Thanks. I double checked your 90mm on 5x7 = 18mm on 35mm calculation
since I didn't think off the top of my head that 90mm on 5x7 would be
quite that wide. I found the respective angles of coverage to be within
a fraction of a degree. :-)
Chuck Norcutt
Dawid Loubser wrote:
> 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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