DV101: What Makes a Good Lens?
Sizing up how to properly size up your optics.
By Jay Holben
Many new-to-DSLR shooters are encountering the situation in which their lens choices have gone from using a single fixed zoom lens — typical of most similarly priced camcorders — to employing a wide wealth of fantastic optics choices. And navigating the waters of the options can be daunting.
So, what is good? What is bad? And how to you choose the best lens possible?
Carl Zeiss now offers two different options for DSLRs, the first option, comprised of Distagon T* and Planar T* lenses, which are re-tooled SLR lenses, and the Zeiss CP lenses (seen above), which are far more expensive but specifically designed for cinema-style shooting.
Meanwhile, Schneider, which most people now know for their high-quality filters, has returned to their roots with a new brand of lenses specifically created for digital cinematography, the Cine-Xenar Mk II (seen below).
These two manufacturers are the top of the game when it comes to cine-style lenses for DSLRs, but what about the rest? Does this mean that all other lenses aren’t any good? Do you always have to spend a fortune to get a good lens?
Well, no, but understanding what really makes a lens good is a complicated process.
Recognizing the Problems
Every lens is different. No lens is perfect and no one lens is perfect for every job. By the very nature of optics, every lens is a collection of compromises to optimize its performance for a specific task or range of tasks. Here are some of the characteristics that you need to examine before making a lens choice:
• Build quality
• Mechanical Quality
• Chromatic aberration
• Fall off
When it comes to build quality, inexpensive lenses generally have a plastic build, while better quality ones have an aluminum (or other metal) build. There should be a metal lens mount. The lens should feel well constructed and robust, even if lightweight.
Regarding mechanical quality, the lens mechanisms should move smoothly, easily and uniformly, without rough spots, grinding or easy spots. Test the focus ring, zoom ring and aperture controls for smooth, easy action.
Chromatic aberration (CA) is a defect that happens when a particular wavelength of light entering the objective is not focused by the lens at the same point as are the other wavelengths. It is represented in the image by a ghosting or fringing of color around the edges of an object. This is the most common optical defect and is generally corrected by high-quality lens coatings. CA is most visible when shooting at wider apertures.
Distortion takes place when light rays don’t strike the lens in parallel patterns. They diverge or converge as they strike the lens turning what should be straight lines in the image into convex (barrel distortion) or concave (pin cushion).
Breathing is a defect whereby the focal length seems changes slightly (zooming effect) when focusing the lens. This happens most often with zoom lenses, but can also happen with primes.
Fall off is a defect that happens when the center of an image is brighter than the edges, also called vignetting. It is most notable at wide apertures.
These characteristics are the benefits and strengths of the individual lens:
Speed is determined as the widest aperture possible on that lens. f1 being a theoretical widest aperture and an extremely fast lens. f5.6 being a slow lens. Prime lenses are most often faster than zooms because they require far fewer optical elements. A high-quality zoom lens has a constant aperture across the entire zoom range.
Resolution is the lens’ ability to reproduce details, typically measured by Modulation Transfer Function (MTF) charts. Most photographic lenses are “optimized” for the center of the image to be of the highest resolution. On an MTF chart, the far left of the scale represents the center of the lens and the right end of the scale represents the far edge. It is presumed that the other side of the lens mirrors the optical qualities — although, in reality, this isn’t always the case.
Contrast is often misunderstood with regard to lenses. This doesn’t refer to the lens’ ability to reproduce tones between black and white, but more to differentiate between similar hues in close proximity. Contrast is closely related to resolution as together they define the lens’ overall apparent sharpness.
Lens color varies, sometimes considerably, from lens to lens — even within a family of lenses. There are many factors that contribute to the lens’ color including the individual glass elements, their various coatings, the age of the coatings and so forth. Some lenses have a warmer tone, some a cooler.
Bokeh has become an over-used term. It is actually coined from the Japanese work boke meaning “blur” or “haze,” and photographers use the term to describe the quality of the out-of-focus aspects of an image through a given lens. Bokeh is primarily defined by the number of blades in the lens’ iris. The more blades, the softer and more circular the out-of-focus highlights become.
This discussion on optics will be continued in an upcoming issue of DV.