Archived posting to the Leica Users Group, 2006/11/25
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]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 >