Sony 135mm f/2.8 [T4.5] STF ( SAL-135F28 ) - Full Format Review / Lab Test - Analysis
Lens Reviews - Sony Alpha (Full Format)
Article Index
Introduction
Analysis

Distortion

The Sony lens produces mild pincushion distortion of ~0.5%. This is usually negligible.

Vignetting

Full format lenses tend to produce quite a bit of vignetting on full format DSLRs. However, this does not apply to the STF lens! This was quite surprising for us so we repeated the test but with the same results. The amount of vignetting is absolutely irrelevant even at max. aperture. This is a unique characteristic!

MTF (resolution)

The lens produced very impressive figures in the MTF lab. The center quality is already exceptionally high at T/4.5 (f/2.8) whereas the border/corner performance is "just" good to very good. Stopping down to T/5.6 or T/8 increases primarily the border/corner quality although the center receives a slight boost as well. Beyond f/11 diffraction is the limiting factor.

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)

Lateral chromatic aberrations (color shadows at harsh contrast transitions) are very well controlled with an average CA-pixel width (@ the borders) around 0.3px. This is usually nothing to worry about and, surprisingly, not any higher than in the APS-C format scope.

Purple Fringing

At max. aperture the lens is slightly prone to produce purple fringing in extreme contrast situations. This is not unusual for large aperture lenses but it can be disturbing at times.

Longitudinal (Axial) Chromatic Aberrations (LoCA)

LoCAs (non-coinciding focal planes of the various colors), sometimes called "bokeh CAs", can be a visible in certain situations. You may notice the slightly magenta-to-green color shift in the foreground and background of the sample crops below. The effect is gone by T/8.

Move the mouse cursor over the f-stop marks below to observe the respective LoCAs
T/4.5 T/5.6 T/8

Bokeh

Due to time constraints we weren't really able to look into the full format behavior but the APS-C results should remain intact except maybe for a slight deterioration of the highlight shapes towards the corners of the full format image circle.

As mentioned the design of the lens has been optimized to achieve a silky bokeh (out-of-focus blur) and the STF does not disappoint here. At T/4.5 (f/2.8) the bokeh is smoother than on any other lens tested to date. As expected the effect deteriorates somewhat when stopping down but usually you will prefer to use the max. aperture anyway.

Verdict

The Sony 135mm f/2.8 [T/4.5] STF remains the ultimate cream machine - also in its native full format scope. Regarding the quality of its bokeh (out-of-focus blur) it outclasses all other lenses tested to date so if this is a #1 priority for you just bow to the inevitable and go for it to enjoy images as smooth as silk. That said it doesn't stop here. The lens provides superb results on APS-C DSLRs but it also performs impressively within its native full format. The center quality is excellent at max. aperture and the borders/corners are certainly "sharp enough" here. At T/5.6 the results are very impressive across the image frame. Neither vignetting, distortion nor lateral CAs are something to worry about in field conditions. It is not a perfect lens though - it has its share of "bokeh CAs" (Longitudinal (Axial) Chromatic Aberrations) and it can produce purple fringing in extreme contrast situations. These are minor aspects from a global perspective though. A more significant issue may be the handling of the lens. It is a manual focus lens and you don't even have a focus guidance via the focus indicator in the viewfinder. Due to the special apodisation design the lens "loses" 1 1/2 f-stops (max. T/4.5 @ f/2.8) so it may not be the right lens for available-light photography (although super-steady-shot should help here). Regarding the sum of its (special) qualities the verdict can only be "Highly recommended"!

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