Our Answers to ten common questions will ease your mind.
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?
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.
My lens is called a "10X" zoom. Why?
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.
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.
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.