Welcome to the first of an ongoing series where we dig deep to answer your questions about photo gear and taking pictures. Today, we'll look at whether it makes more sense to buy two shorter zooms or a single long zoom lens that covers the entire focal range.
Recently, one of Adorama's Facebook fans asked a question about lenses that went something like this:
"Last year I bought a starter DSLR with two zoom lenses, an 18-55mm and 55-200mm "kit" zoom. I really like the long range of focal lengths but as I started using my camera I realized that I hated having to constantly switch lenses. I've missed shots because I was busy juggling lenses. Dust gets on the sensor when I change lenses, and I have to clean the sensor frequently. It's a pain, and I'm thinking about trading the two zoom lenses for a single 18-250mm lens. Is this a good idea, or not? Is there a difference in image quality?"
That's a great question, and the answer really depends on the needs of each individual, so there's no “right” answer (although in the case of this particular reader, he may have answered his own question and is simply looking for confirmation). There are several advantages and disadvantages to each approach, but there are also specific, measurable differences in image quality when comparing two short-range zooms with a single long-ranger.
How does this long-range zoom lens compare...
...with these two shorter zoom lenses, which collectively cover approximately the same focal range?
Canon, Nikon, Pentax, and Sony all make low-end zoom lenses that cover both shorter ranges and a long-range zoom. The typical “kit” lens is an 18-55mm f/3.5-5.6, and a low-cost tele zoom might be a 55-200mm or 55-300mm f/4-5.6. The major camera makers (as well as indie lensmakers such as Sigma, Tamron and Tokina) also make long-range zooms, typically 18-200, 18-250, or 18-300mm that are surprisingly compact. Pricewise, these long range zooms tend to cost approximately what a combination of an 18-55mm and 55-200mm would cost. There are similar focal range breakdowns for enthusiast-level lenses (which feature better quality glass and construction) and pro-level lenses (which feature constant f/2.8 apertures and the best optics and build).
To compare apples to apples, I'm going to focus on kit-grade lenses designed for DSLRs with APS sensors—the least expensive options out there. That's where the differences are most pronounced, and where the question comes up most frequently, although the test results and other factors can be applied to lenses for full-frame cameras and higher-end models.
A typical 18-55mm kit lens, when purchased separately, costs in the $100-150 range, while a typical low-end tele zoom will set you back around $200. A long-range zoom of similar construction and quality will cost around $300-400, so when comparing the overall cost of two lenses against one longer one, price is roughly equal.
The combined weight of two shorter-range zoom is roughly equivalent to that of a single long-range zoom. A Canon 18-55mm kit lens weighs 200 grams and the Canon 55-250mm lens weighs 390 grams, for a total of 590 grams. The Canon 18-250mm lens weighs 595 grams—a wash, weightwise. (Specs for other brands work out similarly.) However, you don't always need to bring both lenses with you at all times, so you have some control over the overall weight of your camera kit; the heavier long-range zoom could be harder to handhold for extended periods, a disadvantage for long zooms.
Clearly, the best argument in favor of a single, longer lens is convenience. You don't have to juggle two lenses. Your picture-taking kit is self-contained, and if you don't remove the lens, you avoid the possibility of dust collecting on the sensor and affecting your images. If that's the most important criteria for you, go for it!
Why, then, is a DSLR with a single superzoom lens a better choice than, say, buying an EVF camera with a built-in long-range zoom lens? The answer is that a DSLR has a larger sensor than any EVF superzoom currently on the market. EVFs tend to have sensors that are the same size as the kind you'd find on a pocket-sized compact, which means they'll produce inferior overall image quality, especially when you want to shoot in low light using higher ISO settings.
Finally, we come to the elephant in the room: Is there a difference in image quality between two shorter zooms and one longer zoom covering the same focal range? The answer is that it's a mixed bag but that two shorter lenses have an edge.
I ran comparisons of several manufacturers' short-range zooms against long-range zoom lenses using the lens test result comparison tools provided by our lab partner, DxOLabs. DxOLabs tests for sharpness, chromatic aberrations, vignetting, and optical distortion. I won't go into any specific comparison, but instead will talk about general patterns that I saw in each of these test results, because the patterns are more or less the same across all manufacturers' lenses in this specific class and range of focal lengths.
Generally, the 18-55mm lenses delivered the best optical performance overall, with the best sharpness at most apertures in the centers and edges. The 55-200 or 55-250mm lenses delivered less impressive results. The 18-200mm lenses performed similarly to the 55-200mm lenses, but across the longer focal range. So, going head to head, the 18-55mm lens was notably better than the 18-200mm lens in the overlapping focal range, while the 55-200mm lenses delivered a performance that was roughly equal to that of the long-range zoom in its overlapping focal range.
Let's break it down further:
Above, from left to right: All shot at approximately the same focal length, the 18-55mm showed almost no distortion, the 55-250mm lens had pronounced pillow distortion, and the 18-200mm lens displayed obvious pincussion distortion.
Distortion: All of these lower-cost lenses displayed linear distortion. The long-range zooms tended to have more pronounced pillow distortion at the shortest focal lengths than the shorter zooms, and tended to have distinct pincussion distortion at the full tele extension—more so than the 55-200 or 55-250 at maximum extension.
The curvy lines on the right side of the chart show the range of “fringing” different levels of chromatic aberration can have on a hard, contrasty subject.
Chromatic Aberration: This was a mixed bag. All lenses tested displayed above-average chromatic aberration, which shows up in an image as colored fringing along the edges of contrasty subjects. The degree of aberration varied from lens to lens and brand to brand. The good news? Unless you plan on making enlargements above 11x14 inches, you probably won't see it.
At widest apertures at 50 or 55mm, vignetting was moderate...although on the 55-250mm lens, it was a bit more noticeable, as illustrated here.
Vignetting: At almost all focal lengths, vignetting (darkening of the images towards the corners) was more pronounced with the long-range zoom at the widest and smallest apertures. Middle apertures (f/8 and f/11) showed virtually no vignetting in any of the lenses.
Sharpness: Sharpness is measured in lines per millimeter based on a standard target shot under consistent light conditions. When shooting at middle aperture settings, the lenses all showed similarly good image quality especially in the middle of the image. The closer you went towards the smallest or largest aperture, the softer image quality got in the corners of the image and to a lesser extent in the middle. This is expected behavior for all lenses, but the long-range zoom showed the most softness at the edges at the longest settings compared to the shorter-range tele zoom. Quality in the 18-55mm range was more or less equal between the short-range zoom and the long-ranger.
The bottom line? If you are going to spend around $300-400 (total) for a focal range of around 18-200mm, you can expect variable apertures and OK, but not stellar performance. And if you split the focal range into two lenses, you'll get better overall image quality in the 18-55mm range. If you're planning on making prints 8x10 or smaller, you probably won't notice a difference.
Software to the rescue?
Many (but not all) of the optical imperfections can be reduced or eliminated via software that compensates for specific lenses' problems. DxO Optics Pro has hundreds of downloadable modules that compensate for pincussion and pillow distortion, color fringing, and even vignetting. It can also improve sharpness. If you are willing to pay the expense and put in the post-production time to improve your images, this is an option that can make the differences described above less important.