There is a lot of science behind the Tilt-Shift Photography
Simply said, that's when we use techniques and lenses that could tilt and shift.
Those are special types of lenses allowing to, shortly said, move their optical elements in the horizontal and vertical axes, independently. Imagine you want to photograph a skyscraper and you're in its base. When you look up it looks like a trapezium and not a rectangle. This, for instance is a great use case for the shifting feature of such lenses. So there is a stigma those lenses are used only for architectural photography. However, I know photographers, who are in love with their TS lenses and use them for all purposes, travel, portrait photography, etc...
Of course, the tilt-shift effect could be achieved in post-processing by using graduated filters, you gonna need to apply few of those over each other several times. That effect is quite popular actually as it creates a "miniature" effect since the focus plane is quite narrow. Controlling that plane is where the Tilt comes into the picture ;)
Here are few images with such effect and at the bottom I have put one image with and without it so you could compare. Generally, high-view-angle photos are the best suitable to play with that effect. If you are a train models collector, go have some photos of train stations from above and apply that effect, it's fun! :)
P.S. Just to mention that the most of the good modern mobile image editors have that tilt-shift software effect as, of course, they don't have the physical space to have that done optically. :)
Have a creative day!
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Copyright: Damian Hadjiyvanov, Light Captured
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Have a great day, stay safe, stay strong and let your muse be always on your shoulder!