Studio Tests - 35mm full frame (FX)
The Sigma puts in a decent, if not quite outstanding performance on full frame; again it's best at the shorter end of the zoom, but suffers from slight softness, chromatic aberration, and relatively high pincushion distortion at 200mm. However central sharpness is otherwise high, and vignetting notably low. Again we see the same symptoms of focus shift on stopping down as observed on the DX format, resulting in slight under-representation of the lens's true resolution at intermediate apertures. Interestingly, the Sigma does not suffer from the combination of strong vignetting and extreme corner softness at longer focal lengths and that we saw with the Nikon 70-200mm F2.8 AF-S VR on the same body; however it doesn't match its superb central resolution either.
|Sharpness||Sharpness results on full-frame look somewhat similar to APS-C - naturally central sharpness is higher, but this is tempered by a drop-off towards the corners of the frame at F2.8-4. Again best results are obtained towards the wider end of the range, with the lens returning good but not exceptional results at 200mm.|
|Chromatic Aberration||Chromatic aberration is practically absent at 70mm, but increases steadily towards longer focal lengths; it also tends to increase noticeably on stopping down. At 200mm it's as much an issue on full frame as it was on DX, and something of a weak point.|
|Falloff||We consider falloff to become perceptible when the corner illumination falls to more than 1 stop less than the centre. Vignetting is distinctly low for this class of lens, at just 1.3 stops even wide open, and falling to 1 stop at F4; overall an impressive performance.|
|Distortion||Distortion on full frame ranges from 0.8% barrel at 70mm, through neutral point at 85mm, to -1.8% pincushion at 200mm. This final figure is a little high, and could occasionally be an issue in real-world shots.|
Full-frame compared to DX
Eagle-eyed viewers will no doubt have noticed that the MTF50 sharpness data at any particular focal length/aperture combination is distinctly higher on full-frame when compared to DX. This may at first sight appear unexpected, but in fact is an inevitable consequence of our presentation of the sharpness data in terms of line pairs per picture height (and thus independent of format size).
Quite simply, at any given focal length and aperture, the lens will have a fixed MTF50 profile when expressed in terms of line pairs per millimeter. In order to convert to lp/ph, we have to multiply by the sensor height (in mm); as the full-frame sensor is 1.6x larger, MTF50 should therefore be 1.6x higher.
In practice this is an oversimplification; our tests measure system MTF rather than purely lens MTF, and at higher frequencies the camera's anti-aliasing filter will have a significant effect in attenuating the measured MTF50. In addition, our testing procedure involves shooting a chart of fixed size, which therefore requires a closer shooting distance on full frame, and this will also have some influence on the MTF50 data.
Specific image quality issues
As always, our studio tests are backed up by taking hundreds of photographs with the lens across a range of subjects, and examining them in detail. This allows us to confirm our studio observations, and identify any other issues which don't show up in the tests. Our test sample of the lens was in Nikon F mount, and we tested it using an array of camera bodies (Nikon D60, D300, D700, D3).
The complex optical design of 70-200mm F2.8 lenses generally makes them rather susceptible to flare under unfavorable circumstances, and the Sigma is no exception. It's still capable of handling most everyday situations perfectly competently, but can occasionally run into serious problems, especially with strong light sources just outside of the frame.
Our two 'real-world' flare examples show the Sigma behaving in a broadly similar fashion to the Tamron 70-200mm F2.8 Macro. With the sun placed in the corner of the frame at wideangle, the image shows a diffuse flare spot pattern running diagonally across the frame. In contrast, with the sum placed just outside the field of view at 200mm, large areas of the image can be covered in bright veiling flare, essentially making the shot unusable. However once again it's important to bear in mind that these examples are somewhat manufactured 'torture tests', and are not exactly typical of common shooting situations.
|70mm F16, Nikon D3||200mm F2.8, Nikon D3|
Background Blur ('bokeh')
One genuinely desirable, but difficult to measure aspect of a lens's performance is the ability to deliver smoothly blurred out-of-focus regions when trying to isolate a subject from the background, generally when using a long focal length and large aperture. The Sigma produces smooth and attractive bokeh for macro shots, but is also prone to giving unsightly red/cyan fringing around slightly out-of-focus regions. Once the subject distance increases, the background can also become somewhat 'busy' and distracting. Overall, the Sigma isn't too bad here, but neither is it best in class in this regard.
