A Viewfinder Darkly

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Three Way Standard Zoom Lens Review

August 30 2011

by Philip Northeast

A three way lens review of standard zoom lenses explores a couple of popular misconceptions about Digital SLR kits lens quality.

DSLR kit lenses do not have a very high reputation for image quality with some new and potential DSLR photographers. In this lens review we compare test results from DxOMark for a typical kit lens with two contenders from specialist lens manufacturers. This also explores the contradictory myth common on internet forums to only use the same brand lenses as the Digital SLR camera body.

Tamron 17-50mm zoom

The Digital SLR 18-55mm zoom kit lens is common on cameras with an APS-C size digital sensors and offers focal lengths ranging from wide angle for scenery and group portraits, through to mild telephoto for individual portraits.

In this test we have Nikon 18-55mm zoom, often sold with their entry level DLSRs, up against a Sigma 17-50mm and Tamron 17-50mm zooms.

DxOMark tests analyse the RAW file produced by the digital sensor in the camera body.
In this lens review all the lenses were tested on a the same model camera, in this case a Nikon D300s APS-C DSLR, so any differences in overall performance are due to the lens.

DxOMark test score summary

Initially the DxOMark Score suggests it as a no contest, with the Sigma and Tamron lenses showing a clear advantage in the overall score.

However, lens performance varies with aperture and focal length. Lenses work their best at an intermediate aperture rather than wide open (small f number), or at very small apertures (large f number). To take account of these variations the DxOMark conduct their tests over a range of settings. Analysing the detailed results reveals the settings where there are shortcomings in the performance of each lens.

Distortion

This is a measure of the lenses ability to reproduce straight lines, often called barrel distortion. The DxOMark results show a grid of straight lines, where there is lens distortion it  shows as curves in the lines.

Distortion measured at shortest focal length

All the lenses show some degree of distortion at their shortest focal length in the results above.This is typical for wide angle lenses on DSLRs with APS-C size sensors.

Distortion measured at a focal length of 35mm

Zooming the lenses out to a focal length of 35mm reduces the distortion  dramatically.

Chromic Aberration

This manifests itself as coloured fringing around the edge of objects, particularly in the corners of photos.

Chromic Aberration with aperture wide open at the shortest focal lengths

All three lenses are affected by a degree of Chromic Aberration at  short focal lengths and wide apertures. The Sigma has less obvious yellow areas in the results display indicating mild Chromic Aberration, while the Tamron’s pronounced yellow areas suggest widespread Chromic Aberration. The Nikon has a much cleaner image centre with the Chromic Aberration closer to the corners than the other two. There are tinges of red in the extreme corners suggesting high levels of Chromic Aberration in this area for the Nikon.

Chromic Aberration with the lenses stopped down to f8 aperture

When we stop the lens down to f8 the Chromic Aberration in the Nikon 18-55 becomes more widespread throughout the photo. The Tamron 17-50mm zoom  improves to now have less Chromic Aberration than the Sigma 17-50mm zoom.

Chromic Aberration with aperture of f8 and focal length of 35mm

Finally, increasing the focal length to 35mm reduces the amount of Chromic Aberration in all three lenses with the Tamron having a much deeper green all over the image, while the Sigma and the Nikon have a yellow tinge over much of the image area. This suggests widespread, but low level, Chromic Aberration.

Resolution

This how well the lens can distinguish fine lines to give a measure of the detail it can show.Resolution measured in the corners of the images

This graphic shows the resolution of the three lenses in the corners of the photos, they do perform better in the centre of photos.  Left to right shows the resolution as focal length changes. More dramatically, up and down shows the difference due to changing the lens aperture.  The Tamron is again the best performer with a very distinct best area for apertures from f5.6 to f11. This is also the best area for the Sigma and the Nikon..

Vignetting

This is a darkening of the image in the corners.

