 BetterPhoto Member |
Zoom Lens I want a longer zoom, but am very confused about APO and ASP, etc. Looking at Sigma 70-300, but there are two different lenses that vary a lot in price. What's the diff, and what's better for me? I take pictures of my kids a LOT.
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Jon Close |
The non-ASP and non-APO versions are strictly economy models, and will produce acceptable (but not great) prints up to 4x6. ASP refers to "Aspherical" lens elements. The surface of most lens elements follow the arc of a circle or sphere. Lens elements that are not spherical (ie. aspherical) can reduce the total number of elements required in a lens design. They can deliver better performance while reducing weight and size. Aspherical lenses maximize optical performance while minimizing lens size and weight. Aspherical lenses help reduce some of the problems usually associated with wide angles and zooms, such as lens flare and edge distortion. They also help deliver higher quality telephoto images. APO lenses use apochromatic design and Special Low Dispersion glass (SLD) for minimum color aberration and ultimate telephoto image quality, improving contrast and sharpness. Apochromatic (APO) telephoto lenses greatly minimize color aberration, a phenomenon created by different wave lengths of light refracting at different angles. This causes each color to have a slightly different imaging point, resulting in poor image
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John A. Lind |
I find the use of "APO" by some lens makers in their advertising interesting and amusing. Virtually all but the absolute cheapest photographic lenses have been "apochromatic" for decades, since before W.W.II! A term often used then was "achromatic" and it means the same thing. Perhaps what they are referring to is better chromatic aberration correction. However, slides I've seen from the early days of Kodachrome (circa 1930's) show no sign of objectionable chromatic aberration. Even before color films with orthochromatic B/W it would have caused problems with resolution if a lens was not apochromatic. Any lens with three or more elements (and the designs for some date to 1890-1900) is typically well corrected for chromatic aberration. IMHO don't worry about whether a lens is labeled "APO." None of my numerous lenses, most over 15-20 years old (one is 45 years old) have an "APO" designation. I have found zero measurable chromatic aberration in any of them in projecting slides from them to a 50 inch screen (about 35X magnification).
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John A. Lind |
Some remarks about the "SLD," also called "ULD" by other manufacturers. This can be important in a zoom lens with many elements. Each glass element absorbs some of the light traveling through it. More glass means potentially more absorption. Too much absorption results in loss of image contrast. This is not usually a problem for decent primes made since W.W.II with the advent of lens coatings. They typically have no more than 5 to 7 elements. Even a single coating on each side of each group allows sufficient light transmission that any further improvement is hardly noticeable. However, zooms are much more complex with 2 to 3 times as many elements and pose the problem of much more glass to absorb light. The "SLD" or "ULD" glass absorbs less light improving light transmission from subject to film. Some of the high end zooms with SLD/ULD elements have contrast normally found only in primes.
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