LENSES-All Models
Chapter 3 of "Instruction Manual
for RB Super Graflex &c"
The lenses supplied by Graflex with Graflex cameras are selected as
the best available for the requirements of both black-and-white and color
photography.
Most lenses on Graflex cameras are in barrel mount, (either normal
or sunk), although certain models will accommodate lenses in
shutters The RB Super D Graflex also accepts lenses in automatic diaphragm
mount. (See page 7.)
Lens Aperture
Lenses (Figure 9) are normally equipped with an iris diaphragm
between the cells, which forms a many- sided opening whose diameter is
altered by the rotation of a ring outside the lens. The diaphragm controls
1 the amount of light passed by the lens, and 2 the depth of field.
Figure 9 |
The size of the opening of the diaphragm is indicated
by a series of numbers associated with the diaphragm-control ring; these
are the f/ numbers, expressing the relationship between the focal
length of the lens and its effective aperture--(not the diameter
of the diaphragm opening). The rated speed of a lens is its maximum
relative aperture, as indicated by the f/ number engraved on the front
cell-usually as a part of the name of the lens. Equal f/ numbers have substantially
equivalent light-transmitting values on all lenses regardless of focal
length. The larger openings, passing larger quantities of light, are associated
with smaller numbers such as f/2.9, f/3.5, or f/4.5. The smaller openings,
passing less light, are represented by larger numbers like f/22 or f/32.
Full information about the use of the diaphragm will be found under the
heading "Correct Exposure" on page 20. |
Focal Length
The focal length of a lens is the distance, when it is focused
on infinity (very distant objects), from the plane in which the image is
formed to the rear nodal plane of the lens (in standard lenses, generally
located near the diaphragm) . Like the maximum relative aperture (f/ number),
the focal length is usually engraved on the lens cell. It may be expressed
in inches, millimeters or centimeters.
The size of image given by a lens at any particular working distance
varies with the focal length: for instance, if a 3-inch lens gives a 1-inch
high image of an object, a 6-inch lens in the same position will give an
image 2 inches high of the same object.
The "normal" lens for a given size of negative has a focal
length approximately equal to the diagonal of the negative. The Revolving
Back Graflex and other reflex cameras require focal lengths slightly greater
than "normal" to allow clearance for the mirror when it swings
up.
The greater the focal length the farther the camera can be placed from
a subject for obtaining a given image size, with improved perspective.
The greater the focal length of a lens, the less its depth of field
for a given working distance and aperture. This is discussed fully under
the heading "Depth of Field" on page 19.
Long-Focus Lenses and Telephotos
When it is not possible to approach close enough to a distant or semi
distant subject, as in sports photography, long focal-lengths are desirable
to give as large an image as possible on the negative. Telephoto lenses
are long-focus lenses specially designed so as not to require as great
bellows extension as ordinary lenses of the same equivalent focus, thus
making the camera more compact and usable. Telephoto lenses can be used
with most models of the Graflex; full information is under "Interchanging
Lenses" on page 8.
Wide-Angle Lenses
These lenses have focal lengths materially shorter than "normal"
for the negative they are designed to cover, special optical design making
possible a sharp Image over the entire area of the negative. They
cannot be used in Graflex, or other reflez, cameras because they
do not provide clearance for the swing of the mirror.
COATED LENSES
The coating of lenses is one of the most recent accomplishments in the
optical industry. A coated lens will have a brownish-purple color on the
surface. This color is similar to the tarnish or iridescent color that
was previously considered detrimental in lenses. The process of coating
deposits a metallic salt, such as magnesium fluoride, on the surface of
the lens in an extremely high vacuum.
The coating of lenses is done only on the air glass surfaces and not
on the cemented surfaces. The advantages of coating photographic lenses
are an increase in transmitted light by a reduction in light loss caused
by internal reflections, a definite increase in brilliance or contrast
of the Image and the elimination of flare spots due to multiple reflections
within the lens. Every surface of a lens that is coated will increase the
light transmission However, with most standard lenses, this Increase In
light transmission will not be noticeable due to the exposure latitude
of most of the films currently available. The added brillIance due to the
coating of the lens surfaces should be noticeable and should result in
finer photographs.
The coating which is now being applied to lens surfaces is almost as
hard as the average optical glass. While no special precautions are necessary
for cleaning the surfaces of coated lenses, they should be handled as carefully
as all fine lenses. All surface grit and dust should first be removed with
a soft camel's hair brush before the lens is cleaned with a fine lens tissue
or a soft cloth such as a well-laundered linen handkerchief. Do not apply
pressure on the cloth. Simply breathe on the surface of the lens and wipe
gently with easy circular motions. If a lens cleaning fluid (only those
specifically recommended by lens manufacturers) is to be used, do not apply
it directly on the lens surface. Instead place a drop or two on the lens
tissue or cloth and wipe the lens with the moistened material.
DEPTH OF FIELD
Although we may focus most precisely on a certain portion of an object,
we all know that things slightly nearer to the camera as well as some slightly
farther away may also appear sharp in the final print. The field of sharp
focus extends farther behind the object than in front. This range of distances
within which all objects appear equally sharp is called the depth of
field.
As in the case ot stopping motion to be considered in the following
section, our acceptance of sharpness will depend upon the nature of the
print and how it is viewed; and the sharpness to be demanded of a negative
for making such prints must depend upon the amount of en largement contemplated.
Depth of Field, therefore, is not a precise physical quantity which can
be measured. Most lens manufacturers publish tables under the heading "depth
of field," or less correctly "depth of focus," which some
photographers find useful. The great majority of camera users, however,
find observatton of the imaqe on the ground glass and the following easily-remembered
facts to be enough of a guide for all practical purposes.
- The greater the distance to the object focused on, the greater the
depth of field for a given focal length and relative aperture.
- The greater the focal length of the lens, the less the depth
of field at a given relative aperture and distance.
- The greater the lens aperture (the smaller the f/ number), the less
the depth of field for a given focal length and distance.
- The sharp field is deeper behind the object than in front of it.
If you want the greatest possible amount of material to appear sharp
in your picture, stop the lens down as far as you can without requiring
an exposure time so long that movement becomes objectionable. If, on the
other hand, you wish to minimize the background or nearer objects by purposely
throwing them out of focus, open up the lens to one of its larger apertures.
If you want the maximum sharpness at the precise point on which you focus
without regard to objects nearer or farther away, choose a lens aperture
approximately half way between the greatest and the smallest which your
lens can give.
When using the automatic diaphragm of the Super D Graflex, bear in mind
that the depth of field will be greater when the lens has stopped itself
down than when you focus wide-open.
[Next: Using the Graflex Cameras]
[Up: RB Graflex Instruction Manual]
[Back: The Camera Back]
Brought to you by Graflex.Org