How to avoid keystone effect - make the camera look vertically on the page

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L.Willms
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How to avoid keystone effect - make the camera look vertically on the page

Post by L.Willms »

I disagree with the opinion voiced repeatedly in another thread, that the camera has to be centered above the center of the page to be scanned, order to avoid the "keystone" effect, that is a trapezoid distortion.

Actually, the optical axis of the camera has to stand perpendicular (vertical) on the plane to be imaged.

And that is not too difficult to find out: put a mirror on the surface which is to be photographed, them move the camera around until the lens is in the center of the image shown in the viewfinder or display, and you are set.

I got this tip from a professional photographer whom I consider a good friend of mine, and who now operates a finca on Mallorca as a cultural center and as a base for on location photo shootings on Mallorca.

Make a test pointing with your camera on the house on the other side of the street, and you should find that the window frames seen on the edges of your image are well aligned, as long as you hold your camera horizontally looking on the wall. Only when you look up to the higher floors you will see the "keystone" effect.
duerig
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Re: How to avoid keystone effect - make the camera look vertically on the page

Post by duerig »

I think you are right that keystoning strictly refers to the distortion caused when the camera is not looking straight at the page. And while it isn't the only kind of distortion, it is a kind of distortion that can be readily eliminated.

There are still good reasons to want the camera to point to a particular spot on the platen rather than just straight at the platen. When you set the zoom of the camera, you are zooming in on the center wherever the camera points at. So having the camera pointing at the (approximate) center of the platen or of the book page means you can zoom in as much as possible while keeping either the entire platen or the entire book in view.

The mirror technique you refer to is very useful, but there are two ways to sharpen it.

First, if you paint a dot on the mirror, you can center that dot in the viewfinder in addition to centering the reflected image of the camera lens. If you do this, the camera is now not only aligned pointing directly at the platen, but also pointing at the position of that dot in particular. If the mirror is a standard size and the dot is in the exact center, you can place the mirror with the aid of an alignment card to point the camera at a known repeatable position on the platen.

Second, if you use a crosshair instead of a dot, you can also lock in the camera's rotation. This is not quite as useful because the software deskewing methods out there seem to be quite accurate. But if you align the crosshairs up with a crosshair in the viewfinder, then you may find that your scans are completely straight on their own and do not need any software deskewing.

I recommend the camera trick for anyone whose scanner maintains a fixed geometry between the camera and the platen. A small craft mirror, a marker, a printed image on cardstock, and a craft knife is all you need to perfectly align your camera. I include a mirror and alignment card in my kits, but it is also easy to make on your own.

-Jonathon Duerig
L.Willms
Posts: 134
Joined: 21 Sep 2016, 10:51
E-book readers owned: Tolino Shine
Country: Germany
Location: Frankfurt/Main, Germany

Re: How to avoid keystone effect - make the camera look vertically on the page

Post by L.Willms »

duerig wrote:There are still good reasons to want the camera to point to a particular spot on the platen rather than just straight at the platen.
Sure, I agree that the camera should center on the center of the book's page. At the intersection of the two diagonals.

As you said, for freedom of zooming and also because camera lenses tend to have quality losses at the edges of the imaged area. But not for avoiding the trapezoid distortion (keystone effect).
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