By Joe Kashi, for the Redoubt Reporter
Although it’s often said that people tend to see life through the emotional equivalent of a distorted lens, there’s rarely a good reason to use a distorting lens in your photography.
To achieve crisp results with any camera requires good optics used to best advantage. In the field, only very few, costly lenses approach their theoretical maximum resolution. So we need to make the best use of what’s readily affordable and available.
This week, we’ll examine several rules of thumb for getting the most of any lens. These are not hard-and-fast rules to be observed under all circumstances, but rather a starting point for your own use and experience.
There are hundreds, if not thousands, of lens designs currently on the new and used market. We’ll point you to some of the best professional websites that specialize in testing lenses.
Following are some of my favorite sites. These are the most reliable and comprehensive sources of objective tests and data I’ve found:
- http://www.slrgear.com, http://www.lensrentals.com (good general information about camera optics), http://www.lenstip.com, http://www.dxomark.com (DXO makes the lens-testing programs used by many other lens testers and their data is among the best and most comprehensive), http://www.ephotozine.com, http://www.photozone.de and http://www.cameralabs.com. Several sites have interactive graphical displays that show how each area of the image responds to changing zoom magnifications and lens apertures, the two factors most critically affecting the sharpness of a lens in daily use. I particularly like those found at http://www.slrgear.com and the direct lens comparison and sharpness “field view” features of http://www.dxomark.com.
The image quality of every optic, whether cellphone camera or no-expense-spared prograde lens, is affected by a number of considerations, some of which pull against each other. Just as important as raw “resolution” are precision assembly, and contrast and color rendition characteristics.
Poor quality control is an unfortunate but pervasive problem. Cost alone isn’t a good indication of whether a new lens is correctly aligned when made. Misalignment (usually called “decentering”) occurs nearly as often with brand-name optics. Even moderate misalignment of one or two internal glass elements during manufacture may ruin a lens.
Decentering typically shows up as one side of the image being noticeably and consistently less sharp than the center and other side. When severe, decentering is a valid basis for returning the lens. Be sure to test every new lens as soon as it is received. There’s usually only a short period of time to return a bad lens.
A highly textured and detailed flat surface, such as a wooden fence, can be used as your test subject. Be sure to face the flat object squarely, at a 90-degree angle, when taking your test shots. The test won’t be accurate if you don’t aim your camera at a perpendicular angle where each edge is the same distance from you and your camera.
If you should somehow obtain that perfectly assembled, sharp lens, by all means retain it indefinitely. Take care to protect it from being knocked about in transit and in use. Don’t decenter a good lens.
When a lens minimizes stray light randomly reflected internally from its sides or glass elements (usually termed “flare”), that lens is said to have good contrast and appears sharper. In fact, a higher-contrast lens often is sharper in use because flare tends to reduce contrast and submerge the fine detail that we see as resolution.
Zoom lenses with their many internal glass elements often have the worst problems with flare, especially at wider, brighter lens apertures. Often, using an intermediate lens aperture helps.
Try to avoid flare in the first instance by minimizing strong light sources directly visible within the image area. Sometimes bright light striking the front of a lens at an angle causes even worse flare. Using a lens hood is the best and least-expensive method to avoid these off-axis flare problems.
Modern optical coatings help a great deal. Every lens maker has its own formulation, although some are more effective than others. Pentax already used an excellent process for many years, so its more recent “HD” coatings provide only moderate additional benefit. On the other hand, Olympus’ recent “Zero” coating process resulted in a noticeable improvement in both flare and overall sharpness, even though the underlying lens designs did not change.
Every type of lens coating results in somewhat different color characteristics, so if you think that different lenses render identical colors differently, you’re probably right. Objectively accurate color rendition is rare. Color rendering qualities are basic to each lens design and can’t be changed by users. You can, however, at least partially compensate later with PhotoShop, Lightroom or other post-processing programs if you know how your lens renders color.
Simple single-magnification lenses tend to be sharper than zoom lenses, which necessarily have more complex optics that are more difficult to design and to adequately assemble at an acceptable selling price. Virtually all zoom lenses are sharper at some magnifications. Usually, the wider-angle and midrange magnifications tend to be sharpest, though not always.
Zoom lenses, particularly consumer-priced lenses with a wide magnification ratio, tend to be the least sharp overall because they need to balance the greatest number of contending factors. For the same reasons, less complex single-magnification “prime” lenses tend to be smaller and sharper overall. Rather than one or two zoom lenses, I personally prefer to use a range of prime lenses whenever feasible and affordable.
There are a few notable exceptions, particularly Sigma’s excellent new 18- to 35-mm f/1.8 zoom sold for many different camera makes. Although not quite as sharp as the best prime lenses, this new Sigma zoom is making waves, and rightfully so, providing better quality than comparable OEM lenses at a significantly lower cost.
Most lenses have a narrow range of lens aperture settings where sharpness, contrast and other image quality factors are at their best. Even though every lens is theoretically sharpest at its widest, brightest aperture, that hardly ever occurs in reality.
Generally, optical aberrations have the most detrimental impact on image quality at or near a lens’ widest, brightest aperture. To some extent, this can be overcome through complex design and costly manufacture. At the other end, sharpness starts to unavoidably degrade at very small lens apertures on the order of f/11 to f/16. That’s a basic limitation of physics known as diffraction.
Every lens, even the bottom of a Coke bottle, has an optimum aperture, where you’ll find the best balance of contrast, and center and edge sharpness. It’s typically found at intermediate apertures one to three stops smaller than the widest opening. Sharpness is usually better in the center, dropping off toward the edges. Edge sharpness improves rapidly at intermediate lens apertures. By referring to the lens test sites listed in the extended web version of this article, you’ll find data showing the best apertures for your lens model. Remember, though, that this is only a starting point for your own tests.
Older lenses for full-frame film cameras are usually best, and in fact often still very good, when set between f/6.3 to f/8. Lenses designed for current APS-C digital SLR cameras tend to do best when set to f/5.6 or so, while smaller Micro Four-Thirds lenses typically prefer the f/4 to f/5.6 range.
- Around town: Kenai Peninsula College’s art students are exhibiting their new work at the Kenai Fine Arts Center between Feb. 15 and Feb. 28, with a free public opening reception from 3 to 5 p.m. Feb. 15. Gallery hours are noon to 5 p.m. Wednesdays through Saturdays.
Local attorney Joe Kashi received his bachelor’s and master’s degrees from MIT and his law degree from Georgetown University. Many of his technology and photography articles can be accessed through his website, http://www.kashilaw.com.