Optic Nerves? Afraid to buy the wrong lens?

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Question:
Lensmakers almost always quote the number of elements and groups of elements in their lenses, often with diagrams showing the relative position of the groups. Why? What can I learn from them?

Answer:
Very little, in fact. Those diagrams are largely a matter of curiosity, unless, as one lens- maker spokesperson said, “they’re pulled out as justification for higher pricing.” (Translation: The more elements, the bigger the bill.)

Before the zoom revolution of the 1950s and 1960s, most lenses had four elements. In a cost-cutting move, some lensmakers tried to get away with three-element formulas, resulting in increased aberrations and decreased edge sharpness. Today, however, there’s little or no correlation between the number or configuration of a lens’ elements and its optical quality.

Question:
My lens is called a “10X” zoom. Why?

Answer:
The “X” refers to its zoom ratio. A 10X lens has a 10:1 zoom ratio; its longest tele focal length is 10 times that of its widest. To determine a zoom ratio, divide the short focal length into the long. The popular 28–300mm superzooms, for example, have a zoom ratio of about 11:1. (300mm ÷ 28mm = 10.7X). The higher the X number, the more versatile the zoom.

Don’t confuse zoom ratios with magnification powers, which are also communicated with the letter X. The magnification power of a lens tells you how the size of an image cast by a lens compares to the size of the actual object under focus. A 2X macro system, for example, can reproduce an image of an object on film or image sensor at twice its actual size.

Question:
I own a digital SLR with an APS-sized sensor and am getting conflicting information about whether to buy “digital” or “full-frame” lenses for it.

Answer:
Consider both. The difference between full-frame and digital is in the size of the circular image cast by the lens, called its image circle. A digital lens throws a circle with a diameter large enough to cover the area of an APS-sized digital sensor, about 15x22mm. The image circle cast by a full-frame lens is larger and wide enough to cover a full-frame of 35mm film, or 24x36mm.

Full-frame lenses will be compatible with any future full-frame SLRs, film or digital, that share that lensmount; they are also compatible with any 35mm film SLRs from the same manufacturer. Digital-only lenses offer more options for your DSLR at the wide end of the focal-length spectrum. There are many more ultrawide zooms from camera makers and third-party lensmakers in digital-only versions (10–20mm, 11–18mm, or 12–24mm, for example) than full-frames.

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