There are many ways in which digital cameras are different from film
cameras, but the thing that makes them digital is the imager. These
digital imagers use different technologies (which we won't get into)
and come in various sizes.
Imagers, often referred to as CCDs (regardless of technology), are
measured on the diagonal. For the most part they are much smaller than
a frame of 35mm film, and sizes are often specified as 1/x inches (at
least here in the States) - such as 1/1.7", which translates to roughly
0.6" or 15 mm.
A lens of the same focal length will provide different amounts of
coverage with different sized imagers, so it's common for the
manufacturer to specify focal length range both in actual mm and "35mm
equivalent" - what lens would provide the same view on a 35mm film
camera. This makes it much easier to compare the coverage between
digicams.
While really cheap digicams often have an optical viewfinder and a
fixed lens, the more popular models come equipped with a zoom lens and
and LCD viewfinder. Every lens will have three specifications: focal
length, aperture, and close focus. (Some entry-level digicams don't
focus at all.)
Focal Length
The "normal" lens on a 35mm point-and-shoot camera is in the 35-40mm
range, while the norm on 35mm SLR cameras is 50mm. Most point-and-shoot
35s with zooms start at about 38mm, and most digicams match that with
zooms starting in the 38mm equivalent range.
While this is the norm, a few digicams provide wider coverage,
sometimes to the equivalent of a 28mm or even a 24mm lens on a 35mm
camera. (2002 examples: Minolta Dimage 7i, Nikon Coolpix 5000,
discontinued Canon PowerShot A50 all go to 28mm equiv. More recent
example: Kodak EasyShare P880, with a wonderful 24-140mm equiv. zoom
range.) The extra coverage can be great in tight quarters or when
shooting a broad expanse. One argument in favor of wider coverage is
that while you can crop your final image to make up for a "too short"
lens, you can't put more coverage into a picture you've already
taken.
On the long end, zoom digicams may only provide the equivalent of a
60-70mm lens (like my first digicam, a Canon PowerShot A50) - or as
much as 380mm, as on the Olympus C-700.
You'll want to analyze your own needs in picking the right range.
This is one place where a local camera shop that knows digital cameras
can be a big help.
Add-on Lenses
Some digicams can also accept conversion lenses to provide even
broader coverage or greater magnification. While useful, they are one
extra piece of equipment to carry and are generally a nuisance to mount
on your digicam. Those made especially for your camera will provide the
best quality; the quality of third party conversion lenses will vary.
When possible, get all the range you're going to need in the standard
zoom.
The SLR Lens Multiplier
For those shooting the expensive SLR digicams with interchangeable
lenses, keep in mind that very few digital SLRs have an imager as large
as the standard 35mm frame. Because of this, whatever lens you attach
will typically provide 50-60% more magnification than they would with
35mm film or a full-frame imager, so your 28mm or 35mm lens may be
needed to provide the coverage you expect from a 50mm lens.This can be
especially trying if you need extremely wide lenses, since a 15mm
super-wide may act like a 24mm lens on a digital SLR. (Your 50mm lens
becomes a nice low light 75-80mm lens, and my favorite - an 85mm f/1.8
- gives you 135-135mm of reach with great light gathering.)
Aperture
An aperture is an opening. In a lens, the aperture determines how
much light passes through to the film or imager. Apertures are often
called f-stops, and the lower the number, the more light a lens lets
through.
With zoom lenses, the amount of light transmitted will often
decrease as the lens is zoomed. In such cases, the lens will typically
be specified as f/2.0-2.5 (or whatever is correct for that lens),
indicating the maximum aperture at the short end and long end of the
zoom range.
Most 35mm point-and-shoot cameras with zooms have pathetically small
apertures, often somewhere around f/5.6 on the short end and f/11 at
the long end. This is one of the reasons a lot of people recommend 400
or even 800 speed film in these cameras.
Most digicams have faster lenses than that, which helps make up for
the fact that most imagers provide equivalent sensitivity to 100 to 400
speed film. The fastest lens built into a digicam today (2002) is the
f/1.8-2.6 3x zoom (equivalent to a 40-120mm zoom) on the Olympus
Camedia C-2040 and C-3040. There are several digicams with lenses in
the f/2.0-2.8 range.
There are two reasons aperture is important. The wider the aperture,
the more likely you'll be able to shoot without flash. Further, the
wider the aperture, the more range you'll have with your flash.
As digicams move to providing greater light sensitivity (some now
offer the equal of 3200 speed film, albeit usually with a lot more
noise and sometimes only at reduced resolution), these faster lenses
will provide a real advantage over 35mm point-and-shoot cameras.
Close Focus
Some low-end digicams have no focus at all. Others will focus in the
normal picture taking range - down to 4', 3', and maybe even 2'. Some
offer macro focus as close as the front of the lens. For any shooting
closer than 3', you'll want to use through-the-lens viewing, whether
digital or optical, rather than an optical viewfinder.
Next: The Imager and Digital Zoom
