If you remember my article from 2014 about "Bokeh Dreams", you may have noticed the unique bokeh of the Sony 135/2.8 STF. Now Venus Optics, an ambitious lens manufacturer from China, designed a new "smooth trans focus" potrait lens, the Laowa 105 mm f/2 STF.
STF stands for "Smooth Transition Focus" and was intruduced by Minolta in 1999. It means an optical design with dual diaphragm and an apodization element as shown in this schematic diagram:
Comparing conventional design with smooth trans focus design (source: Venus Optics)
You can think of the apodization element as a kind of inverse center ND filter which darkens from center towards the borders. It sits between the dual diaphragms. The rear one has 9 aperture blades, the front diaphragm is stepless with 14 blades giving a perfectly round shaped aperture.
In conjunction with the apodization element, the dual diaphragm gives this lens the ability to produce an extremely smooth and pleasing bokeh. The lens in Nikon mount version weighs about 785g (without hood and caps) and has about the same size and weight as the Sony STF 138/2.8. For further technical details, please visit http://www.venuslens.net/laowa-105mm-f2-smooth-trans-focus-stf-lens/
In this article, we will compare it to the Sony 135mm f/2.8 (T4.5) STF and the Zeiss Batis 1.8/85 on a Sony A7RII. Another comparison to the Nikon AF DC-NIKKOR 105mm f/2D can be found here.
From Sharpness to...
All candidates provide excellent sharpness right from open aperture, what you can examine when you click on the images in order to open their Flickr page where you can access higher resolutions including full size images. More important, these portrait lenses are almost all about...
Please notice that the three candidates do not have the same focal length but as the only other STF lens available on the market has 135mm, there is not much choice. The third candidate is the Zeiss Batis 1.8/85 which is currently (not only) my favourite portrait lens on the Sony A7RII as well as on the A6300. On the A6300 it gives an equivalent viewing angle to a 135mm full frame lens so most photos taken with the Batis were shot on the Sony A6300.
The lenses were used at different distances to the subject as well as with different distances to the background. As mentioned, you can click on the images in order to access other resolutions:
The following examples show a little more variance in subject sizes as the shooting position had to be changed a bit and in this case the Batis was used without crop on the Sony A7RII:
As you may have seen, the bokeh of the STF lenses is significantly smoother than the bokeh of the Batis 85.
Looking on the night bokeh and the way how punctual light sources are rendered when out of focus, you may consider the influence of the apodization element as the circles of confusion are not neutral, instead they get darker from center to borders:
CAs like color shifting or purple fringing are very well controlled on the Laowa 105, as you can see in these examples:
Laowa 105mm STF @ f/2.0 T3.2 on Sony A7RII
I would put that lens into one league with apocromatic corrected lenses.
- Since 1999, the Minolta / Sony 135mm f/2.8 (T4.5) is the classic STF telephoto lens implementation available only for A-mount and providing an unbeatable smooth bokeh together with outstanding sharpness. It is quite sensitive against flares and contrast reduction when shooting against bright light and with 135mm it is already quite long for a portrait lens. With a transmission of T4.5 it is no low light lens, so you should be careful to have enough light when shooting with short shutter times.
- The Zeiss Batis 1.8/85 is my favourite portrait lens due to its good overall performance. It provides AF (including the most helpful eye-AF) and optical stabilization and for that performance it is quite compact and lightweight. Bokeh is sometimes a little bit harsh and nervous and CAs are less good controlled compared to the STF lenses. Flare resistence and contrast are significantly higher when shooting against bright light due to ZEISS's well known T* coating.
- The Laowa 105 STF delivers a bokeh as smooth and pleasing as the Sony STF together with very good sharpness right from open aperture but with 105 mm focal length it is more flexible than the longer Sony STF. It is also quite sensitive when shooting against the light. With a transmission of T3.2 it provides about one stop more light than the Sony STF helping to keep ISO lower when shooting with short exposure.
The Laowa STF will be available from April 2016 with Nikon F, Canon EF, Sony A and E-mount (covering FE full frame) and from May 2016 with Pentax K mount. You can also choose Sony FE, Fuji X and mFT mount with an adapter (charged additionally). Please see http://www.venuslens.net/product/laowa-105mm-f2-smooth-trans-focus-stf-lens/ for details and inquiries about other dealers.
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