Black-and-white negative film
Kodak Technical Pan is the film of choice for most purposes. Technical Pan can
be processed to give a wide range of contrasts according to need. When used at lower
contrasts a large range of tones are retained in the image; at higher contrasts small
differences are accentuated that pictorial films have more difficulty recording. Table 1
contains a set of developing conditions for Technical Pan at different contrast levels,
and typical examples of their uses. When in doubt, lower contrast will preserve more
information and can be followed by printing on high contrast paper.
Contrast Level | Speed rating (ASA) | Developer | Typical Use |
A Low(=pictorial) | 25 | Technidol LC; | pictorial applications, e.g., copying figures, photo-graphing autoradiographs. Also good for eye imaginal discs after DAB staining |
B medium | 50 | HC110;, dil. F 8 mins (20°C) | most other uses, e.g. sectioned material, in situs or embryonic cuticle preps in dark field |
C high | 100 | HC110, dil. D 6 mins. (20°C) | Polytene chromosomes |
D very high | 150 | D19, undiluted 4mins (4°C) | embryonic cuticle preps. in phase contrast |
Speed Rating (ASA) | 21°C | 24°C | 27°C | 29°C | |
A | 1600 | 7.5 | 6 | 5 | 4.5 |
B | 3200 | 10 | 7.5 | 7 | 5.75 |
C | 6400 | 12 | 9.5 | 8 | 6.75 |
Colour negative film
Colour negative films have some advantages over black-and -white. They
contain more information, and are developed and printed commercially both rapidly
and with little effort to yourself. We have preferred Fujicolour 100, but are trying the
new high-definition Kodak Ektar films. The main disadvantage is loss of control over
the printing process. Only the standard (low) contrast level is available, cropping and
enlargement are expensive, and variation in colour balance and exposure are hard to
avoid. These last two points result from the strong red colour of the negatives, which
must be filtered out by the printer. Inexpensive commercial labs invariably expose and
balance prints automatically with respect to some standard. Consequently there is
little point bracketing the exposure of your negatives or using colour filtered
illumination, since the printer will likely correct all of this. Printing your own requires
equipment and skill.
Colour slide film
Colour reversal (slide) films do not suffer these disadvantages, since what you
get back is the film you originally exposed, and the processing is invariant. Accurate
prints can readily be made from slides if required. The most used films are
Kodachrome and Kodak Ektachrome. These two have a different construction that
affects their properties. Kodachrome comprises three separate light-sensitive layers,
each of which is processed separately to generate one of the primary colours. Also a
non-reflective backing reduces halation. The resolution and colour-rendition of
Kodachrome are unparalleled. Also the thorough nature of the processing effectively
removes uncoupled dyes, increasing the stability of the image. In Ektachrome films all
the colour reactions occur within the same layer, limiting the accuracy of the colour
reproduction, but making processing easier. Ektachrome films are available in a
variety of speeds. Also, Ektachrome slides are supposedly more stable in intense
illumination, such as found in a slide-projector. We tend to use Ektchrome 160ASA
routinely, because of its speed and ease of processing, Ektachrome P800/1600 for
fluorescent images, and Kodachrome only when the very highest standards are
required.
Daylight/Tungsten films
Ektachrome is availible balanced for daylight or tungsten lamp (3200K)
illumination. The colour balance of a lamp changes with its output. With an Axiophot
it is therefore best to use the tungsten light source always, and use the neutral-density
filters to adjust the light level. An 87A filter (which approximately corresponds to the
blue microscope filter) converts tungsten illumination for use with daylight film.
"Professional" films
Both Kodachrome and Ektachrome are availible in a "Professional" grade that is
slightly more expensive. These offer better quality control, more natural colour in the
case of Ektachrome, and more accurate speed rating. These films are sold and stored
refrigerated, and there is little point paying the extra unless you intend to do likewise.
Reciprocity
At low light levels the response of photographic films (like X-ray films) ceases to
be linear. The exposure time necessary increases out of proportion to the decrease in
illumination, an effect called "reciprocity failure". Typically this becomes significant
for exposures >1sec. The axiophot control panel will correct for this effect if a
reciprocity factor, specific for each film, is entered. Some reciprocity factors
recommended by Zeiss are:
KODAK Technical Pan 3
EKTACHROME 160 5
EKTACHROME 400 (=P800/1600) 6
For colour films, however, reciprocity failure will be variable with different
wavelengths, a problem not completely overcome by these exposure compensations.