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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 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.


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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