Tamron's SP AF17-50mm f/2.8 XR Di II VC lens is designed exclusively for cameras with APS-C sized sensors and covers the popular standard zoom range of focal lengths. On Nikon cameras it is equivalent to 26-78mm in 35mm format, while on the Canon EOS 40D body we used for our tests, the equivalent range is 27-80mm. The fast f/2.8 maximum aperture applies across the focal length range.
The SP AF17-50mm f/2.8 XR Di II VC lens. (Source: Tamron.)
The optical design is relatively complex, with 19 elements arranged in 14 groups. Three compound aspheric elements (ASL) are included to reduce the size and weight of the optical system while maintaining imaging performance. In addition, use of XR (Extra Refractive Index) glass has enabled Tamron to ‘optimise the overall optical power distribution' and reduce the size of the lens. This lens also includes two LD (Low Dispersion) elements to correct axial chromatic aberrations and chromatic aberrations due to magnification.
The above diagram shows the position of the various elements in the lens. (Source: Tamron.)
Tamron's proprietary Vibration Compensation (VC) image stabilisation mechanism is also built into the optical path. It's a tri-axial system in which three driving coils move internal elements within the VC lens electromagnetically, based on signals originating from three steel ball bearings. The VC compensating elements are held in place by contact with these bearings and move parallel to the image plane via electronic control.
The above diagram shows the components in the VC stabilisation system. (Source: Tamron.)
The system in this short zoom lens is an adaptation of the VC mechanism in Tamron's high-power zoom lenses, created by reducing the size of the VC unit while increasing its torque. This has enabled Tamron to include stabilisation in such a fast lens without making it significantly bulkier or heavier. Although it is larger and heavier than most manufacturers' standard kit lenses, the larger glass elements required for the faster f/2.8 maximum aperture contribute significantly to increasing the size and weight of this lens.
The latest BBAR (Broad-Band Anti-Reflection) multi-layer coatings have been applied to all the cemented surfaces of lens elements to reduce internal reflections. They also increase light transmission in both short and long wavelengths, improving sharpness, colour reproduction and colour balance.
Build quality is good for the price tag. Most of the barrel is made from high-quality black polycarbonate and the lens has a solid metal mount plus the characteristic gold ring around the middle of the barrel. A petal-shaped lens hood is provided, along with front and rear caps. This lens accepts 72 mm diameter filters.
The minimum focusing distance is 29 cm across the entire zoom range. This provides a maximum magnification of 1:4.8 (equivalent to 0.21x) with the 50mm setting. Although this is not true macro performance, it provides adequate scope for close-ups, particularly with the 50mm focal length setting.
In addition, the fast f/2.8 maximum aperture provides excellent opportunities for sharp, handheld, close-up photography at slow shutter speeds and gives good control over depth-of-field in close-up shots. Together with the VC stabilisation, it also provides a bright, shake-free viewfinder image and improves focus tracking performance.
Handling
Because of its size and weight, the review lens was a comfortable, well-balanced match to the EOS 40D body we used for our tests. It protruded approximately 95 mm from the camera body without the hood. Adding the hood extended the overall length to 142 mm.
The focusing ring is 15 mm wide and located just behind the front of the lens. It carries a 10 mm wide broadly-ridged rubber coating and distance marks in metres and feet are engraved on its trailing edge.
Since it's driven by a micro-motor (rather than ultrasonically), you can hear the motor grinding away as the lens is moved during autofocusing. But, otherwise, focusing is acceptably smooth and fast enough for most shooting situations.
Although internal focusing prevents the front element from rotating, the focus ring itself rotates in AF mode. This can be disconcerting. In manual mode, it turns through a very short arc (roughly a quarter of a turn), which leaves little scope for precise manual focusing.
