FROM: w.j.markerink@a1.nl (Willem-Jan Markerink)
SUBJECT: Re: Mamiya 7 lenses and IR mark?
DATE: Mon, 22 Feb 99 17:58:12 GMT
ORGANIZATION: A1 Internet news-server
NEWSGROUPS: rec.photo.equipment.medium-format

In article <19990222023447.23722.00001444@ng116.aol.com>,
   mamiya7ii@aol.com (Mamiya 7II) wrote:
>Dear Group:
>>
>>I am considering buying a Mamiya 7 system. I must admit to being spoiled with
>>Leica glass for the last few years. There are no IR marks on a Leica lens,
>>because the lens is corrected well into the IR.
>>
>>Is it true that the Mamiya lenses have an IR mark? Are these lenses well
>>corrected through the spectrum?
>>
>>I am having a difficult time finding a review of these lenses - I am most
>>interested in the 65mm. Any references?
>>Pete
>
>                                                                      
>Hmmmmm,
>Sounds a bit to coy for me, Pete. There is no such thing as any glass being
>fully spectrum corrected, and that certainly includes the Mamiya 7 optics that
>have been designed from the ground up with CAD technology using only the best
>low density glass. My humble suggestion is to rent an M7 and make your own
>judgement. Lee

The reason why (modern!) Leica optics often don't have an IR-index is 
because Leitz found it confusing/misleading for the following reasons:
- it is only valid at one particular distance setting, most often infinity
- it is only valid for a certain spectral range, while there are two 
different IR films (Konica up to 850nm, Kodak up to 950nm)
- it also depends on the filter that is used....a strong IR filter 
increases the need for correction, while a mild orange/red filter hardly 
requires correction
- if people focus through a dark red filter, they already compensate 
slightly, so there is less additional correction needed than the IR-mark would 
indicate

--
Bye,

Willem-Jan Markerink

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





From:             "Willem-Jan Markerink" 
To:               infrared@a1.nl
Date sent:        Sun, 9 May 1999 01:38:59 +0000
Subject:          Re: focus
Send reply to:    infrared@a1.nl

On  8 May 99 at 13:43, Llamapix@aol.com wrote:

> Hi all.  A question regarding focusing which is not addressed in anything 
> I've read about IR so far.  I am shooting Kodak HIE with a 25 red filter.    I use
> a Canon AT-1 with Canon 28mm and 50mm lenses.
> 
> I have been shooting portraits with IR.  At first I shot some semi low-light
> pictures using larger apertures like f/3.5  In spite of careful focusing and
> offsetting to the IR mark, it seemed the subjects' faces were slightly out of
> focus.  I then theorized that perhaps since the film was being exposed by both
> visible and infrared light, that by using the offset focus I was therefore leaving
> all the visible light rays out of focus.  Is this the case?

Yes.

This one of the main problems of using IR-correction marks....the
correction needed not only depends on 1) sensitivity of film (the
Konica Hexar has two IR-correction settings, one for 750nm (Konica),
and one for 850nm (Kodak)), but also 2) on the filter used....the
stronger the filter, the smaller the spectral range you are
recording, and the easier it is to pin-point a certain correction
for it....without any filter, you would have to bridge a very large
range (350-950nm), hard to cover with a non-APO tele without
stopping down....but luckily HIE more or less requires a #25 red
filter already (which leaves only the smaller range 600-950nm).

Factor 3 in setting an IR-mark is the distance....normally the mark 
is set for infinity, which makes close-focus sometimes guess work, 
especially with complicated optical designs....zooms in particular.

All 3 factors combined are also the reason why Leica doesn't put 
IR-marks on their modern lenses anymore.... 

> Since then, I have been shooting with smaller apertures like f/8. and then 
> setting the focus in between the normal and IR focus marks.  The greater 
> depth of field easily encompasses both focus marks.  Yet in some of the 
> pictures people's faces still seem a little soft.  Is this just the nature of IR? 
> Is it because of the high IR reflectance from people's skin combined with halation
> perhaps that makes them appear soft?  What are other people's experience with IR
> and portraits?

Skin comes across fuzzy indeed, that's part of HIE's
characteristics....keep in mind you can more or less look through
someones skin if he/she has pronounced veins (and a light skin
color)....;-)) 

--                 
Bye,

Willem-Jan Markerink

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


[note: 'a-one' & 'en-el'!]





From:             "Willem-Jan Markerink" 
To:               infrared@a1.nl
Date sent:        Tue, 8 Jun 1999 14:54:10 +0100
Subject:          Re: Nikon - Dot or no dot ?
Send reply to:    infrared@a1.nl

On  8 Jun 99 at 3:37, danzig@pacific.net.sg wrote:

> >This topic, especially the macro-aspect, could keep optical 
> >engineers talking for hours, without any hard rules for your 
> >specific lens, so ... from WJ
> 
> 
> Actually WJ, I BEG to differ ;) 

Begging won't get you far with me....:-))

> Focus on your subject, the stripe represents visible light focus.  
> Move the focussing ring by the distance that lies between dot and 
> the stripe ( works better if you actually mark the area on the 
> focussing ring directly in front of the stripe, so you know exactly 
> how much to move the focussing ring  ) Move it that distance 
> towards the dot . That is the correct focus for IR. 

Mmm....not really, at least not 100% accuracy with 100% certainty.
The problem is that the IR dot is a compromise, affected/fouled by 
several factors, like: 
- film sensitivity (absolute limit in nm; note that Konica Hexar has 
two AF correction settings, one for 750nm, one for 850nm)
- filter used for final image (relative limit in nm)
- filter used for composing (correction applied by human eye)
- object distance (correction can differ with distance, the more complex 
the design, the harder to estimate)
- focal length (note that some APO zooms are only true APO at one end 
of the zoom range, confirming this theory)

And all this is what caused Leica to remove the IR-dot on their 
modern lenses....they felt that it gave a false sense of accuracy.

--                 
Bye,

Willem-Jan Markerink

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


[note: 'a-one' & 'en-el'!]

If you have any question, remark, comment, want to share some philosophy or just want to express your opinion about these pages, feel free to send email to: w.j.markerink @ a1.nl