Everyone likes a bargain, so price is always a factor in choosing a lens. However, there can be very significant differences in prices between lenses which on first glance may appear to be very similar.
What makes some lenses much more expensive than others when they seem to cover a similar focal length range and are similar in speed?
Why, for example, should I buy a Canon EF 70-300mm f/4-5.6L IS USM ($1,500) when there's the Canon EF 75-300mm f/4.0-5.6 III ($180)?
Or what about the Sigma 17-50mm f/2.8 EX DC OS HSM ($670)? The Sigma 18-50mm f/2.8-4.5 DC OS HSM ($200) is so much less expensive!
Finally, there's the Nikon AF-S DX Zoom-NIKKOR 17-55mm f/2.8G IF-ED ($1,425) which is a lot of money compared to the Nikon AF-S DX Zoom-NIKKOR 18-55mm f/3.5-5.6G ED II ($115). What are we paying all that extra money for? Or if we go for the lower-cost lenses, what in performance and/or quality do we lose?
(Note: Prices quoted are accurate as of Nov. 29, 2011)
The name (and letter) game
First there are a few obvious and important differences which are revealed in the names of the lenses, if you know how to read them!
With Canon lenses, the more expensive telephoto zoom includes the letters "L," "IS," and "USM," while the less expensive lens doesn't. "IS" is Image Stabilization, which means that the lens can compensate for some movement of the camera while shooting which can significantly improve the sharpness of the image in some situations, especially when shooting hand held and in low light. "USM" signifies the lens has an UltraSonic Motor for autofocus. That means AF is faster and quieter then the less expensive lens, which uses a conventional motor. It also means that you can manually focus the lens without having to switch from AF to MF mode.
Finally the "L" is Canon's designation for lenses which use special elements (aspherics, low dispersion glass and/or fluorite elements) which are expensive but improve performance. "L" lenses are also more rugged and are built to stand up to professional use. The "EF" signifies that both lenses cover a full 35mm frame ("EF-S" lenses can only be used on APS sensor cameras)
Reading the letters in the case of Nikon lenses reveals that the more expensive lens is "IF-ED" which means it uses an internal focusing mechanism and special "ED" glass to lower optical aberrations. The more expensive lens is 0.5 stops faster at 17mm (f/2.8 vs f/3.5) and a full 2 stops faster at 55mm (f/2.8 vs. f/5.6). Nikon "DX" lenses are designed for use onAPS sensor cameras. Neither lens is optically stabilized, which would be indicated by the letters "VR" (vibration reduction) in the name of the lens.
Sigma lenses appear more similar and are closer in price than either the Nikon or Canon examples. Both have Optical Stabilization "OS" and Hyper-sonic motors (HSM). The main differences you can tell from their names is that the more expensive lens is an "EX" lens (Sigma's designation for lenses with "... superior build and optical quality..."), it zooms slightly wider (17mm vs 18mm), and is about 1.33 stops faster at 50mm (f/2.8 vs f/4.5). Both lenses are from Sigma's DC series, which means they are designed for use only APS sensor cameras.
Differences to look for
Differences between an expensive lens and a similar less expensive lens might include one or more of the following:
- More features such as internal stabilization and a faster focusing motor
- More solid construction leading to better durability in hard use.
- Faster optics for better performance in low light or to better blur backgrounds or to allow faster shutter speeds to stop action.
Better image quality due to the use of more expensive glass and more complex optical designs
The first three factors can often be determined by reading the lens specifications. However the last factor, better image quality, can't be determined from the lens data sheet. Sometimes the manufacturer will publish MTF (Modulation Transfer Function) plots, which to a trained eye can reveal likely differences in image quality, but there's no real substitute for side-by-side lens testing.
Case Study: Sigma 17-50mm f2.8 EX DC OS HSM vs. Sigma 18-50mm f/2.8-4.5 DC OS HSM
How do these differences translate into lab test results, and actual real-life images? Sigma was kind enough to loan me samples of the 17-50mm f/2.8 EX DC OS HSM and the 18-50mm f//2.8-4.5 DC OS HSM in Canon mounts so that I could compare image quality using my Canon 7D and see what you get for the higher cost of the 17-50mm f/2.8.
Sigma 18-50mm f/2.8-4.5 DC OS HSM(left); Sigma 17-50mm f2.8 EX DC OS HSM (right)
One way to find out how two lenses compare without having them in your hands is to look at their test results on DxOMark.com. You can select up to five lenses to compare side-by-side. While not all lenses have been tested yet, DxO has tested hundreds of lenses, many of them on multiple cameras at different resolutions. I checked and luckily, DxOMark has tested both the Sigma 17-50mm f/2.8 and the Sigma 18-50mm f/2.8-4.5 on a Canon 7D. You can go here and investigate the lenses of your choice in depth. This comparison chart provides an overview of DxOMark's test results for our case study lenses:
As you can see, the 17-50mm f/2.8 outperformed the 18-50mm f/2.8-4.5 in all categories. My comments below address image quality in more detail, and are based on a series of images of a resolution test target taken at multiple apertures and focal lengths using an EOS 7D camera. As you can see, they confirm the above results.
