Archived posting to the Leica Users Group, 2006/11/25

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Subject: [Leica] M8 & Infrared //Erwin Puts article on the subject
From: abridge at gmail.com (Adam Bridge)
Date: Sat Nov 25 15:45:23 2006
References: <200611241934.kAOJYJiv075075@server1.waverley.reid.org> <000e01c7106f$889ddd90$bf34a33e@symke> <37433C73-86F1-4550-B882-6A3FF7BAFA19@comcast.net> <p06230906c18e38cbc07d@10.0.1.7> <4cfa589b0611251057k30ede90lb55c685d833f6991@mail.gmail.com> <p06230908c18e4aef00fe@10.0.1.7>

Thanks for the instructional reply which I greatly appreciate, Henning!

Adam

On 11/25/06, Henning Wulff <henningw@archiphoto.com> wrote:
> 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
>   |[ ]|     http://www.archiphoto.com
>
> _______________________________________________
> Leica Users Group.
> See http://leica-users.org/mailman/listinfo/lug for more information
>

In reply to: Message from jim.christie at sbcglobal.net (Jim Christie) ([Leica] M8 & Infrared Photograph)
Message from s.jessurun95 at chello.nl (animal) ([Leica] M8 & Infrared //Erwin Puts article on the subject)
Message from len-1 at comcast.net (Leonard Taupier) ([Leica] M8 & Infrared //Erwin Puts article on the subject)
Message from abridge at gmail.com (Adam Bridge) ([Leica] M8 & Infrared //Erwin Puts article on the subject)