Arca Swiss 69 fc

Hi to the forum.

Has anyone had a problem using a 45mm lens with the Arca 69FC. An email I received mentioned that there were problems with

“using ultrawides requires a camera that is able to have precisely parallel lens and film planes. The depth of focus (not depth of field) is extremely shallow as the lens moves that close to the film plane, and if your 4x5 is off "Zero" or "Parellel" even by a hair, you could be introducing unwanted scheimpflug, or worse yet, extreme top to bottom, or side to side focus error. I recall reading some in-depth info about parallelism on the Photo.net Large Format Forums”.

Could this create a problem, some additional input would be appreciated.

Will the Arca 69 rfh adapter also take a 67 rfh? Has anyone ever measured the rise that they get with a 47mm lens.



Thanks
Karl

Extreme wde angle lenses are pretty demanding in several ways. This is one of them, but it isn't some invisible peril waiting to bite you. If forewarned, you should be able to guard against it.

I haven't used the camera, however there some problems associated with medium format
view cameras.

Alignment:

The depth of focus is not dependent upon focal length. The formula for two-way depth of focus
is: depth of focus = 2 x f-number x required circle of confusion

If you were to make a fairly sharp (6 line per mm) 8X 16x20 print, you'd need a circle of
confusion of 1/48 mm (or 48 lpmm). To get that sort of resolution, the lens must be used at
it's optimal aperture of f/22. The two-way depth of focus would be 2 x 22 x 1/48 = 0.9 mm.
In other words, planarity between the standards must be maintained at +/- 0.45 mm,
assuming the lens mount, board mount and rollfilm holder - film plane are perfect.

If one uses 4x5, the required coc is about 1/2 that required for 6x7 for the same size print,
or 24 lpmm. An average LF lens has no problem with that resolution when stopped down
to f/45. The depth of focus would be 2 x 45 x 1/24 or 3.75 mm. For the 8x10 format, the
same approach may be used to give a depth of focus of 2 x 90 x 1/12 = 15 mm.

If you double the lens focal lengths (45 mm, 90 mm and 165 mm) and stop them down
to the same degree (f/22, f/45 and f/90), depth of field for a same size print is the same.
However, the tolerance for error (reliability) is much greater for the larger formats. Those
systems are not pushed anywhere near the limits of resolution, and physically, it's easy
to get movements without stress on the camera.

Focus:

To sharply focus at 48 lpmm, you need to use a loupe of approximately 12x power, since
the average person's vision is limited to around 4 lpmm for medium and low contrast scenes.
Less power is required for the larger formats (though I use a 10x), however, the corners are
much easier to check. Also, you don't miss much on an 8x10 GG.

Focus error:

This is related to focal length. The formula is 1/focal length = 1/extension + 1/subject distance.
A 1 mm overextension error for a 45 mm lens focused at infinity results in focus at 2 meters.
For 90 mm and 165 mm lenses, focus would be at 8.2 m and 27.4 m. You can do calculations
for other distances.

Summary:

The bottom line is that if you want to use medium format, the tolerances required are those
of a fixed body MF camera like a Hassy SWC or the 6x12 cameras, or the "solid body" cameras
similar to a Silvestri. For a 45 mm lens at 8x, the accuracy and gradation of focus should be
the equivalent of a 35 mm camera. Those are some of the technical reasons why the larger
formats have dominated high quality architechural photography.

Hi to the forum

Thanks for the answers, one then has to be very careful with 69 cameras. The maths was interesting, obvioulsy those that are careful will get good images. Something to think about.

Karl