Introduction

I have felt for some time the need to write something about digital cameras, now that these have become very useful for weather photography. While digital cameras are not suitable for all kinds of photography, in many cases they are suitable, and sometimes even preferable over traditional film cameras.

The Nikon D100 (pictured above) and D70 are good D-SLR (digital single-lens reflex) cameras for weather photography. The D70 is cheaper, newer and in some aspects better than the D100.

Resolution

Probably the most important issue is which digital resolution (number of sensor pixels) would give the same resolution as a 35mm-frame of, say, slide film. I believe that a 10 to 12 megapixel digital sensor gives the same resolution as a 35mm film when scanned with a professional film scanner. The highest resolution obtainable from a 35mm frame is about 4000 DPI, which corresponds to an image of about 5000x3000 pixels (15 megapixels). However, most lenses are not capable of obtaining such sharpness in the first place. 12 megapixel sensors are available at the time of this writing (January 2005).

However, a good slide film such as Fuji Velvia 50 is able to resolve about 100 lines/mm (its spatial resolution), if with decent contrast. This means that you'd be barely able to resolve a grating with 100 lines per mm on the film, or one line per 10 micrometer. If you'd calculate the sensor resolution necessary to resolve the same using a digital sensor, the number turns out to be higher than 12 megapixels. Assuming you need two pixel rows to resolve one line (a dark line bordering a brighter line), there are 36mm * 100 lines/mm * 2 pixels/line = 7200 pixels in the longer dimension of the 35mm frame (which measures 36mm by 24mm). So, in total, you'd need on the order of 7200 * (2/3) * 7200 pixels = 34.56 megapixels to resolve 100 lines/mm.

The Canon D-SLR cameras offer outstanding performance with their low sensor noise levels. The 300D (pictured above), 10D and 20D cameras all have CMOS sensors. The 300D is the cheapest digital SLR available, with a street price of around US$800.

The difference between these numbers has to do with how film responds to different contrast levels. Film has no discrete uniformly sized grains or pixels; if contrast is high, the grains are smaller, resulting in higher sharpness. Digital sensors don't have this issue; they always have the same resolution for any contrast level. Also, in going from a one-dimensional spatial resolution to a two-dimensional resolution, the above calculation doesn't hold so well. In addition, most digital sensors are smaller than 35mm film, so the effective resolution is higher.

My conclusion is that a digital camera with a sensor of between 10 and 20 megapixels will be equal to, or outperform, the sharpness obtained by most print and slide films of 35mm format for most contrast levels.

With the availability of 12-megapixel digital SLRs, 35mm film has already been approached or surpassed as far as resolution goes. These cameras cost thousands of US$ and are thus not readily available for everyone, but it is only a matter of time before such resolution cameras do enter the consumer market.

Sensor sensitivity

CCD and CMOS sensors don't respond to light the way film does. Our eyes see light levels logarithmically over a wide dynamic range (we can see some things in the middle of the night as well as in broad daylight, which is a huge range of brightness!). Print and slide film behave more linearly, but still somewhat logarithmically. Digital sensors however respond quite linearly and this can cause some unwanted effects, especially with weather photography.¹

Thermal noise increases with sensor sensitivity, as demonstrated here (using a Nikon D100 camera). But digital SLR cameras are getting better than film used to be, and quite decent exposures can still be made at ISO 800, especially with the newer lower-noise D-SLRs.

Digital noise

All digital sensors are subject to electronic noise, of which there are various types. The most notable noise is thermal noise, which causes an effect similar to the coarse grains seen in high-speed film. Thermal noise is more noticeable if the sensor is used at a more sensitive setting. This noise is also more prominent on smaller-sized sensors. The larger sensors commonly found in digital SLRs suffer less from this noise than do the smaller sensors in point-and-shoot digital cameras for the consumer. In fact, the newest digital SLR cameras show virtually no noise even at 400 ISO sensitivity, and can usually be used at 800 ISO without noticeable loss in image quality.

Another type of noise has to do with hot and cold pixels on a sensor. Hot pixels manifest themselves as bright colored pixels of red, blue or green. These become more obvious if the exposure time is long such as several seconds or minutes, but they can be removed by doing a so-called dark-frame exposure, during which an exposure of equal length is made with the shutter of the camera closed. The dark frame is then simply subtracted from the photo to remove the hot pixels. Clearly this is highly impractical when doing long exposures such as 10 minutes, but dark frame subtraction can also be done by computer later, using a dark frame obtained earlier at the same temperature, ISO setting and exposure time.

Digital cameras for weather photography

The following table lists some subjects that you may be photographing and whether digital or film is better suited.

In compiling this table I looked mostly at the dynamic range (in brightness) of the subject (film advantageous over digital if the dynamic range is large), ease and cost of use (digital always advantageous) and sensitivity vs. noise/graininess (digital advantageous if minute- to hour-long exposures are made with a low-noise digital camera). Also, I assumed that a digital camera is used of the same resolution as film.

In general, a low-contrast subject is better to photograph digitally, as are all astronomy subjects.

Lightning photography can be done both digitally or with film, although I find film giving slightly better results, due to the intense brightness of a lightning discharge. Digital sensors tend to saturate the brightest parts of the image unnaturally, resulting in completely white areas that don't gradually merge with the lesser densities. This is also noticeable in sunset pictures taken digitally, and halo photos that show the sun.

Digital sensor sizes vs. film

A major difference (at present) between digital SLR cameras and film cameras is the difference in film/sensor size. Since most of the digital imaging sensors found in digital cameras are smaller than film, all 35mm-film camera lenses you use on the body will have a 35mm-equivalent focal length that is longer than when used with 35mm film. Usually this factor is about 1.5 or 1.6, meaning a 28mm lens for film will effectively be about 42mm when used on a digital SLR. For long focal lengths this is not a problem, since the factor only extends the ranges of focal lengths you can use, but it does limit the use of wide-angle lenses.

Summary

Since weather photography in its most usual sense includes taking pictures of clouds, sunsets and perhaps lightning, I can say that once you know how to expose properly, a digital camera can do a great job, as long as your digital camera has some sort of manual mode, it has bulb mode or long time exposures, and has a tripod mount. Also, for long time exposures such as astronomy and lightning, it is imperative that you do dark-frame subtraction later using dark frames obtained at other times, so you can maximize your time available for photography in the field.

For more exotic subjects such as halos and the sun's green flash film is (in my opinion) still preferable to today's digital cameras, although digital cameras do a good job on most of these subjects.

Which camera to buy

I suggest, without any doubt on my mind, that you buy a digital camera if you are just starting with weather photography. This will avoid so many problems that film photography gave in the past, and you can see the results immediately. Digital SLRs (DSLRs) are far superior to digital compact cameras (more versatile and less noisy), so if you can spare the money, buy a DSLR.

1. CMOS sensors (which are programmable) may likely be made to respond logarithmically in the future. Whether this is really possible and affordable, time will tell. This would be a major advantage, since an image obtained by a sensor with a logarithmic response (large dynamic range) can always be made to have more contrast (less dynamic range) by digital manipulation, while the reverse is generally not possible.