Sony 28-75mm f/2.8 SAM (SAL-2875) - Review / Lab Test - Analysis
Lens Reviews - Sony Alpha (Full Format)
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Introduction
Analysis

Distortion

The Sony 28-75mm f/2.8 produces pronounced (~2.6%) barrel distortion at 28mm changing to slight pincushion distortion at 75mm (~1%). At 40mm both forces even out resulting in distortion-free images. A quite decent performance for a full format standard zoom lens actually.

Move the mouse cursor over the focal length text marks below to observe the respective distortions
28mm 40mm 75mm

Vignetting

The Sony lens struggles somewhat regarding very heavy vignetting at max. aperture which peaks beyond ~2EV at 28mm. The falloff is reasonably well controlled at f/4 and good beyond. The issue is somewhat tamed at 40mm but it increases again towards the very long end of the zoom range.

MTF (resolution)

The 28-75mm f/2.8 has a bit of a mixed resolution characteristic. The image quality is weak at f/2.8 - less so in the image center but the borders and especially the corners are soft. The contrast level leaves also something to be desired at and beyond 40mm. The situation improves significantly when stopping down though. The global image quality is already very fine from f/4 at 28mm. At 40mm and 75mm you better stop down to f/5.6. The peak performance is reached around f/8 with a very good quality across the image frame. There's only a marginal amount of field curvature.

Over time we tested quite some 28-75mm samples - two native Sony lenses and two Tamrons. They all showed a varying degree of centering problems. From a potential buyer's perspective this is probably more of a showstopper than the potential image quality.

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)

The lens produces a decent degree of chromatic aberrations (color shadows at harsh contrast transitions). The average CA pixel width at the borders is around 2px at 28mm - this is visible in field conditions but acceptable within the full format scope. Lateral CAs are a lesser issue at 40mm and especially at 75mm.

Bokeh Fringing (Longitudinal Chromatic Aberrations (LoCA))

LoCAs (non-coinciding focal planes of the various colors) are primarily a problem with ultra-fast prime lenses. The Sony may be a comparatively fast zoom lens but it doesn't really run into this issue. You can only spot marginal traces of bokeh fringing at f/2.8 and they're already gone by f/4. This is nothing to worry about.

Move the mouse cursor over the f-stop marks below to observe the respective LoCAs
f/2.8 f/4 f/5.6

Bokeh (out-of-focus blur)

The Sony 28-75mm f/2.8 SAM produces an Okayish bokeh for a zoom lens but it has its flaws. Out-of-focus highlights show a slight onion-like sub-structure and when looking at the image borders, the circular shape deteriorate somewhat to a edgy "cat's eye" shape at large aperture settings.

The general blur quality is quite smooth in the foreground (to the right below) but rough in the background (to the left below). There's also some obvious smearing towards the extreme corners.

Verdict

The Sony 28-75mm f/2.8 SAM is an affordable alternative to the Zeiss ZA 24-70mm f/2.8. Technically it isn't all that far away from its bigger brother - both have a weakness at max. aperture specifically with respect to the corner resolution. However, the Zeiss is generally more snappy here (higher contrast) - this aspect is not exactly a strength of the Sony lens. The contrast receives a boost when stopping down and the lens is capable of delivering very good results across the image frame at medium aperture settings. The light falloff is hefty at f/2.8 but the issue improves rapidly when stopping down a bit. Taking all these issues (contrast, soft corners, vignetting) into account you should better consider it to be a 28-75mm f/4 - starting from f/4 it is actually a very decent lens. The distortion characteristic is pretty good for a standard zoom lens although you will be able to spot some barrel distortion at 28mm. The lateral CAs (color shadows) are a visible albeit not extreme issue in the lower part of the zoom range. Bokeh fringing (LoCA) is basically a non-existent which is quite unusual actually. The quality of the bokeh (out-of-focus blur) is just acceptable but the Zeiss isn't any better actually. The build quality is Ok but nothing to rave about either.

Despite its flaws it is still an interesting lens when taking the quite affordable price into account. That said .. you may as well go for the much cheaper Tamron AF 28-75mm f/2.8 XR which features the same optical design, a similar build quality and a conventional AF rather than Sony's SAM. Based on what we've seen so far you should watch out for centering issues in any case.

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