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.
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
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$800.
The difference between these numbers has to do with how film
responds to different contrast levels. Film has no discrete
uniformly sized grains or pixels; if contrast is high, the grains
are smaller, resulting in higher sharpness. Digital sensors don't
have this issue; they always have the same resolution for any
contrast level. Also, in going from a one-dimensional spatial
resolution to a two-dimensional resolution, the above calculation
doesn't hold so well. In addition, most digital sensors are smaller
than 35mm film, so the effective resolution is higher.
My conclusion is that a digital camera with a sensor of between
10 and 20 megapixels will be equal to, or outperform, the sharpness
obtained by most print and slide films of 35mm format for most
With the availability of 12-megapixel digital SLRs, 35mm film
has already been approached or surpassed as far as resolution
goes. These cameras cost thousands of US$ 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.
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
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.
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.
macro photography (rime, snow crystals etc)
artistic photography (multi-sun exposures etc)
light bridge, gegenschein, lunar libration clouds
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.
Lightning looks better on slide or print film than digital. This
flash of lightning was photographed using Fujichrome Provia 100F (left image) and
digitally using a Nikon Coolpix 5700 (right image). Note the subtle brightness
changes in the slide exposure where the lightning exits the cloud base; the digital
photo shows an unnatural abrupt brightness change due to the linear response of the CCD
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.
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
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.