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FOCIMETER
Antoine Claudet (1797–1867) fi rst demonstrated the
Focimeter in 1849, and exhibited the instrument at the
Great Exhibition in London in 1851. His Focimeter
was one of three distinctively different instruments
of the same name designed in the nineteenth century
to perform different but associated functions. Claudet
displayed the device as part of a group of instruments he
had designed to enhance the predictability of the pho-
tographic process. Alongside the Focimeter, he showed
his Photographometer, an early light measuring device,
and the Dynactinometer which measured the relative
powers of lenses. The Focimeter was developed as an
aid to photographers in achieving perfect focus on the
daguerreotype plate when using the achromatic lenses
then currently employed in photography. In the words
of the text accompanying the display of the instrument,
Claudet wrote
It is impossible to obtain well defi ned photographic pic-
tures without previously ascertaining the exact position
of the photogenic focus, which is easily done by taking
the image of the focimeter on a photographic surface,
and comparing the segments of the apparatus with the
image, then on the ground glass and the photographic
surface.
The narrow spectral sensitivity of the daguerreotype
plate meant that chemical and optical focus did not
always coincide when using optics which were not
fully corrected. Alongside the instrument itself, Claudet
exhibited a group of daguerreotypes demonstrating the
differences between the visual and photogenic focus
and their variation, underlining the effectiveness of the
Focimeter in achieving perfect chemical focus.
The instrument consisted on a series of numbered
‘fl ags’ located radially around a short pole. The Focime-
ter was set up at the subject position, and the pole aligned
along the axis of the taking lens. The fl ags were separated
along the pole at fi xed distances, and the camera was
focussed on the central one, usually numbered ‘4.’ The
exposed and processed plate was checked to ascertain
which of the fl ags was most precisely in focus. If fl ag 4
was sharpest, then a fully corrected lens was in use. If fl ag
4 was not the sharpest, then the difference between opti-
cal and chemical focus could be ascertained by checking
the distance between fl ag 4 and the sharpest fl ag.
Michael Faraday gave the same name to a device he
designed in 1860 for Trinity House—the body responsi-
ble for British lighthouses—to test the precision of lens
manufacture and the accuracy of lens and lamp align-
ment in lighthouses. His Focimeter was made for him
by William Ladd, the London-based optical instrument
and microscope maker, and the device greatly increased
the effectiveness of lighthouse beams.
By the end of the nineteenth century, a third device
had emerged, once again being normally referred to as
a ‘Focimeter.’ The origins of this third ‘Focimeter’ can
be traced back to Thomas R Dallmeyer’s ‘Focometer’
which was an optical test bench designed to ascertain
lens focal length and check the extent to which aberra-
tions had been corrected. It is unclear when ‘focometer’
gave way to ‘focimeter’ as the normally used term.
A sophistication of that device, for measuring the
strength and characteristics of ophthalmic lenses, is
now an established tool of the science of optics, and
‘focimetry’ or ‘focometry’ is a branch of that science.
John HannavyFOCUSING
Ensuring precise focus has always been one of the
fundamental challenges of photography. However, the
ability to focus an image on to a surface predates pho-
tography by centuries.
While the earliest descriptions of the camera obscura,
in use since the eleventh century, did not include a means
of focusing the image, by the late 18th century, the focus-
ing camera obscura was a relatively common artist’s tool.
While some camera obscura designs included a limited
facility to focus the lens, the majority were, like the much
later ‘box camera’ of fi xed focus construction—being
focused on infi nity. By the years immediately preceding
the invention of photography, however, the typical camera
obscura was, like the early production models of photo-
graphic cameras, of sliding-box construction with two
boxes sliding, one inside the other, to enable the operator
to select the required plane of focus.
On the focusing camera obscura the larger box, the
camera body, normally contained the viewing screen,
while a sliding lens box moved in and out of the front.
With the photographic camera, however, the design
was generally reversed, with the lens panel and camera
body (the larger box) remaining fi xed to the tripod
while the rear box carrying the focusing screen and
sensitive material was moved in and out to achieve
precise focus. Once focus had been achieved, the two
boxes were ‘locked’ together with a thumbscrew. That
was the design adopted by Alphonse Giroux for the fi rst
production model of the daguerreotype camera in 1839,
and one which dominated camera design in the early
years, despite its bulk.
There were, however, several cameras in which the
box carrying the lens was movable—including one of
the fi rst cameras to be fi tted with a focusing scale, de-
signed by George Smith Cundell in 1844.
In contrast, several of Henry Fox Talbot’s fi rst and
smallest cameras—his ‘mousetraps’—were of fi xed
focus designs, although others did incorporate lenses
which could be focused in a sliding sleeve.
One of the fi rst cameras to be fi tted with a mechanism
for assisting precise focusing—focus being adjusted by