Sigma AF 120-400mm f/4.5-5.6 DG HSM OS - Full Format Review / Lab Test Report - Analysis
Lens Reviews - Canon EOS (Full Format)
Article Index
Introduction
Analysis

Distortion

The Sigma lens produces a very slight degree of pincushion distortion throughout the entire zoom range. This is excellent for such a lens.

Move the mouse cursor over the focal length text marks below to observe the respective distortion
120mm 200mm 300mm 400mm

Vignetting

Vignetting is a critical topic for most lenses when used on full format DSLRs but it's not an overly significant issue on the Sigma lens. The problem is, of course, still visible at ~0.9-1.2EV at max. aperture but it remains comparatively moderate. It is usually sufficient to close the aperture by 1-1.5 f-stops to reduce the issue to a non-relevant degree.

MTF (resolution)

The Sigma lens delivered a rather mixed performance is our MTF lab. Normally you'd expect the sweet spot to be at the short end of the range but things are not all that great here. The center performance is on a very good level but the borders/corners are soft at f/4.5 and f/5.6. The border quality is good at f/8 but you should stop down to f/11 for a decent performance across the image frame. The situation is fairly similar at 200mm although it is sufficient to stop down to f/8 for a relatively even performance here. Moving towards the long end of the range we can observe that the center resolution drops at f/5.6 and the contrast could also be better here. However, the border/corner resolution is on a good+ level at all medium aperture settings. The center quality can be boosted to very good levels by stopping down to f/8.

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

Lateral CAs are generally very well controlled - the SLD elements used in the optical design seem to be pretty effective reducing this aberration. An average CA width of less than ~1px at the image borders it is usually nothing to worry about in field conditions.

Bokeh

Ultra-long tele zoom lenses tend to have a rather mediocre bokeh and the Sigma doesn't shine here either. Out-of-focus highlights have a nervous "onion-like" inner zone. The highlight disc are circular in the center but they deteriorate towards the usual "cat's eye" in the border/corner image portions. Stopping down improves the border/corner discs to a more circular shape but the aperture edges get more obvious.

Out-of-focus objects in the foreground (left sample crop below) have a rather nervous rending whereas the background blur (right sample below) is more smooth.

Verdict

The Sigma AF 120-400mm f/4.5-5.6 DG HSM OS is a consumer grade tele zoom lens with a relatively decent albeit not spectacular performance. The resolution figures are generally fine in the image center whereas the borders/corners are just soso in the lower part of the zoom range. That said the border/corner performance is, of course, not overly relevant for such a lens so you may forgive this weakness. The contrast level is somewhat reduced beyond 300mm @ f/5.6 but stopping down helps to get more punchy colors (although it will never reach the contrast level achievable with long prime lenses). Distortions are basically a non-issue in field conditions. There's some light falloff at max. aperture but the issue is not overly extreme and mostly gone when stopping down a little. The quality of the bokeh is rather mediocre but this is quite common in this lens class.
The build quality is very good - everything is tightly assembled and most parts are made of metal. This is certainly something that you won't really expect from a consumer-grade lens. Sigma came a long way in terms of AF performance but the HSM ("Hypersonic Motor") AF is both very fast and silent and basically as good as the AF on the Canon EF 100-400mm f/4.5-5.6 USM L IS. The OS ("Optical Image Stabilizer") is pretty efficient but comparatively noisy. All-in-all the Sigma AF 120-400mm f/4.5-5.6 DG HSM OS may not be able to touch the stars optically but you're still getting lots of bangs for your bucks here.

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