Archived posting to the Leica Users Group, 2006/11/25
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]At 10:57 AM -0800 11/25/06, Adam Bridge wrote:
>Henning, I've done no IR at all, ever. In reading comments I notice
>that there are often lens scales showing IR focusing differences. Does
>this vary from lens to lens? Is there a way to compute the difference
>and create a spreadsheet or do I do it manually and make a table of
>results? Does the setting for infinity change? Longer wavelengths
>focus closer or further than visible light? I don't remember and my
>mind can make reasons why its either.
>
>Thanks for answering basic questions.
>
>Adam
Hi Adam,
It will vary from lens to lens and with different filters, but in
general try setting the distance that you measure opposite the f/4
with a moderate IR filter, such as an 89b or equivalent (Hoya 72 or
Leica IR or B+W 092). A stronger IR filter, such as an 87 will need a
bit more correction, and some lenses start getting noticeably soft.
That's why it's a good idea to use smaller apertures such as f/8 or
f/11. That will cover you for the missed focus plane as well as the
uncorrected lens aberrations at those wavelengths.
An ideal IR filter would be one that allows only a small part of the
IR spectrum through, because the usual IR filters, blocking visible
light only, then allow a wide range of IR wavelengths to get to the
sensor, and if the sensor is one like the M8, wavelengths up to
1000nm get captured, all the way from 700. The problem is that 900nm
will focus on a different plane than 750, so a certain smearing will
occur with most lenses. If a filter had it's nominal sensititvity at
800nm and transmission at 750 and 850 of 5 to 10% of that at 800, you
could focus for 800 and get sharper files.
APO lenses are a different issue, and may or may not focus IR
correctly. APO correction means that the lens is corrected for three
different wavelengths, but you rarely can find out which ones, or how
far the focal length drifts off the design length at longer
wavelengths. In general APO lenses are better at IR. I haven't used
any Leica APO lenses for IR, as my main interest is in the wider
focal lengths.
--
* Henning J. Wulff
/|\ Wulff Photography & Design
/###\ mailto:henningw@archiphoto.com
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