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6) What's generally the sharpest lens aperture?
Usually it's right around the middle of the aperture range; traditionally, the rule has been 2 to 3 stops down from maximum aperture, and our tests support this pretty much across the board.
For example, if you look at our SQF chart for the Pentax 70mm f/2.4 Limited DA AF lens, you'll see the best (that is, highest) numbers fall around f/5.6 to f/8, and they drop off from there. Turn the chart sideways, draw a line over the red zones, and you'll get close to a classic bell curve.
This surprises people who confuse absolute sharpness with depth of field, the zone in front of and behind the subject of a photo that is acceptably sharp. Depth of field is maximized at small apertures, such as f/16 and f/22. But if you examine the image closely, you'll see the point of focus is less sharp than in pictures taken at middle apertures. This is due to diffraction, an image-degrading effect exacerbated by small lens openings.
7) Do heavier lenses perform better?
Not necessarily. Some lightweight (and light-price) lenses perform very well, especially at their optimal apertures. For a typical f/3.5-5.6 kit lens such as the 18-55mm Zoom-Nikkor AF-S DX, that's around f/11 -- perfect for landscapes.
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| © Ed Nunez |
But there are drawbacks. Says Ed Nuñez, who used that lens for the photo at right, "It doesn't have a focus-distance scale, like pro lenses have." So he couldn't save time by setting the lens to hyperfocal distance. "I had to manually focus and use the depth-of-field preview button on my camera, or just autofocus in a section of the scene and manually move the focus until I got the feeling that everything would be right." Still, he got some nice pictures with a 7-ounce lens.
8) What is "equivalent focal length"?
In a DSLR, a sensor the size of a frame of 35mm film (24x36mm) is "full-frame," while the smaller APS-size (17x25mm, with variations) is more common. (The Four Thirds system format, 13.5x18mm, is even smaller.)
Put the smaller imager behind a full-frame lens, and it'll capture only the center of the image -- in effect, cropping. Since most shooters still remember 35mm film, it's accepted practice to state a conversion factor to show what the equivalent lens for 35mm would be. (Soon people will be so familiar with their digital format that this will be unnecessary. We'll know that an 18mm lens is a moderately wide angle without converting it to "27mm equivalent.")
Equivalent focal length is not the same as coverage -- the size of the image circle a lens throws onto the sensor. Coverage is determined by lens design, independent of focal length. "Digital-only" lenses designed for APS-size sensors throw a smaller image circle than full-frame lenses. If you use a digital-only lens on a full-frame camera, the corners of the picture are likely to be dark because the image circle won't cover the whole frame. Using a full-frame lens for an APS-size sensor is no problem, because the full-frame lens has coverage to spare.
9) So shouldn't I buy full-frame lenses for a DSLR with an APS-sized sensor?
Our test results here aren't very conclusive. We recently retested the full-frame 28-135mm f/3.5-5.6 Canon EF IS USM ($410, street) to see how it would do using just the APS-size "sweet spot." The difference? Statistically insignificant. Since that's a legacy lens designed for film, we also tested the new 14-24mm f/2.8G Nikkor ($1,800, street) at APS size as well as full frame. Again, the difference was meaningless.
Then we tested the new 28-300mm f/3.5-6.3 Tamron VC lens ($600, street). While it provides full-frame coverage, it's intended mostly for APS-sensor DSLRs. We found insignificant differencecs at the two shorter focal lengths, but improvements of one full grade at 200mm and two grades at 300mm.The Tamron lens also showed improvement in flare and light falloff when used on APS format.
But this isn't a given for all lenses, either. The fact is that APS digital-specific lenses can and do deliver comparable performance to the "sweet spot" of a full-frame lens. So, if you own full-frame lenses, you'll get fine results from them on your APS-sensor DSLR. But it won't pay to buy full-frame lenses for your DSLR just to get better image quality.
10) Which lens-based image stabilization system is best?
While the details differ among Image Stabilizer (Canon), Vibration Reduction (Nikon), MEGA Optical Image Stabilization (Panasonic/Leica), Optical Stabilizer (Sigma), and Vibration Compensation (Tamron), the principle is the same: A motorized lens group counteracts motion detected by gyro sensors in the lens. Our tests show that most give you about 2 to 3 stops of extra handholding advantage. Stabilization is more effective with telephotos than wide-angles, where you may gain only 1 stop. Why? Short lenses magnify less and thus show little motion blur.
Stabilization improves with each successive generation. We've gotten the best test results (up to 4 stops) from the latest Nikon VR II technology. Do not expect its rivals to let the matter rest.
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