Re: Focus corrected IR-filter

["Willem-Jan Markerink" <w.j.markerink@xxxxx>]

On  8 Dec 96 at 22:08, REMKiksAss@xxxxxxx wrote:

> Does anyone know if there are filters like that for 35mm??? I'd like to use
> something like that on my Canons. I also have a Bronica that it would be
> useful on. Is it focus corrected for just IR or does it refocus everything.
> Canon "L" lens have special glass that makes all light focused on the same
> plane, is that what these do?

It just acts as a slight diopter, on top of being a dark IR filter.
So it affects all colors, not just IR, although only IR is passed 
because of its color (mmm....not one of my smartest sentences 
lately).
The special glass you mention doesn't change things, it is the
non-special glass in your ordinary lens which you need to compensate
by refocusing.

There are several solutions for this, the most common is refocusing
to the IR mark on your lens distance scale, a mark that is found on
many lenses (lately only on primes, not on zooms (too complicated)).
Additional extraction of the bellows of 1/4% of focal lenght is a
similar method.
Focusing on a bright lit razor blade while looking through an opaque 
filter is another method, as is using slight diopters (like the 
Rollei, but still not sure how much diopters you need, and whether 
you need a different one for tele vs wide angle).

Stopping down is the most easy solution, but also not always
practical (picture me this winter, standing on my ski's while
shooting with a 200/2.8 & a #87C....this is the very edge of hand
hold IR photography)

The only problem where none of the above methods other than stopping 
down works, is when using Ektachrome IR. The IR spectrum still 
requires an IR correction, yet that would throw the rest of the 
spectrum (which is also used with this film, unlike HIE) out of 
focus. My German literature clearly indicates that sharpness with 
Ektrachrome IR vastly improves when using APO lenses.

Note further that not all Canon L lenses are 'IR-focus proof', you
need the expensive high end ones that use fluorite; my 200/2.8 for
example has still such an IR-focus mark. 
This is also a nice way to qualify the too often used APO: if it 
still requires IR-refocus, then its APO qualities are minimal.

PS: I have some nice ASCII art from Andy Davidhazy on my homepage,
where these focus phenomena for different types of lenses are
explained:

http://www.a1.nl/phomepag/markerink/mainpage.htm


--
Bye,

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      The desire to understand 
is sometimes far less intelligent than
     the inability to understand


<w.j.markerink@xxxxx>
[note: 'a-one' & 'en-el'!]

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Topic No. 2