Films For Lunar Photography

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Of all the film formats on the market, the most common these days is the 35 mm, or 135, as it is properly known. There is a vast array of colour negative, colour positive and black and white negative films easily available in the 135 format. Most cameras, particularly most of the highly desirable single-lens reflex (SLR) cameras, use this film format. So, my recommendation is to select a camera that uses 35 mm films.

Having settled that, what type of film should we select for our lunar photography? It is always nice to have colour photographs but that is hardly essential in the case of the Moon. Even better if we can give slide shows of our photographs, though colour prints are sometimes more convenient. Certainly they are more portable than a slide projector and screen. However, it is now easy and fairly inexpensive to get prints run off from colour positive (= 'transparency', also known as 'slide') films.

Films come in different sensitivities, or speed ratings. Surely light is almost always at a premium in astronomical photography, so a 'fast' (= sensitive) film is desirable?

Is that it, then? We need a fast colour transparency film, from which we can order a set of prints if desired as well as having them mounted as slides? Unfortunately, things are not that straightforward if we wish to get the best possible results. Take a look at Figure 4.1, first viewing it from a distance and then up close. It is a print I made from a fast colour transparency film. The horrible 'pebbledash' effect is known as photographic grain. Obviously it limits the amount of detail that can be shown. Prominent grain is an unfortunate characteristic of fast films. 'Slower' (= less sensitive) films have a much finer grain structure and allow finer details to be resolved (grain size is not the only factor which determines a film's resolution of detail but it is an important one).

The film speed is quoted as an ISO (International Standards Organisation) number. The number is in two parts, the first corresponding to the arithmetic ASA number that most modern photographers are familiar with and the second a logarithmic value, equivalent to the old DIN number. For instance Ilford's FP4 black and white negative film has a speed of ISO 125/22°. In this book I propose only referring to the arithmetic part of the ISO number. Hence a film of ISO 250 is twice as sensitive (twice as 'fast') as a film of ISO 125. The ISO 250 film would record an image of half the brightness in the same exposure time as would the ISO 125 film. Alternatively, it could record an image of the same brightness in half the exposure time.

As an aside, I ought to mention that there is an effect called reciprocity failure, whereby halving the brightness necessitates using more than double the exposure time. However, this effect only becomes really significant for light levels low enough such that exposures of more than several seconds are needed. We photographers of the Moon need not concern ourselves too much with reciprocity failure.

Of course, we must not forget that the ISO 250 film will have a poorer resolution and noticeably larger grain structure in the final photograph. As far as photographic emulsions are concerned, resolution is expressed in 'lines per millimetre'. If a grid of fine black lines with white spaces between were imaged onto the film, then when the film was processed it could only show the grid if the spacing of the lines were greater than a certain figure. For instance FP4 film, if processed according to the manufacturer's instruc-

Figure 4.1 The deleterious effects of photographic grain.

Figure 4.1 The deleterious effects of photographic grain.

tions, has a resolving power of 145 lines per millimetre (this is the manufacturer's quoted value). In other words, if the spacing of the lines were less than 1/145 millimetre the film could not resolve them as separate and a grey area would be seen instead of the grid of separate black and white lines. A fast (ISO 1000, or more) film might only resolve about 40 lines per millimetre. We might not be terribly interested in photographing grids of black and white lines but we are certainly concerned to record the finest possible details on the Moon.

Another factor we should consider is the levels of contrast we can expect in our final photographs but I defer a discussion of this until Section 4.6. If we are using a colour film, the accuracy of the colour reproduction is also of some concern. Not all colour films will give natural looking reproductions of the Moon's subtle tones. In fact, do we need a colour film at all, since most people see the Moon as stark blacks, greys and whites, anyway?

After the foregoing generalities it is now time to settle on some specifics. There are a number of techniques you might employ to image the Moon. You might be wishing to take the most detailed possible photographs of the Moon through your telescope. At the other extreme, you might just want to photograph the phases of the Moon, and possibly eclipses, with a simple camera mounted on a tripod. In the following notes I detail how you might go about it and in each case I make suggestions of the specific film types you might first like to try. My intention, though, is for you to treat my recommendations as only the first step; merely enough to get you started. Once you gain some practical experience my only advice, then, is that you experiment for yourself and refine your own techniques. Good luck!

4.2 TRIPODS AND TELEPHOTO LENSES, FOCAL RATIOS AND EXPOSURES If you want to picture the Moon against a particular asterism, or perhaps a grouping of planets, then you will need a larger field of view than you will get by imaging through your telescope. Setting the camera on a tripod and taking photographs using its standard lens, or a telephoto lens, would be best. Of course, the rub is that the image scale will be such as to reproduce the Moon at rather small size on the final photograph.

The image scale, I, measured in arcseconds per millimetre, produced by an optical system of effective focal length F is given by:

where F is measured in millimetres. F is the effective focal length because this takes into account any additional optics, such as teleconverter lenses, etc. The Moon's apparent diameter is approximately 2000 arcseconds. This provides a useful 'rule of thumb' in that the diameter of the focused full Moon is approximately 1/100 of the effective focal length of the optical system. If we are imaging the Moon at the 2 m focus of a large telescope then the Moon will appear approximately 20 mm across on the film. If we use a 200 mm telephoto lens then the Moon will appear just 2 mm across. With the standard photographic lens of 50 mm focus the Moon's image will span only 0.5 mm.

Of course, this is just the size of the Moon on the film. If, for instance, a print is made from the film, it will be enlarged when printing. However, the

Table 4.1. Image scales and corresponding angular sizes of field covered by a 135 format film frame for lenses of various focal lengths.

Image scale Focal length (mm) (arcseconds/mm)

Angular field (degrees)

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