The Salt Lake herald. [volume] (Salt Lake City [Utah]) 1870-1909, 1900-02-23

lens is a single convex lens. A convex
lens VIII cause the rays of light coming
from an object or group of objects TO
converge until at a fixed distance they
form sharp, clear image. This dis
lance Is called the focal length Of the
/ lens, and is greater or less according
l 10 the curvature of the surfaces of the
lens, being shorter in a lens having
strong curves and longer when the
curves are weak. If we were to exam
inc the whole of the picture formed
by such single lens it would be found
that while the circular image is quite
large, only a small portion in the CEN
Ter would appear Sharply defined, While
an eVer-increasing amount of distortion
would appear from the center outward.
/ While no Photographic lens has ever
yet been made which would give absO-
Lutte sharpness over its entire image
circle, the modern anastigmats designed
by Professor Able of Jena very nearly
do so. It viii thus be seen that one
of the most valuable features of lens
Is the ability to produce a sharp, nom
distorted image over the whole image
circle, and the more nearly a lens will
do this th- more valuable it is. Coupled
with covering power" as it is called,
there should be rapidly The speed of
lens depends upon the number Of
rays of light which it will converge
upon a given area. Light proceeds from
every luminous object, equally in all
/ directions, hence all lenses having the
same diameter viii receive an equal
amount of light. It is the manner in
which the lens cOnverges the light
which determines the speed. The pho
tOgraphic plate is affected more or less
quickly as the light which falls upon
I it is more or less intense. The intensity
/ of light varies inversely with the square
of the distance from it's source. For
/ practical purposes we may consider the
| source of light falling upon the photo
graphic plate to be at the lens Then,
suppOSing two lenses of the same diam
! eter and having the same size image
circles to differ in focal length by one
half, according to the law of intensity,
/ the image formed by the lens having
/ the shorter focus win be four times as
! bright as the other the illumination
will be four times as intense, and the
/ lens will secure a picture in One-fOurth
the time required by the other.
For rapid photography, and this in
cludes the greater part of amateur
! photography, it is obvious that the lens
should have the shortest possible focus
and at the same time be able to pro-
! duce a perfectly sharp image over the
whole plate. The difficulties of the OPP
I tician begin when he attempts 10 COM
! bine these desirable features.
To return 10 our single convex lens, a
portion Of its indistinctness can be done
away with by placing an Opaque stop.
with a small circular opening in il. in
front of the lens. The smaller the open
ing the larger the area of sharpness
will be; at the same time the volume of
light will be corresponding reduced
a, h. Ia e1a Tho J.
cal solution is the use of stop of
moderate size and a lens having an
image circle much larger than the dag
Onal of the plate Of the camera, thus
allowing the indistinct portion of the
image circle to fall outside the limits
of the picture TO be used. Under these
| immutable conditions it will be seen
that only a lens of very limited speed
can be had when the double convex
| form is employed. This is the cheap
| est form of lens made. The simple
meniscus lens is an improvement on
/ the double convex form. which 1t re-
/ sen1bles, with the exception that the
surface toward the object is curved in
! ward instead of outward. This in
/ creases the area of sharpness in the
1 image circle and permits the use of a
/ lens at shorter focus and greater aper-
| ture.
The rays Of light which fall upon the
portions Of a lens farthest from the
| center and at the most Oblique angles
are the ones which are most acted upon
| by the lens. and which, unless some
! special provision is made for them in
! the construction of the lens, cause in
distinctness in the image and make it
/ necessary to shut them out by means
! of stops; therefore. the lens which can
bring the greatest number of these
marginal rays to aid in the formation
Of a sharp image Will be the most rapid
and give the least distorted image.
The next step in this process IS the
employment of two lenses Of different
| shape cemented together to form one
! lens. One lens is Of crown glass with
comparatively low refTacting power,
and the other of flint glass Of great re
I fractive action. The resulting lens is
called an achrOmatiC lens, and is su-
| periOr to either the double convex or
meniscus, as it's image circle has a
! larger sharply defined area and a
much shorter focus may be employed
/ on the same plate, thus giving greater
speed. These single achrOmatic lenses
/ are also made double convex and me-
/ niscus. Of which the meniscus is the
/ better. Having proceeded so far, it is
easy TO see that still further results can
/ be obtained by the proper combination
of two achromatic lenses, constructed
/ for the purpose, into one compound
! lens. The simplest form 10 that known
/ as 'reCtillnear." in which two meniscus