Picking the Right 35mm SLR

Introduction to Lenses

Dan Knight - June 2002, updated

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).

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