|200mm F4, Nikon D3||200mm F2.8, Nikon D3|
|50% crop, upper right||50% crop, upper right|
This lens is different to the previous 70-200mm zooms we've tested, which have all been optimized to give near-zero chromatic aberration in the middle of the range, with low levels of red/cyan fringing visible at 70mm and 200mm. However Sigma have taken a different approach, and minimized CA at 70mm instead. Fringing then increases steadily in magnitude through to 200mm, and at this point it can be distinctly visible on both DX and FX formats at high contrast edges (black on white, as shown here, it typically the worst-case scenario). There's good news for users of high-end Nikons though; the automatic CA correction of the D3, D700 and D300 works well with this lens, leaving practically no visible fringing in camera JPEGs.
|200mm F8, Nikon D3 (RAW + ACR)||200mm F2.8, Nikon D300 (RAW + ACR)|
|100% crop, top left corner||100% crop, top right|
The original Sigma 70-200mm F2.8 EX design started life with a minimum focus distance of 1.8m, which was reduced dramatically to 1m with the launch of the 'Macro' version. This latest 'Macro II' iteration retains that 1m minimum focus distance, but features a modified layout, supposedly to improve performance in the close-focus range. We were impressed with the Tamron 70-200mm F2.8's macro capability, and so had high hopes for the Sigma; unfortunately these haven't quite been realized. Instead the lens suffers from some major issues limit its usefulness for closeups.
Firstly, sharpness suffers significantly at close distances; realistically you need to stop down to at least F5.6 to get any kind of detail. Secondly, the lens suffers from significant, and very unsightly 'bokeh chromatic aberration', which means that regions just ahead of, and behind the field of focus can be surrounded by strong red and cyan fringing (and unlike lateral CA, this can't be easily corrected in software). Finally, and most problematically, the lens suffers from a significant focus shift on stopping down; we measured this as approximately 1.5mm away from the camera per F-stop at 200mm and closest focus, which means that if you focus critically with the lens wide open then stop down to F11, the point of sharpest focus will shift a full 6mm away from the camera. This may not sound much, but in macro terms (where the entire depth of field can be just a few millimeters), it's an awful lot.
The set of images below illustrate these issues, using 50% crops from the centre of the frame, at 200mm and around 1.4m focus distance, using the Nikon D300 body (i.e. DX format). The F2.8 shot is generally soft, but sharpest focus is on the match head third from the front of the frame; the matchsticks towards the top left of the image (i.e. behind the field of focus) show cyan bokeh CA, whilst those towards the lower right show red bokeh CA. The F5.6 shot shows that the image has sharpened up considerably, but focus has shifted noticeably backwards, and bokeh CAs are still clearly visible. Finally the F11 shot shows the focus shifting further, with the fifth match head now the sharpest; and the original focus point is right at the front of the overall depth of field.
|200mm F2.8||200mm F5.6||200mm F11|
In general use, bokeh CAs can be genuinely problematic, especially when shooting at a long focal length and large aperture to isolate the subject from the background. The sample below illustrate this clearly; the in-focus region shows little CA, but is distinctly soft and lacking detail, however regions in front of the focus field show strong red fringing, whereas those behind the focus field are surrounded by cyan halos; both are highly visible against the green background.
|200mm F2.8, Nikon D300||100% crop, in-focus region|
|100% crop, red bokeh CA||100% crop, blue bokeh CA|
The practical consequence of this behaviour on macro shooting is that it's advisable to focus towards the front of the desired depth of field (as opposed to the centre, which is usually correct at higher magnifications), check the effect carefully using depth of field preview or live view if possible, and confirm the results in image review. It's also best to stop down a bit for better sharpness and to minimize bokeh CAs, although this obviously negates the subject isolation benefits of shooting with a fast lens at a wide aperture.