Center of frame at 17/18mm
- At the 17-50mm f/2.8 showed higher sharpness and contrast.
- At f/4 the two lenses were closer in performance, though the 17-50mm f/2.8 was still slightly better
- At f/5.6 it was hard to tell the difference between the two lenses
Corner of frame at 17/18mm
- At f/2.8 the 17-50mm f/2.8 showed less vignetting and higher sharpness than the 18-50/2.8-4.5
- At f/4 the sharpness of the 17-50mm f/2.8 improved, but there was not much change for the 18-50mm f/2.8-4.5
- The 17-50/2.8 again increased in sharpness with stopping down to f/5.6 and f/8, while the sharpness of the 18-50mm f/2.8-4.5 did not significantly improve
Center of frame at 28mm
- Wide open, the 17-50mm f/2.8 was sharper than the 18-50mm f/2.8-4.5
- Stopped down the f/5.6 the 18-50mm f/2.8-4.5 showed a significant improvement, though the 17-50mm f/2.8 still gave the sharper image.
- At f/8 the two lenses were very similar
Corner of frame at 28mm
- Wide open the 17-50mm f/2.8 was sharper than the 18-50mm f/2.8-4.5 and showed lower vignetting
- Stopping down to f/5.6 and f/8 improved sharpness in both lenses, but even at f/8 the 17-50mm f/2.8 still produced the sharper image.
Center of frame at 50mm
- Wide open the 17-50mm f/2.8 gave sharper images. The image from the 18-50mm f/2.8-4.5 (at f/4.5) was a little low in contrast.
- Stopping sown improved both lenses (especially the 18-50mm f/2.8-4.5) and by f8 the two lenses were gain similar in sharpness.
Corner of frame at 50mm
- At f/2.8 the 17-50mm f/2.8 image was a little soft, but it sharpened up significantly when the lens was stopped down.
- At f/4.5 (wide open for the 18-50mm f/2.8-4.5) the 17-50mm f/2.8 image was significantly improved over that at f/2.8 and was sharper than that from the 18-50mm f/2.8-4.5.
- Both lenses showed improvement when stopped down further and by f/8 both lenses gave images of similar sharpness.
Real-world example images
To better give you some idea of the difference in image quality the image below contains 100% crops from the edge of the frame at 18mm. It's pretty clear that the more expensive 17-50mm f/2.8 lens yields a sharper image. However even when the image doesn't look so sharp it's important to remember that these crops represent a very small part of the image (approximately 1/400th of the total area). At smaller print sizes, the differences in sharpness might well pass unnoticed by most viewers.
Relative image quality at 18mm at the edge of the frame
It's evident that stopping down from f/2.8 to f/5.6 results in a small improvement, but the more expensive lens is sharper wide open at f/2.8 than the less expensive lens is when stopped down to f/5.6.
The image crops below show the image quality in the center of the same images. At f/2.8 the more expensive lens is sharper and shows higher contrast. However in contrast to the situation at the edge, this time stopping down the less expensive lens to f/5.6 results in a large improvement and an image quality on a par with that of the more expensive lens.
Relative image quality at 18mm in the center of the frame
To summarize, at all focal lengths and all positions in the frame, at wide apertures the 17-50mm f/2.8 outperformed the 18-50mm f/2.8-4.5 in both sharpness and contrast. When stopped down to f8, the two lenses were pretty close in sharpness and contrast at the mid and long end of the range, though the 17-50mm f/2.8 was still clearly sharper in the corners when the lenses were zoomed out to their wideangle settings.
For typical amateur use, where large prints are not required and where the image quality in the center of the frame is most important, the Sigma 18-50/2.8-4.5 is very good value for around $200. The image quality would be good enough for most users, especially if the lens is stopped down a stop or so from maximum aperture. However there's no doubt here that you get a significantly better lens for the extra cost of the Sigma 17-50/2.8. It's clearly the lens of choice for critical professional use and where you need to shoot wide open and maximize image quality all the way out to the edges of the frame.
What about the other brand lenses? Whether you're shooting Nikon, Canon, Sony or another brand lens, its' a pretty safe bet that you'll find similar differences in performance and image quality based on how much you spend. You can research unbiased lab test results and compare different lenses that you're interested in by visting DxOMark.com's lens test database. How much you're willing to compromise on image quality to meet a tight budget—or how much you're willing to pay for the best optics you can afford—is up to you.