On 2008-09-26, James Silverton wrote:

[...]

I haven't got the nerve (or ability, probably) to perform surgery on a
camera but how expensive a camera do you need for IR photography? I have
been impressed by the artistic quality of many IR photographs but not
their sharpness and those were taken with conventional film cameras.

IR rays come to a focus further from the lens than visible rays do, so
after visually focusing you need to adjust the distance setting on the
lens to compensate for the difference [1]. But if your IR-passing filter
isn't restricted to a narrow waveband, there will still be some IR rays
with much longer wavelength than others getting through to the
film/sensor and they won't all be focused at a single point. Normal
camera lenses are designed to overcome this 'chromatic aberration' for
visible wavelengths, but they don't do it for UV or IR. So IR pictures
are seldom as sharp as those the same equipment can make using only
visible light. Using a small aperture can help a little - at the cost
of a longer exposure, of course.

[1] Some lenses have an IR focus index as well as the visible-light one;
after focusing visually, move the focusing ring so that the distance next
to the usual focus index is next to the IR one instead. If there isn't an
IR index, use the 'closer' depth of field indicator for f/5.6. Of course
with an auto-focus-only or fixed-focus compact camera, you're stuck with
what the camera does, which will be 'wrong' - but the large 'depth of
field' that goes with a tiny sensor might offset the problem somewhat.

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-- ^^^^^^^^^^
-- Whiskers
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