Fitting the unit to
the camera
There are two methods of fitting the
unit to your camera. The conventional way is to remove
the lens from the camera and screw the unit in its place
by turning the complete assembly round until it is fully
screwed home into the camera front, then release the
small knurled screws which allow you to adjust the unit
and re-lock square with the camera body. The lens is then
screwed onto the front panel of the bellows
unit.
A better method is to first remove
both the front and rear lens flanges by unscrewing the
four knurled screws sufficiently to allow these flanges
to be lifted up and out. These lockscrews must be
unscrewed until their points are completely withdrawn
from the large recess in order to clear the tapered edge
of each flange. Now select the rear or male mount and
screw, or bayonet, into the camera body. Screw or bayonet
the front flange onto the lens, which may then be placed
into position on the unit and lightly secured by the two
knurled lockscrews. Now the complete bellows unit can be
fitted to the camera body by placing the camera onto the
back of the unit, making sure that you enter the dovetail
of the flange into the top of the recess moving it down
into the working position and finally securing square to
the camera by tightening the two knurled lockscrews. Do
not overtighten these screws as a light pressure will
prevent the unit from twisting, and too much force will
mark the tapered edge of the flange, so preventing
accurate location in time. Be sure to keep the flange
dovetails clean and free from grit in order to ensure
perfect alignment.
Ultra Close-up
Photography
The previous description of fitting
the B.P.M. Universal Bellows Unit is correct for ultra
close-up photography. With a 50 mm. camera lens and the
maximum extension of 150 mm. a magnification of 3x image
size can be attained. It must be understood that at these
high magnifications the depth of focus is very shallow,
and the subject extremely close to the lens. It is easy
to obtain unsharp pictures due to camera shake and the
unit is provided with two tripod holes in order that this
may be overcome. When fitting to a tripod, ensure that
you select the hole which is closest to the centre of
balance of the unit and camera.
When working with the bellows at its
the smaller end of extension, it may be found that the
pillars foul the subject being photographed. In this
case, remove the lens and unit from the camera by
releasing the four knurled screws and turn the unit round
to bring the pillars underneath the camera, thus allowing
a clear area in front of the lens with no protrusions.
The focus and lock knobs now come on opposite
sides.
One very convenient method of close-up
photography is to first decide the area you wish to cover
by focussing on a rule. Lock the unit and leave set, now
approach the subject until it appears sharp in the
viewfinder, then make the exposure.
Focusing Blind
Spot
Every bellows extension has a blind
spot in its focus range. This is due to the closed
thickness of the bellows plus the frame and mount. The
B.P.M. Bellows Unit has bee specially designed to keep
this to a minimum and comlete with screwed flanges its
fully closed extension is no greater than 30 mm. With
bayonet type flanges this dimension may be slightly
greater depending on the type of camera mount.
Finding the
Exposure
When a lens is extended beyond the
normal working position, as in ultra close-up
photography, the effective f stop or aperture is no
longer that which is engraved upon the lens barrel. This
alteration of the aperture is always constant, being due
to the increased distance of the nodal point of the lens
from the film plane. It alters the effective value of the
aperture in accordance with the inverse square law. A
formula for obtaining the increase in exposure becomes
simlpy focal length of lens plus extension. divided by
focal length of lens, then square the result.
( (LFL + EXT) / LFL ) 2
The following three examples will
explain.
Example number 1 ( (Lens 50 mm. +
Extension 50 mm. ) / Lens 50 mm. ) = 10/5 = 2 x 2 = 4x
factor.
Example number 2 ( (Lens 50 mm. +
Extenson 100 mm. ) / Lens 50 mm. ) = 15/5 = 3 x 3 = 9x
factor.
Example number 3 ( (Lens 50 mm. +
Extension 150 mm. ) / Lens 50 mm. ) = 20/5 = 4 x 4 = 16x
factor.
The extension can be read from the
right hand pillar in centimetres. This scale is
approximate, and should only be used as a guide. This is
due to the slight variations in the lens mounts and
relative positions of the nodal points of the lenses
which vary in different makes.
Telephoto or Long
Range Photography
Most lenses that have a focal length
of not less than 75 mm. or longer than 125 mm. will make
a satisfactory telephoto lens. Longer types may be
accommodated by fitting an extension tube on the front of
the bellows unit if necessary.
This includes a large selection of the
older type lenses intended for plate or roll film
cameras, with or without focusing mounts. To make a lens
of this type focus on infinity when used on the B.P.M.
Bellows Unit, it is important that the distance from the
rear of the lens mount to the focal plane is greater than
the total distance of the camera plus Bellows Unit at its
shortest extension. Likewise, a lens that has a focal
length that is greater than the total extension provided
by the camera and fully extended Bellows Unit will not
focus at all unless an additional extension tube is
fitted as mentioned previously.
Fitting Special
Lenses
[This section is about the
supply of specially made flanges for non-standard screw
mounts. As this service is no longer available (as far as
I know) I have omitted it]
Slide
Reproduction
The B.P.M. Bellows Unit is supplied
complete with a special frame [Note: this was the
case when the units were avilable new but is not
necessarily the case when the bellows are purchased
second-hand] to enable the user to reproduce colour
transparencies, make black and white negatives from
colour slides and monochrome slides direct from
negatives. Simply clip the frame between the two pillars
as in the illustration, and rack the lens to a position
that just covers the area of the slide. Avoid placing the
frame too far away as this will result in including part
of the unwanted mask. Once the approximate lens position
is found, it is easier to move the slide frame to obtain
final focus. Illuminate from the rear by daylight,
photoflood or flash, according to the type of film in the
camera, making certain that the camera is not pointing
directly at the light source or sun. By far the most
satisfactory illumination is achieved if a white card is
placed at 45 degrees to the direction of the light to
reflect the light on to the rear of the slide. A good
lens hood is also desirable to exclude unwanted side
reflections. Correct exposure can be ascertained by
placing an exposure meter close up to the slide and
making a direct reading. To reproduce a 35 mm. slide the
same size, a total extension of twice the focal length of
the lens will be required. This means that the meter
reading must be multiplied by 4. For example, a direct
meter reading of a quarter of a second at f/11. The
correct exposure would be 1 second at f/11 or a quarter
of a second at f/5.6.
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