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Re: [OM] [OT] Center filters

Subject: Re: [OM] [OT] Center filters
From: Dawid Loubser <dawidl@xxxxxxxxxxx>
Date: Mon, 20 Jul 2009 15:59:33 +0200
Chuck, the way I understand it, it purely has to do with the distance  
the light has to travel
to the film plane (with the symmetrical LF lenses). The Schneider  
Super-Angulon XL I use (and I suspect
most modern wide angle LF optics) have a neat trick where the aperture  
is optically magnified (increasing the apparent size)
as you move off-axis, it's quite neat to see when looking into the  
front of the lens.

However, this still can't account for the fact that the light has to  
travel more than 2x as far to the corners of
the film plane, than what it has to the centre. This, and only this,  
causes the light falloff, and it's this that the
centre filter tries to correct.

"Wide Angle" is perhaps a vague term, but I would say consider it  
similar to 35mm terms, any LF lens which gives you
an angle of view wider than 35mm or so on an OM could be considered  
"wide angle" - and I think usually around here you'll
start getting a bit of light falloff. But only in the super-wides does  
this become problematic. (90mm is "super-wide"
on a 5x7in or 6x17cm camera, about as wide as an OM 18mm).

There are no wide-angle enlarging lenses with an angle of view  
comparable to the wide-angle lenses available for
the LF cameras (the widest are about 40mm-equiv in 35mm film terms) so  
this problem does not exist during
enlarging. Of course, you can still fit a standard falloff correction  
lens to the enlarger if you want to
cause or correct light falloff during printing, I think there are many  
available.

The filter that goes on the camera is dark in the centre, to bring the  
apparent brightness in tune with the corners.
So, to correct this, you need the opposite filter on your enlarger  
(one that is dark in the corners, to block
light in the corners, bringing up the brightness to centre level).

It's not perfect though, so best is to shoot with the correct filter  
on the camera. A new Scheinder Centre filter
alone costs more than a comprehensive OM starter kit (body + couple of  
lenses) !! But it's perfectly matched
to the lens, and really high quality.

I bought my Linhof Technorama 617S for about double of that the centre  
filter on the lens alone
would cost me new :-)


On 20 Jul 2009, at 1:49 PM, Chuck Norcutt wrote:

> Thanks.  What defines wide angle?  Also, if printing large format from
> an enlarger is there an equivalent center filter that can be applied  
> to
> the enlarger lens?
>
> Chuck Norcutt
>
> Dawid Loubser wrote:
>> Hi Chuck,
>>
>> Yes, it's very common to use centre filters for all wide-angle large
>> format
>> photography, unless you really want extreme dark corners for effect,
>> or if you're
>> not shooting colour slide film, which means the darkening is often
>> corrected during
>> printing or digitally post-scan.
>>
>> I never shoot my 6x17 camera without the centre-filter (well, I did
>> last weekend for the
>> first time, still have to see how the images came out).
>>
>>
>> On 20 Jul 2009, at 2:57 AM, Chuck Norcutt wrote:
>>
>>> With all the discussion of 617 cameras today I finally discovered  
>>> the
>>> term "center filter".  I had a hunch what it was but verified the
>>> hunch
>>> that it was used to control vignetting when I discovered this.
>>> <http://www.bhphotovideo.com/c/product/149254-REG/Linhof_022302_Center_Filter_95mm_for.html
>>> This raises a question in my mind whether the use of center  
>>> filters is
>>> common in large format photography with wide angle lenses.  After
>>> all, a
>>> 6x17cm negative is basically a horizontal 2-1/4" wide slice out of a
>>> 5x7.  Vignetting tat the corners of a 5x7 negative must be even  
>>> worse.
>>>
>>> Chuck Norcutt
>>> -- 
>>
>>
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