Optic Nerves? Afraid to buy the wrong lens?

Our Answers to ten common questions will ease your mind.

Optic-Nerves-Afraid-to-buy-the-wrong-lens
Optic-Nerves-Afraid-to-buy-the-wrong-lens

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.

Question:
What is an "internal focusing" lens, and what are its advantages?

Answer:
An internal focusing (IF) lens focuses by shifting select elements (usually elements in front of the lens diaphragm) within and independent of the outer lens barrel. With IF lenses, the outer barrel doesn't change in length or turn as the lens is focused. This makes IF lenses well-suited for use with lens-mounted accessories such as ring lights and filter systems.

Lens shades can also be larger and thus more effective, because the autofocus motors in IF lenses don't have to turn them. Other benefits of IF lenses: They're generally smaller and closer-focusing than comparable non-IF lenses.

Question:
I know that digital-only lenses cause corner vignetting when used on 35mm cameras, but I thought 35mm lenses worked on DSLRs without a hitch. So I was surprised, when I put Sigma and Tamron lenses on my new [Canon] EOS Digital Rebel XT, and the camera's LCD flashed "Error Code 99." What are my options?

Answer:
Most often, Canon's Error Code 99 indicates incompatibility between a camera and lens. (Canon 35mm film cameras similarly blink the battery symbol.) A temporary fix is to shoot only at maximum aperture. The error code may disappear and normal readouts will return to the LCD.

Long-term, the issue can be solved inexpensively by returning the lens to its U.S. distributor to have its flexboard (or microprocessor) updated. Most independent lensmakers offer this for current lenses at no cost; you pay only the return postage. Call the customer service department at Sigma (800-896-6858), Tamron (800-827-8880), or Tokina (THK Photo, 800-421-1141) for more info.
Question:
What are aspheric lens elements, and what do they do?

Answer:
Put simply, they're lens elements that are not sphere-shaped. Most camera lens elements have arc-shaped concave or convex front or back surfaces (see diagram); they're spheric. For many lens types, spheric elements can be combined to transmit light without introducing areas of fuzziness or bent lines (a.k.a. aberration and distortion).

However, with some lens types (usually ultrawide and large-aperture lenses), optical engineers can't easily control aberration or distortion with spherical elements alone. These lens types require one or more non-spherical ("aspheric") elements. Most aspherics are hybrids of a molded aspheric element made of plastic cemented onto a spheric element of glass. All-glass aspherics are less common because of the time and expense involved in grinding and polishing glass to the correct shape.

Question:
Why do some zoom lenses have a single, constant maximum aperture and others have variable apertures? Which is better?

Answer:
Both offer advantages. As you zoom out, variable aperture zooms admit less light through to the film or sensor. As a result, the f-number changes. Example: A 28-300mm f/4.5-5.6 has a maximum aperture of f/4.5 at 28mm, and as you zoom out, the f-number changes to f/5.6 at 300mm.

Variable apertures let engineers incorporate longer tele settings in smaller, more affordable packages. In the example just cited, a constant-aperture 28-300mm f/4.5 would need to maintain the aperture as you zoom out, and so would require significantly larger glass elements to deliver the 300mm focal length at f/4.5; the resulting lens would be large and expensive. Designing the lens to dim to f/5.6 when zoomed out, however, contains both size and cost.

Because most metering is TTL, the exposure system automatically compensates for the shrinking aperture. From an exposure standpoint, most photographers will find no downside to variable-aperture zooms. However, photographers who set exposure with handheld, external meters typically bracket exposures more when shooting with variable-aperture zooms.

Another potential downside to variable aperture? Depth of field. The smaller apertures at longer focal lengths make it harder to throw a background out of focus as you zoom out.

Question:
How does image stabilization in SLR/DSLR lenses work, and should I spend the extra money for it?

Answer:
For average SLR shooters, camera shake introduces blur in photos made at shutter speeds slower than the reciprocal of the lens focal length. A 300mm lens used without a tripod or flash will usually show blur at shutter speeds below 1/300 sec. Image stabilization lets you shoot at 1/250 sec, 1/125 sec, even 1/60 sec, and still enjoy sharp pictures.

It does this thanks to a floating element (see diagram, next page) whose movements are controlled by a gyroscope-like sensor that recognizes up/down (pitch); and another, sensitive to left/right (yaw) movement. Processors calculate the direction and degree of off-axis camera movement, and the device automatically shifts the floating element to compensate for that movement.

If the lens goes up, the floating element rises as well, bending the incoming light rays to strike the imaging plane from an angle that's relatively uninfluenced by the (slight) camera movement.

As for the second part of the question: We've never met a photographer who regretted investing in IS.

Question:
I know that Pop Photo uses the SQF system to test lens sharpness. I even know that SQF stands for Subjective Quality Factor. But how does it work?

Answer:
SQF was developed by Edward Granger, a professor at the Rochester Institute of Technology and senior scientist at the Eastman Kodak Company during the 1970s, as a better way of measuring lens sharpness than the prevailing system at the time, known as Modulation Transfer Function (MTF). MTF measures a lens' ability to reproduce discrete bars (or lines) in increasingly smaller arrays.

Sharp lenses can reproduce more "line pairs per millimeter" (lpm) than can unsharp lenses. MTF numbers, however, are meaningful only to optical engineers, so Granger set about designing tests that could have real-world meaning to most photographers.

The actual SQF tests are performed on an optical bench: The tested lens is mounted on a movable, computer-controlled stage and focused at infinity. Light from a laser is used to illuminate a metal plate into which very fine crosshairs have been etched-2.5 microns wide for a normal lens, or much smaller than the diameter of a human hair. (Larger crosshairs are used for short focal length lenses; smaller crosshairs, for long lenses.)

The crosshairs are optically altered by a device called a collimator to appear to be at infinity. The collimated light passes through the crosshairs, through the center of the test lens, ultimately striking a high-resolution CCD array where the "light spread" (blur) of the image is measured.

The greater the light spread, the less sharp the lens. We measure the spread from the full range of lens apertures.

After testing the center of the lens, we continue with off-center and edge measurements. The results are compiled, compared, and weighted: Image center readings comprise 50 percent of a lens' SQF score; off-center readings, 30 percent; and image edge readings, 20 percent. (The percentages are slightly different for digital lenses.)

By running the readings through a series of computerized calculations, SQF can quantify lens sharpness and interpolate the rate at which sharpness will deteriorate as image size increases.

Question:
What is ultra-low dispersion glass?

Answer:
A clear, optical-quality glass that goes by many names, it's especially effective at controlling color fringing in tele lenses. Lenses that use it are usually designated "apochromatic." Color fringing, also known as chromatic aberration, is what happens when colors don't focus to the same plane.

In very large blowups, if you see a small halo of color-red, say-along an object in a photo, your lens isn't focusing red to the same plane as the other colors that make up white light. Canon and Tamron's Low dispersion (LD), Nikon's Extra-Low dispersion (ED), Sigma's Extraordinarily Low dispersion (ELD), and Tokina's Super-Low dispersion (SD) glass promise to control the problem.

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