The 35 mm wide zoom ring is located just behind the gold band. It carries a 28 mm wide ridged, rubberised grip and has four focal length markings (17mm, 24mm, 35mm and 50mm) engraved on its trailing edge. Zooming from the 17mm to the 50mm position requires roughly one quarter of a turn and extends the lens barrel by 32 mm.
Two slider switches are located on the main barrel behind the zoom ring. The upper one is the AF/MF switch, while the lower switches the stabilisation on and off. Stabilisation acts in both vertical and horizontal planes; you can't select one or the other. A little way down from the top of the lens barrel on the opposite side is a sliding lock that can be engaged when the lens is at the 17mm position to prevent unwanted barrel extension when the lens is carried pointing downwards. Interestingly, that problem never occurred while we were testing the review lens.
Performance
Edge softening was very evident at wide aperture settings in our Imatest assessments of the resolution of the review lens. However, resolution in the centre of the frame was well up to the capabilities of the EOS 40D's sensor and stopping down reduced the effects of the softening from about f/5.6 on. The graph below shows the results of our Imatest tests.
Diffraction begins to kick in at around f/8, gradually reducing centre resolution while the differences between centre and edge sharpness are simultaneously diminish. For landscape photographers, this suggests the optimal shooting aperture should be between f/6.3 and f/11 to minimise the effects of edge softening while maintaining a small enough aperture to obtain adequate depth-of-field.
Lateral chromatic aberration was negligible at almost all aperture and focal length settings, sliding into the ‘negligible' band for the two shorter focal length settings. In the graph of our Imatest test below, the red line marks the border between ‘negligible' and ‘low' CA.
No significant coloured fringing could be seen in test shots and backlit subjects were competently handled. The only way to make this lens flare is to point it directly into the sun. Some vignetting could be seen at the widest lens apertures with all focal length settings. Edge darkening was measured at almost one f-stop at f/2.8 but stopping down to f/4 effectively eliminated it.
As expected for a wide-angle lens, we found some barrel distortion in shots taken with the 17mm focal length setting but by 24mm the problem was largely resolved. No pincushion distortion was detected at the 50mm focal length setting.
The review lens delivered attractive bokeh for its specifications and price tag. Out-of-focus blurring was mostly smooth, although some outlining could be seen around the brightest out-of-focus highlights at the widest lens apertures. This had largely disappeared by f/4. The VC stabilisation system was a competent performer, enabling us to use shutter speeds as slow as 1/4 second with the 50mm focal length.
Buy this lens if:
- You want a fast standard zoom lens with good build quality at an affordable price.
- You can't tolerate coloured fringing or lateral chromatic aberration.
- You'd like effective image stabilisation.
- You'd like a lens that requires no readjustment when you fit polarisers and graduated filters.
- You want attractive bokeh at wide apertures.
Don't buy this lens if:
- You want high resolution at all focal lengths, along with good edge-to-edge sharpness.
- You need true macro capabilities.
IMATEST GRAPHS
(based on JPEG files from the Canon EOS 40D)
SAMPLE IMAGES
Vignetting at 17mm f/2.8.
Vignetting at 24mm f/2.8.
Vignetting at 35mm f/2.8.
Vignetting at 50mm f/2.8.
Distortion at 17mm.
Distortion at 50mm.
17mm focal length, 1/125 second at f/5.6, ISO 100.
24mm focal length, 1/166 second at f/6.4, ISO 100.
35mm focal length, 1/197 second at f/6.4, ISO 100.
50mm focal length, 1/250 second at f/7.1, ISO 100.
Close-up at 17mm, 1/100 second at f/4.6, ISO 200.
Close-up at 50mm, 1/83 second at f/4.6, ISO 200.
Bokeh at 50mm, 1/100 second at f/4, ISO 100.
Stabilisation test at 50mm, 1/4 second at f/5.6, ISO 200.
50mm focal length, 1/60 second at f/8, ISO 200.
50mm focal length, 1/790 second at f/2.8, ISO 100.
50mm focal length, 1/500 second at f/4, ISO 100.