Re: Infrared Focusing-The true Story?

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

On 26 Jul 96 at 5:29, Russell J Rosener wrote:

> I was discussing focusing for infra red today with a knowledgeable friend.
> He says the official Nikon word is that one should stop the lens down to
> f/8 or smaller aperture, and focus normally, with NO focus shift. He
> assumes that this is because by using some type of deep red filter, and
> focusing through the lens one is already cutting out most blue light for
> focusing. He claims that Leica recommends the same thing for infra red
> film. Supposedly he has used this for years with good results.
> Can anybody verify this method of focusing? It could make life a lot
> easier! 

I will try to verify this on the Leica list, but for a German 
camera this surprises me, since my German IR literature clearly states 
that one should not use a small aperture, due to the disputed 
diffraction increase for IR. The author, Guenter Spitzing, has 
written many books, almost certainly also on Leica cameras. So for 
that reason only he should have know about it.

Beyond that, your friend's reasoning is confusing; yes, blue is
filtered out, but with a red filter you still focus on visible red,
not on invisible IR, for which the correction is ment. Keep in mind
the correction points at the peak sensitivity of Kodak, 850nm.
FWIW: the Konica Hexar AF camera can be set for this correction, 
either for Konica or for Kodak film (750 and 850nm respectively). So 
theoretically one should also distinguish between those two films....

A last point: *if* one relies on DOF for focus correction, you don't 
get the usual '1/3rd in front' - '2/3rd in back' devision of 
DOF....since normal IR correction requires focusing closer (shall we 
start this thread again Andy Davidhazy?....;-)), you 'spoil' focus 
behind the subject, and loose it in front of it. So where your normal 
devision would be 1/3rd-2/3rd, it now might be 1/6-5/6....or 
whatever.... 
And as said in another posting: it always depends on the lens; no 
design, even within similar focal length, is identical, and hence no 
IR focus correction is identical. There are rules of thumb, but the 
size of the thumb differs sometimes....;-))
[I believe the rule of 'bellows extraction' was 1% of focal length, 
right?]

BTW, anyone experience with using diopters to get the same effect?
One of my books suggest a very low diopter, 0.25 or so. That means it 
'refocuses' infinity back to 4m [1/diopter = new distance in meters].
Some weeks ago, Joe Berenbaum pointed me to the stunning fact that 
Rollei IR filters had this diopter build in, mentioned in some Rollei 
literature. After buying a cheap Rollei filter, I can confirm that 
the included filter manual (cute, never saw a filter manual before!) 
also mentions this IR focus adjustment.
However, I cannot see any curve in this filter; one of my fellow 
filter-nuts tried it by reflecting the sun on a far away 
wall....neither side showed any enlargment of the reflected 
spot....nada niente effect. Are there any other ways to proof
that a glass element is not plan parallel?

I spoke to a Rollei tech in Germany about it previously....he never 
heard of this IR correction (!), but said it was possible, but 
thought it would not require more than 0.10 diopter. That sounds a 
bit low to me....refocusing infinity back to 10meter doesn't comply 
with most IR focus marks on lenses....any comments?

Anyone daring to relate the extraction rule of thumb to a diopter 
number?....;-))

--
Bye,

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The difference 
between men and boys
is the price of their toys

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

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Topic No. 11