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Public Lab Research note


Focus shift in bifocal filter?

by warren | January 26, 2011 05:05 26 Jan 05:05 | #61 | #61

Does this image show focus shift? It seems like the infrared is in focus while the visible is not.

see http://www.apogeephoto.com/mag1-4/mag1-4mfir.shtml for a discussion:

"light waves of different wavelengths do not focus on the same plane when passing through a simple lens"

This may be a good reason not to use the bifocal filter system as we're currently prototyping with the Near-Infrared Camera. An alternative is the Stereo Camera using two synchronized cameras on the same balloon.


1 Comments

Yes it shows focus shift and the correct reason it given. (Heavy chromatic aberration) To compensate for this there have been extra markings on older SLR-Lenses because you could not see IR on the viewfinder and the used IR-sensitive film could not be checked without developing it.

Besides using a Stereo Camera setup you could take pictures with and without the filter. Maybe by rotating it between 2 closely timed shoots. Another possibility would be tocheck old table Books on Optical Engineering and search for a matching depth of field at the higher F-Stops. But taking in account the longer exposition times for the IR pics this could be tricky.and lead to over-/underexposure or/and bluring through movement.

There is another way - you could move the focus point with so called pinhole glasses, also known as stenopeic glasses and even use such type of lens with a bifocal grid.

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