There have been major improvements in the technology
used in digital cameras recently and for the first time they rival photographic
film in terms of the quality of images that can be recorded. Digital cameras
can routinely produce images that are good enough for publication and
are widely used by collectors and mineralogists to record both sites and
specimens. We trialled a number of digital images in the article on fluorite
from the Rogerley Mine, Weardale, in the last edition of this journal.
Green fluorite from the northern Pennines is difficult to record on photographic
film and the digital technique had the advantage that it gave much better
colour rendition. Other coloured fluorites from northern England are also
problematic, the famous blue fluorite from Florence Mine in west Cumbria
commonly registers a dirty pink on film (the photo in issue 22 of this
journal was laboriously altered to produce a more realistic match to the
specimen). Scorodite and pharmacosiderite from Cornwall almost never photograph
properly, perhaps due to some small difference in the response function
of the human eye when compared to film. They do work as digital images.
Digital photography is inexpensive and virtually instantaneous and digital
photos will be increasingly used to illustrate articles. The images generated
by most digital cameras are too small to produce the front covers of journals,
but more than adequate for printing at single column width. Colour images
in the UKJMM are printed at 350 dpi (dots per inch). This means that an
image printed at single column width (about 80 mm) needs to be a little
over a thousand pixels across, well within the capabilities of modern
cameras.
Digital cameras are also very useful when combined with microscopes in
high magnification photography. Most collectors have used a stereomicroscope
at one time or another and almost all of us take a hand lens when collecting
in the field. It is hard not to be impressed by the beauty and perfection
of crystalline specimens at high magnification, but photography in this
size range is beset with difficulties. Anyone working at high magnification
has two major problems to contend with: resolution and depth of field.
These are additional to the normal problems of specimen photography, which
boil down to choosing the specimen and lighting it properly. Photography
at high magnification is a compromise between resolution and depth of
field. Reduce the aperture too much (by shooting at a high f-number or
inserting a pinhole into the microscope system) and the image goes soft
(low resolution due to diffraction). Work at full aperture and the resolution
is good but the depth of field is miniscule so almost nothing appears
in focus. That is why most successful high magnification photos are of
flat specimens or isolated single crystals.
Digital technology provides a way to cheat the laws of optics by combining
multiple images of minute specimens. Image processing software makes it
possible, though not easy, to combine only those bits of the image that
are in focus. A single composite image can be made at high resolution
with good depth of field from a stack of images by taking only those parts
that are in focus. This technique is not new, but it is only recently
with the advent of cheap digital cameras and image processing software
that it has come within range of amateurs. In principle, the procedure
is simple. Beginning with the uppermost crystals in focus the photographer
works carefully down through the specimen producing a stack of digital
images one on top of the other. These can be combined by hand in standard
image processing software or using CombineZ, a freeware program which
is available via the internet. Which of these options is best depends
on the type of specimen being photographed, highly detailed images tend
to work better with CombineZ, while those with a few blocky single crystals,
or with a very large depth, are best done by hand.
Microcrystalline assemblages are an important facet of mineralogy, often
comprising rare and unusual mineral species, but photographic difficulties
mean they are not commonly figured. Digital techniques should go some
way toward alleviating the problem. Methods are still at the development
stage but we intend to produce a dedicated article in the near future
when several different optical systems have been tested. In the meantime,
a few illustrations are included on the facing page to show the improvement
in image quality which is possible using the technique.
