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Introduction

Doing weather photography, you will use a wide range of film speed and types. Here is where the problem starts: if you just used one film for all photography, you would have that in your camera and get away with it, but if you use different types of film for different phenomena, you will generally need several cameras, or interchangeable film backs, one for each film type. That is what I have become used to do, and now I have reduced the number of frequently used types of film to two, for two cameras.

Print or slide?

I suggest you use color slide film for all weather photography, especially if you depend on a lab to process your film. If you process your film yourself, or at a well-known lab, you will find that using print film has its advantages as well, and if you scan the photos directly into computer (without making prints), you avoid the trouble of getting the print printed right (labs usually mess this up, making many photographers think that print film is no good. But the frames are generally ok - people usually just don't realize that making a print involves many extra degrees of freedom in color balance, contrast and brightness, and it is those degrees of freedom that can make a print look really bad).

Which brand? I use Fuji film exclusively, because I have become used to its color characteristics, which are especially important in low light. But you should use several brands to test, and decide for yourself which is best. Every photographer has his or her own personal preference when it comes to film brand.

Black-and-white film can also be very worthwhile to try for some applications, but I recommend to stick to color film for most photography.

Film speed (ISO)

Film speed required for most general weather photography (clouds, sunset, halos etc) is generally 100 ISO. This is a good trade-off between film speed (slowness, actually), high-resolution, and practical use (fast enough for most daytime photography without tripod, if you don't have one with you).

For lightning, 100 ISO film is also well-suited, but you might want to also use 200 for more distant lightning, and close the aperture, to make the photo sharper.

For astronomy and other night scenes, you will need 400 or 800 speed film. Here's where the Fuji 400F really serves well; you can use it at 400, 800 or 1600 ISO, and push-process the film accordingly. Hence you would only need to buy this film in large quantities, and it would serve many purposes. Aurora, nocturnal fog and cloud scenes, lunar halos etc are best photographed on 400 speed film; the zodiacal light and Milky Way on 800 speed, and meteor showers, comets, nebulae and gegenschein best at 1600 speed, to name but a few.

I generally buy Fuji 100F and Fuji 400F and can do most photography well with those films. Sometimes, however, I photograph something for which print film is better. I found that green flash photography goes better with print film, as the print film has a wider exposure latitude and is more forgiving for exposure errors (these happen easily when you photograph the sun). For print film I'm used to Fuji Reala 100, and Superia 400 & 800 for backup purposes.

Film size

The 35mm film format is most widely used by amateurs and is widely supported by photo labs, film scanners, slide viewers/projectors, etc. I suggest you use the 35mm format, and only if you decide to go more professional, and can afford it, the 120/220 format.

Do not use APS film, since the resolution is generally too low for weather photography. Only for holidays and tourist use is this film acceptable.

Film reciprocity errors: Schwarzschild effect and color balance shift

As you make your exposures longer than about 1 second or so, you may start noticing that the color balance and sensitivity of your film changes. This is called the reciprocity error, or SchwarzSchild effect. It basically means that if you make an exposure on, say, 1 second at f/8, this gives no longer the same exposure as a 2-second exposure on f/11, as it would be when you do short exposures (say, 1/500 sec or so). Film sensitivity drops for longer exposures, so in general you will need to expose the film longer (sometimes up to a factor of 2 or 3, or 1 to 1.6 stop) than what the light meter suggests.

It is hard to give a rule-of-thumb for the exposure correction, but what I generally find is that for exposures up to a minute on Fuji slide film, overexposing the photo by between 1/2 and 1 stop, depending on how dark you want the photo to be, gives adequate correction. (This is in addition to the 1 stop overexposure I always do with weather photography, since my slides turn out to be very dark if I expose according to the light value.) If you are photographing a night scene, you may want to make a relatively short exposure, so the night scene will look properly dark on the slide.

The other problem you get is a shift in color balance. For Fuji slide film such as Provia and Sensia, they shift towards the blue, since the Schwarzschild effect is weaker for the cyan & magenta coupling layers than it is for the yellow coupling layer. You will start noticing this blueshift at exposures longer than a few seconds. The Fuji Velvia is (among lightning photographers) infamous for its magenta-shift when used at night, and if you look the sensitivity curves of the Velvia (the datasheet is available at Fuji's website), you will indeed see that the magenta sensitivity is higher for long exposures.

You can get around color shifts by using filters, but I generally don't do this, since the shift in color is generally small. I avoid using Velvia at night (at 50 ISO it is too slow anyway).

Film resolution

Many photographers think differently, but print film is actually higher resolution than slide film. Most photographers maintain the opposite. I think this is due to a miscommunication: it depends on the meaning of sharpness. It is true that slide film is sharper for high-contrast. Also, you generally don't see the grains too much, so it obviously makes slide film preferred for weather photography, because of the large areas of uniform color (like the sky). Any graininess would immediately catch the eye there.

However, for low contrast, print film is generally slightly higher resolution, and its grain size is noticeably smaller than that of slide film (but it is much more obvious to the eye). So it depends on your application which film is best; but generally, slide film performs better for weather photography.

MTF curves as a tool in choosing the right film speed and type

If you are interested in the matter of film resolving power (contrast) as a function of sharpness, the MTF curve of a film tells a lot. MTF stands for Modulation Transfer Function, and is the relationship between the contrast resolving power (in %) of a film and the number of lines/mm. In short, a high-resolving-power film will be able to resolve contrast for a large number of lines/mm, and is evidently better. MTF curves vertically plot the response of the film (or lens) as a percentage, against the resolving resolution in lines/mm horizontally. MTF curves of film generally start at or above 100% for low values such as 1 line/mm, and drop off gradually to below 20% (the cutoff-resolving power) around 50 to 100 lines or more, depending on film type and speed.

[The MTF curve is a handy tool to study the resolution characteristics of a film. The figure shows a fictive, example MTF curve. The curve drops below 20% (the cutoff-response) at a resolution of a spatial frequency of 100 lines/mm. Note that the response may be above 100% for low resolutions.]

What does this all mean? Slow films such as 50 or 100 ISO have higher resolving power for larger number of lines/mm than higher speed films. Hence, they are able to resolve details at higher resolution, i.e. they are high-resolution films.

The resolving power does not measure the grain size; for example for print film, the grain size can be smaller than that of same-speed slide film, even though the slide film has a higher resolving power for high contrast situations (like a tree in front of a light background).

The MTF funtion itself is also a function of exposure, but I should leave that outside the scope of this document. Most MTF curves you will encounter (if any at all) are valid for daylight photography.

Conclusion

In a nutshell, I can't give you any preferred film for all situations with weather photography - but I can say that slide film will generally yield best results, simply because the graininess is less evident and the processing is much simpler than for print film. Slide film has a smaller exposure latitude and thus is less forgiving for exposure errors, but practice and good light metering will help a lot.