how to keep a rectangle rectangular?

[myriam]

Hi,

I'm doing some filming where my DV camera (a panasonic GS65) is fixed to the ceiling on top of my desk and it films down whatever is hapenning on the desk. It's very important for me that objects on the desk look square or rectangular if they are square or rectangular, and that they do not "bulge out" towards the middle. In the beginning I was expecting to have a lot of problems with this, but as it turned out my camcorder can perfectly render a 15-by-20 rectangle at a distance of about 4 feet (to my great surprise) without any bulging out in the middle (if you bring the distance down to about 2.5 feet you start to see some serious bulging). In fact I've had the *opposite* problem: the corners of the rectangle seem nearer or larger to the camera than the rest of it, though they are technically further away. I should mention that the camera is looking straight down onto the rectangle from above. This makes the rectangle "sag in" slightly instead of "bulging out". I have two questions:

1) What is causing this "sagging in" effect? How is it possible and how can I adjust to get rid of it?

2) I'm thinking of changing camcorders to go to HD, maybe buying the HDR-HC1. How can I tell in advance what the perspective will look like, and whether I will have problems with either sagging in or bulging out?

There may be some technical vocab for the "sagging in" and "bulging out" effect I'm describing. Please fill me in.

Thanks!

-M

[junkmalle]

the corners of the rectangle seem nearer or larger to the camera than the rest of it, though they are technically further away. I should mention that the camera is looking straight down onto the rectangle from above. This makes the rectangle "sag in" slightly instead of "bulging out". I have two questions:

1) What is causing this "sagging in" effect? How is it possible and how can I adjust to get rid of it?

Sounds like the camera's lens is causing a pincushion distortion:

http://www.dpreview.com/learn/?/Glossary/Optical/Pincushion_Distortion_01.htm

You can usually reduce this by placing the camera farther away and using more zoom. Bulging out is called a barrel distortion.

2) I'm thinking of changing camcorders to go to HD, maybe buying the HDR-HC1. How can I tell in advance what the perspective will look like, and whether I will have problems with either sagging in or bulging out?

Try them out? Read reviews? Now you know what to look for.

[myriam]

Thanks. Changing the distance of the camera to the subject is not really an option for me, though. Can different lenses be used to alleviate to alleviate either a pin-cushion or barrel effect?

-M

[junkmalle]

Thanks. Changing the distance of the camera to the subject is not really an option for me, though. Can different lenses be used to alleviate to alleviate either a pin-cushion or barrel effect?

-M

If your camera lets you use different lenses you can try them. A wide angle lens may be less likely to pincushion than the typical zoom lens at it's widest setting.

There is one other possibility although I've never tried it. In a pinch, you can use a magnifying glass (convex lens) right in front of a camera as a macro lens. In theory you could use a concave lens in front of the camera to shorten the focal length. That will let you increase the zoom on the camera, getting it more towards the middle of its zoom range -- and maybe reducing the pincussion effect.

[edDV]

I agree this is an optical problem for now. Be aware that DV pixels are never square.

It would help if you better described what you are trying to do.

[junkmalle]

It would help if you better described what you are trying to do.

A sample image with a big chess board would be good!