloaded multiple apps trying to find a match and I cannot find one. Someone please help.
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Learning how to determine which filters a person used in processing their images is tricky. Also, there are a number of filters that can produce the same results with various settings. (For example, everything you can do with a levels filter can also be done with a curves filter.)
The best way to learn this skill is to use the filters and to experiment with them to see how they work. A good starting point is to generate a black to white linear gradient and try it with different filters. For example, when you move the black point up in the Levels tool, what happens to the gradient? What about when you move the white point down?
Some filters may require more complex experiments to understand. One method I've used with more complex color filters is to make an image with approximately all the colors in it. I do this by starting with a square image in Photoshop and dividing it up into some n x n smaller squares. (I usually choose a power of 2, such as 64 x 64. 2^6 = 64.) I then make a vertical linear red gradient in each square and a horizontal green gradient in each square. Finally, I fill each square with a solid shade of blue. The first square has 0 blue, the next has slightly more, until the last has the maximum amount. Then I apply a color filter and see what it does to the colors.
There are other more intuitive ways to learn the filters such as just applying them to your images and trying things to see what happens. Watching a good tutorial online (even a free one) may help, too.
Regardless of how you go about learning, the bigger question is "What are you trying to accomplish by figuring out which filters were used on a particular image?" If you're trying to recreate a look, your best bet is to learn the filters in your favorite editing tool. You can often create a look without knowing exactly what was done to the one you're trying to copy. Also keep in mind that it may not be possible to determine a particular filter because some things could be done in camera or in post, and you may not be able to tell which it was.