Re: Infrared Focusing-The true Story?

[Tom <chowt@xxxxxxxxxxxxxxxxxxx>]

On Fri, 26 Jul 1996, Willem-Jan Markerink wrote:

> 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....

Focusing with a deep red filter will not percetably change the focus of a 
standard photographic lens wrt white light focus. You can easily check 
this yourselves. The reason for this is that most lenses are achromats, 
meaning that they are designed to focus 2 colours correctly on the same 
plane, with all in between colours resonably well. (apochromats are 
designed to focus 3 colours, therefore the increased price). The 2 colours 
are usually chosen with one in the blue and one in the red range. IR is 
beyond the red range, and if you look at the plots of most lenses, focus 
starts deviating more drastically in that area. Again, this is completely 
lens dependant. 

> 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. 

I have never heard this, but it is quite possible for the Rollei TLR's. 
The diopter of the filter must be specific for a certain lens, and since 
the TLR did not have interchangable lenses... but then again, the diopter 
would only be perfect at infinity...

> 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?

photography. 

Cheers,

Tom

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Topic No. 9