The most important part of the camera is the lens. A bad lens will
make poor images no matter how good the rest of the camera is, and a
great lens can get excellent results on an average camera body.
Do You Want to Change Lenses?
The first question in choosing an SLR: Do you want to change
lenses?
There were a few 35mm SLR cameras with built-in zoom lenses, such as
the Olympus IS-10 QD, IS-20QD, IS-3, and IS-30 DLX. These cameras offer
a 28-110mm or 35-180mm zoom lens, which may meet your photographic
needs. These cameras are simpler, more compact, and less costly than
many of the interchangeable lens models.
Of course, just because you can change the lens doesn't mean you
have to. A lot of SLR photographers are quite content with the 28-80mm
zoom that came bundled with the camera - or perhaps a 28-200mm zoom
they chose instead. (Although I had several lenses for my Nikons, I
hardly ever use anything but my Tamron 28-200.)
That brings us to perhaps the most crucial issue in choosing a 35mm
SLR: lenses. You need to have some understanding of focal length (what
do 28mm, 50mm, and 200mm mean?) and lens speed (the aperture, f-stop,
lens opening). We'll start with the latter.
Aperture or F-stop
An aperture is an opening. The larger the aperture, the more light
goes through it, whether we're dealing with a window or a camera lens.
All things being equal, a larger opening will let you shoot at a higher
shutter speed, thus stopping action and reducing camera shake. A
smaller aperture will provide a greater range of sharpness (called
"depth of field") in your photograph but require a slower shutter
speed.
Here's where it gets confusing: The smaller the f-stop
number, the larger the opening. That's because the f-stop (such
as f/1.8, f:2.8, or 1:4.0) is a ratio between the diameter of the
opening and the focal length of the lens (we'll get to that soon). This
number is a reciprocal - the smaller the number, the wider the
opening.
Further, because the aperture is two dimensional, to double the
amount of light allowed through it, the diameter must increase by the
square root of 2 (approximately 1.4) - not by two. This explains the
f-stop scale: 1.0, 1.4, 2.0, 2.8, 4, 5.6, 8, 11, 16, 22, 32, etc.
(Lenses for 35mm cameras rarely go beyond f/32.)
You'll see other f-stops that are usually around the halfway point
between "full" f-stop settings, such as f/1.7, 3.5, 4.5, 6.3, etc.
These are called half-stops.
F-stops are simple math, but they tend to trip up new users. Just
remember that the numbers seem backwards - a smaller number means a
bigger opening and lets through more light - and you've go the most
important concept down.
Focal Length
Camera lenses, whether for 35mm, digital, camcorders, or medium
format cameras, are usually measured in mm. For those a bit rusty on
their metric, there are 25.4 millimeters in one inch.
By convention, a normal lens has a focal length roughly equal to the
diagonal of the film format. Remember the Pythagorean Theorem? It tells
us that the diagonal of a rectangle is the square root of the sum of
the square of the two sides. In mathematical shorthand:
a2 = b2 + c2
or a = √(b2 + c2)
A 35mm negative is nominally 24 mm high and 36 mm wide. 24 x 24 =
576. 36 x 36 = 1296. 576 + 1296 = 1872. The square root of 1872 is
43.27. Thus, a 43mm lens is the theoretical "normal" lens for a 35mm
camera. By convention, the normal lens for a 35mm camera ranges from
40mm to 60mm, with 50mm being the most common.
(For a number of reasons, the lens on non-zoom 35mm viewfinder
cameras tends to be in the 35mm to 40mm range. This makes for a smaller
camera with better coverage, which is very helpful indoors.)
Any lens shorter than "normal" covers a wider area and is considered
a wide-angle lens. The shorter the focal length, the greater the
coverage. A 24mm lens will have roughly twice the vertical and
horizontal coverage of a 50mm lens, a 17mm lens three times as much in
both dimensions!
By convention, any lens longer than normal is called a telephoto
lens, although this isn't always technically accurate (the technical
definition says that a telephoto lens is physically shorter than its
focal length). The longer the focal length, the greater the
magnification. A 200mm lens will make something four times higher and
four times wider than a 50mm lens.
Next: Picking the Right Lens(es).