Darkening of the image corners with a wide aperture

The first DxOMark results graphic shows darkening in the corners of the photo at the shortest focal length and widest aperture.

Less corner darkening with a smaller aperture

The second shows the effect of using a smaller aperture, in this case f8, to minimise vignetting. Here the Nikon has a clear advantage with no discernible vignetting.

Transmission

Here DxOMark show the amount of light coming in through the lens and how it corresponds to the maximum aperture of the lens.  The Sigma and Tamron lenses are clear leaders with their wide maximum aperture of f2.8 over the range of focal lengths. The Nikon has a  best of only f3.5 and this goes down to f5.6 as the focal length increases, but it does not directly affect photo quality.

Summary

The Sigma 17-50mm EX zoom and the Tamron 17-50mm SP zoom are part of both manufacturer’s higher performance range of lenses, indicated by the EX and SP in their full model names. This is reflected both in the higher price and overall higher DxOMark score than the Nikon kit lens.

The Nikon 18-55mm zoom struggles slightly at short focal lengths and wide apertures. At smaller apertures of around f8 and the most of its focal length range it is a competent performer. One of the benefits of the DxOMark site is photographers can find the sweet spot for their lens.

This is where expert photographers work magic with modest equipment, they only operate it in the areas where it produces good photos. They avoid settings, or situations,  where the camera or lens are unsuitable..

Sigma 17-50mm F2.8 EX DC OS HSM Tamron SP AF 17-50mm F/2.8 XR Di II LD Aspherical [IF] Nikkor AF-S DX Nikkor 18-55mm f/3.5-5.6G VR
Aperture f/2.8 Aperture f/2.8 Aperture f/3.5-5.6
Focal range (mm) 17 – 50 Focal range (mm) 17 – 50 Focal range (mm) 18 – 55
Filter diameter (mm) 77 Filter diameter (mm) 67 Filter diameter (mm) 52
Max diameter (mm) 83.5 Max diameter (mm) 73.8 Max diameter (mm) 73
Stabilization Yes Stabilization No Stabilization Yes
AF Motor HSM AF Motor No AF Motor SWM
Zoom type Ring Zoom type Ring Zoom type Ring
Rotating front element No Rotating front element No Rotating front element Yes
Number of lenses 17 Number of lenses 16 Number of lenses 11
Number of groups 13 Number of groups 13 Number of groups 8
Diaphragm blades 7 Diaphragm blades 7 Diaphragm blades 7
Circular aperture No Circular aperture yes Circular aperture Yes
Length (mm) 91.8 Length (mm) 83.2 Length (mm) 79.5
Weight (gr) 565 Weight (gr) 430 Weight (gr) 265

The inherent problems of all the lenses are mainly when the focal length is at the wide angle end of their zoom range and they are at wider apertures. Photographic equipment is always a matter of compromise where gains in one area usually detrimentally effect another aspect of the equipment.

The Sigma and the Tamron lenses are better optical performers over the whole range than the Nikon, but they are larger, heavier and more expensive. If a photographer only rarely shoot extreme wide angles at wide apertures then the Nikon kit lens is very good value for money.

What this does show is photographers can get improved overall optical performance from specialist lens manufacturers instead of relying on keeping the  same brand name on the camera and the lens.

There are other aspects of a lens to consider, such as auto focusing and shake reduction systems. Ruggedness and weather sealing are important for photographers who venture into harsh conditions.

DxO Can Help

Sophisticated photo processing software such asAdobe Lightroom or DxO Optics Pro can compensate for the limitations in lens performance identified in the DxOMark tests.

Prices

Sigma 17-50mm F2.8 EX DC OS HSM  at   Adorama $US699

Tamron SP AF 17-50mm F/2.8 XR Di II LD Aspherical [IF] at Adorama $US434

Tamrom with Vibration Reduction  at Adorama $US599

Nikkor AF-S DX Nikkor 18-55mm f/3.5-5.6G VR  at Adorama $US175

 

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