Sigma AF 24-70mm f/2.8 EX DG HSM (FX) - Review / Test Report - Analysis
Lens Reviews - Nikon / Nikkor (full format)

Distortion

At 24 mm the lens shows a significant amount of barrel distortion (2.68%), which flips over to the pincushion type at 70 mm and with almost 1.3 % can be an issue with certain subjects. At 40 mm the distortion is down to 0.2%, which can be considered distortion-free for most photographic applications. Especially at the wide end the amount of distortion is nothing to rave about, but ok for this lens class.

Move the mouse cursor over the focal length text marks below to observe the respective distortion
24mm 40mm 70mm

The chart above has a real-world size of about 120x80cm.

Vignetting

The lens suffers from quite visible vignetting at 24 mm. Wide open the light falloff is very pronounced and can be decreased by stopping down, surprisingly though even stopped down to f/8 vignetting remains on a rather high level that still can be field relevant. Things look similar, but in summary better at 70mm: very visible vignetting wide open, stopping down helps to decrease the amount considerably. At 40 mm, vignetting is basically no issue, except maybe wide open.

We're performing our vignetting analysis based on (uncorrected) JPEGs straight from the camera. The JPG engine of the Nikon D3x features a rather flat gradation curve, thus has a moderate contrast characteristic, resulting in comparatively low vignetting figures - the corresponding Canon figures are roughly 40% higher due to the more aggressive default contrast setting.

MTF (resolution)

The lens delivered mixed resolution figures on the D3x. The tested sample suffered from a minor decentering issue. The Sigma service was not able to resolve the issue (despite mentioning that this lens would be used for testing) which is why we decided to go on with the test. A perfectly centered sample may show slightly higher MTF numbers with large apertures, especially at the long end of the zoom range.

At all focal lengths the lens is capable to deliver excellent sharpness in the image center when stopped down to f/4 (at 24mm) or f/5.6 (40 and 70mm). Wide open, the lens unfortunately shows significantly less resolution in the image center, less so at 24 mm but very visible at 40 and 70 mm.

Things look a lot worse regarding the image borders and corners. At 24 mm, the border resolution is only fair wide open, but increases to very good values by f/5.6. The extrem corners show even less performance, with poor values wide open, fair at f/4 and f/5.6 and only stopped down to f/8 the resolution reaches good values.

At 40 mm, the lens performs slightly better in the borders and corners, at least from f/4 onwards. Wide open, though, both borders and corners show rather low resolution figures.

Border and corner sharpness drops again at 70 mm, very low values wide open, but unlike at 40 mm stopping down does not gain the same amount of sharpness, only at f/11 the border performance reaches very good territory, in the usual working range of such a lens the results are just fair to good.

Below is a simplified summary of the formal findings. The chart shows line widths per picture height (LW/PH) which can be taken as a measure for sharpness. If you want to know more about the MTF50 figures you may check out the corresponding Imatest Explanations

Chromatic Aberrations (CAs)

Chromatic aberrations (color shadows at harsh contrast transitions) are visible at all focal lengths, with a little higher values at 24 mm. Stopping down does not help to get rid of CAs, however in summary the amount is moderate.
In addition, CAs can easily be corrected in software or by the camera itself (most modern Nikon DSLRs remove CAs themselves if you shoot JPGs).

Bokeh

A standard zoom is not primarily targeted a portraits, however being a fast lens it allows for good subject separation and thus the quality of out-of-focus blur is certainly of interest. We looked at bokeh with our standard test scene at 70mm focal length, which is closest to classic head to shoulder portrait lenses. Field tests showed that the lens delivers very similar bokeh at all focal lengths, though.

Good news first: the blur in front of the focal plane is very smooth. Unfortunately things look a lot worse behind the focal plane, which is what most photographers a more interested in. As you can see below, the blur is rather nervous and produces ugly outlines and halos.

Thanks to 9 rounded aperture blades, background highlights remain their circular shape throughout the whole aperture range. However, they show significant outlining. This may not seem too critical in the test scene below, however if you take a look at the crops of images from the field below, you get an impression of what to expect using this lens wide open.

The only way to get very smooth bokeh with this lens is to make sure you have a massive distance between your main subject and the background. In summary, this is certainly disappointing.

Bokeh Fringing / Longitudinal Chromatic Aberrations (LoCA)

LoCAs (non-coinciding focal planes of the various colors) are a common issue with relatively fast glass. As you can notice below the halos have different colors - magenta (red + blue) in front the focus point and green beyond. Truly "apochromatic" lenses don't show LoCAs but these lenses are very rare especially below 100mm. Unlike lateral lateral CAs, LoCAs cannot easily be fixed in post processing.

At it longest focal length of 70 mm (which we used here) the lens shows a low amount of LoCAs wide open, which is considerably reduced by f/4 already and no longer field relevant at f/5.6 and beyond.

Move the mouse cursor over the f-stop marks below to observe the respective LoCAs
f/2.8 f/4.0 f/5.6 f/8.0 f/11.0




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