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Sony FE 100mm f/2.8 STF GM OSS ( SEL100F28GM ) - Review / Test - Analysis |
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Lens Reviews -
Sony Alpha (Full Format)
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Page 2 of 3
Distortion
The Sony FE 100mm f/2.8 STF GM OSS produces a fairly pronounced degree of pincushion distortions (2.4%) in RAW mode. Given the fact that we are talking about a GM lens, this is a little disappointing. Of course, you can also enable image auto-correction which eliminates the issue.
Move the mouse cursor across the image to view the distortion chart in auto-corrected mode.
Vignetting
One of the "oddities" of the lens is the vignetting characteristic. Either the lens is over-designed or Sony is applying some tricky under the hood. There is no increased vignetting to speak of at f/2.8 (T/5.6) neither in RAW or auto-corrected mode. The remaining light falloff is negligible.
MTF (resolution)
The Sony lens is capable of delivering very sharp results straight from T/5.6. The center quality is outstanding and the outer image field is still good to very good. The peak performance is reached at T/8 where the borders/corners are boosted to very good levels. Diffraction has an impact from T/11 onward. At T/16 the overall quality is still good to very good. However, T/22 (f/20) should be strictly avoided unless really needed.
The centering quality of the tested sample was very good. Field curvature is not an issue.
Please note that we tried to estimate the f-stops of the corresponding T-values. Sony doesn't provide any guidance for this. The reason why the Sony lens shows this variable T-value to f-stop behaviour isn't completely clear. We didn't observe this in other STF lenses that we tested so far. One suggestion is that unlike on other STF lenses, the aperture is further away from the rather extreme APD element which therefore behaves like a secondary aperture. However, we don't really have an explanation that fully convinces us.
Please note that the MTF results are not directly comparable across the different systems!
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 Sony lens produces a fairly low amount of lateral CA (color shadows at harsh contrast transitions) starting with an average CA pixel width of 1.3px at T/5.6 increasing to 1.7px at T/22.
Bokeh
Let's discuss the real purpose of the lens now - the bokeh - thus the quality of the out-of-focus blur.
Out-of-focus highlights are usually rendered as distinctive discs. This is not the case at T/5.6. As you can see below the highlights smoothly dissolve towards their edges (suggesting a very dark outer area of the APD element). This is about as close to perfection as you can get. However, what you can also observe is that the effect is pretty much only visible in full at T/5.6. If you stop down by just 1/3 of an f-stop the effect is almost gone showing the conventional highlight discs again. These highlight discs are still quite nice although they show a bit of an increasing outlining effect the further you stop down. To be fair here - the Sony FE 100mm f/2.8 STF GM OSS is fairly slow thus you will pretty much always prefer to shoot at f/2.8 (T/5.6) anyway.
Just to give you a better idea of what's happening here - for comparison - below are highlights as generated by the Zeiss FE 55mm f/1.8 (which are rather crappy).
Another difference compare to conventional lenses is the quality of the highlights near the borders/corners. Usually you experience a deterioration of the highlight discs towards "cat eyes" whereas the STF lens is still capable of producing these nicely blurred discs here.
Unsurprisingly the general blur quality is also exceptionally smooth in the focus transition zones of both background and foreground. The blur is symmetrical and maintains a high color fidelity.
Bokeh Fringing / Longitudinal Chromatic Aberrations (LoCA)
So-called bokeh fringing is an effect that occurs around the focus point (on the Z-axis). It's visible as halos of different colors in out-of-focus areas - magenta (red + blue) in front of the focus point and green beyond.
The Sony FE 100mm f/2.8 STM GM OSS macro is moderately fast so it's not overly affected by the issue. You can spot traces of this at T/5.6 but they are gone at T/8.
If you traverse through the aperture range below, you will also recognize that the focus remains static (thus no residual spherical aberrations).
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