What is Keystoning

I was speaking with a freind about photography and they mentioned a type of destortion called Keystoning I was wondering if you could explain this for me.

Keystone distortion is what happens when you point a camera upwards at

a vertical surface with the top of the film plane tilted away from the

building.</P> Imagine that there are two parallel vertical lines on

this surface. with the top of the film plane (which is the same as your

groundglass (AKA gg) further away from that surface than the bottom of

the film plane the two lines will no longer appear parallel on the gg

but will appear to converge as they run towards the top of the surface

as viewed on the gg.</P>

This type of distortion is corrected by making the film plane parallel

to the surface.

The reason it's called Keystoning is the resemblance of a rectangle to

the stone at the top of an arch. This stone has sides that are

similar to an upside down triangle although the angle is much less

acute. Anyway this stone supports the entire arch, thus Keystone. A

rectangle, like an office building, shot from below with the lens

tilted up will take on this shape.

 

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Lens rise will minimise or remove this distortion, but keystoning &/or

"falling over backwards" can add a sense of drama and impact to a

photograph of a building, so don't close your mind.

Keystoneing is what happens when you drive through Pennsylvannia

without a map! It also is known to involve less than professional

police officers with cream pies.

One thing to keep in mind when correcting for this type of distortion

is that making the sides of the building parallel to the edges of the

print may look peculiar at times. If your point of view is from a

very low or very high angle, our minds eye expects to see some degree

of convergence. Our everyday "seeing experience" trains us to expect

things to look a certain way. When we over-correct to keep all the

verticals perfectly parallel, it sometimes makes the top of the

skyscraper look wider at the top. On a different note, it's common

to run into keystoning problems when projecting slides from a coffee

table on a screen that drops down from a high ceiling. With the

optical axis of the projection lens not being perpendicular to the

screen surface, the portion of the image that is further away will

seem larger. The projected retangular image will look like a

keystone. There are tripod mounted screens that can lean forward to

correct for this. It also helps in keeping the entire image in focus.