Chances are you've seen a forced perspective shot without even realizing it. The likes of Lord of the Rings and Eternal Sunshine of the Spotless Mind used the effect to create the illusion of objects being far larger, or smaller, than they really are.
With a clever use of angles and positioning a human subject can appear the same size as huge monument, or a small animal seem like a giant.
The process behind taking such a shot is simplicity itself, once the principles behind the technique are known.
When attempting to make two objects of vastly different size seem to be equal, or similar, in scale the trick is to place the smaller object closer to the lens, and the larger object further away. It takes some practice to achieve the correct framing, as getting both objects in focus and correctly exposed can be difficult.
It's best to frame for one object then have the other move closer or further from the camera to achieve the effect.
Increasing the aperture setting allows a further depth of field, so both objects can remain in focus, although on anything but a bright day this will force the shutter speed to be lengthened and therefore increase the possibility of camera shake. To remedy this put the camera on a tripod. This is why the majority of shots of this ilk are taken in daylight and outdoors, as it means the aperture can be increased without affecting the shutter speed.
For example, taking a shot of a person appearing to push back against the Leaning Tower of Piza would need the person far closer to the lens than the Tower.
Another technique to fool the viewer is to use high and low angles along with distance.
A low angle can make the object appear larger, where a high angle can do the opposite. So trying to make a small dog look huge would need a low angle and to be extremely close to the camera.
Vintage horror movies often used this effect in the pre-CGI days to elevate the likes of Ants to a far more impressive scale. Unlike combining two images into one this doesn't need quite as much focus on the lighting, although keeping the lens at a wide angle and having a plain background, such as the sky, which doesn't give away the trick is necessary.
Forced perspective doesn't require any advanced techniques so head out and give it a try today!
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