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Sony 135mm f/2.8[T4.5] STF Lens Review

by Bob Atkins, May 2008


The Sony 135mm f/2.8 STF is a unique lens. The "STF" in the name stands for "Smooth Transition Focus" and is descriptive of the way this lens renders out of focus areas. The lens is designed to give the most pleasing possible "bokeh", a term used to describe both the quantity and quality of the blurred areas of an image.

The unique aspect of the lens is that it contains an apodizing element. Apodization is a fancy way of saying "changing the shape" of something, such as a mathematical function or, in this case, the shape of the aperture stop (iris) of a lens.

Just about all lenses use an aperture stop with a sharp edge, basically a disk with a hole in it. When the lens is stopped down from maximum, the hole typically takes the shape of a regular polygon (e.g., it's a pentagon if 5 aperture blades are used, a hexagon with 6 blades, and so on).

The STF lens uses an apodizing element, which acts rather like a radially graduated neutral density filter, dark at the edges and clear in the center. This apodizing element is shown in gray in the diagram on the left.

One element of the group, which sits right behind the aperture stop is made from an optically absorbing glass. This element is thick at the edges and thin at the center so the light passing through the center is hardly attenuated, while light passing through the edge regions sees significant absorption. This lens component "apodizes" the aperture from a hard edge as would be the case for a conventional lens to a soft edge.

You might wonder why the lens has the strange designation "135/2.8[T4.5]". This is because although the lens has the geometry of an f/2.8 lens, the apodizing element absorbs light so the exposure required is that of an f/4.5 lens. "T" stops the measure of light transmission. Therefore, although this is an f/2.8 lens because the focal length is 2.8x the physical aperture, it's a T4.5 lens because of the light absorption by the apodizing element.

The set of illustrations below show what the aperture of a conventional lens looks like wide open (left) and stopped down using a 5-blade aperture (center). On the right is an illustration of what the aperture of the STF lens looks like. There's a gradual fading in towards the center.

The effect of the apodized aperture is that it changes how the out of focus areas look. This is shown quite dramatically in the out of focus images of a distant street light as shown below:

On the left is the image produced by the STF lens. On the right is the image produced by a conventional lens. These are actual images not simulations. Below is a comparison of the out of focus blur of the 135 STF with 3 similar conventional lenses.

For the above images I took three photos with different conventional 135/2.8 lenses mounted on a Canon EOS 40D, (compare prices) (review). I didn't have the ability to mount them on the Sony Alpha A700, (compare prices) (review), which I used with the Sony 135/2.8[T4.5] STF, but that doesn't really present a problem as the images would have been very similar on the A700. The conventional lenses were a Pentcon 135/2.8, a Hanimax 135/2.8 and a Fujinon 135/2.8, but I don't remember which image was captured with which lens, nor does it really matter. All four lenses were first prefocused at the same close distance (about 2m), then used to photograph a distant barn. It think it's quite clear that the STF lens gives a much smoother image than the conventional lenses.

Operation

The 135 STF is a little unusual in the ways the aperture can be set. There are actually two separate apertures, one manually controlled and one controlled by the camera. For fully automatic operation the aperture ring (see illustration on left) is set to the green "A" symbol. In this mode the lens acts conventionally with the camera stopping down to the aperture set on the camera. It uses 9 blades to give a fairly smooth circular shape.

In manual mode, the aperture can be set between T4.5 (wide open) and T6.7. In this mode the aperture used has 10 blades and gives a slightly smoother circular shape than the 9-blade auto aperture. Though both the auto aperture and manual aperture settings have the same apodized aperture, the slightly more round shape of the manual aperture gives slightly smoother out of focus areas. The manual settings only go to T6.7 because the more you stop down, less of the apodizing element is used. Past about T6.7 the apodizing element has less effect because only the central (clear) part is used.

The Sony 135/2.8[T4.5] STF is a manual focus lens. This is a source of some problems on a Sony DSLR such as the Alpha A700 because there is no indication of focus. The camera can't focus the lens and it doesn't give any signal when the image is in focus. The viewfinder screen has no focusing aids as manual focus SLRs usually had. Since this lens is used to best advantage at full aperture, where the depth of field is at its least, this means that getting exact focus can be a bit of a "hit and miss" proposition unless you have the camera mounted on a tripod and you have time to spend getting the focus right. A viewfinder magnifier can help, though again it's tough to use one with a handheld camera. When I used this lens hand held, wide open, for portrait work, I missed focus on a significant number of the photos. I'd assess my manual focus ability with DSLRs as being above average based on my experience with manual focus lenses on my EOS 40D, so I don't think it's just me that's causing the problem.

Optical Performance

I only had the opportunity to test the 135 STF lens on a Sony Alpha A700, which is an APS sensor DSLR, but on that camera the optical performance was very good indeed. Even wide open at f/2.8 (T4.5) the image was sharp from center to edge with no visible vignetting. I can't comment on full frame corner performance, but I strongly suspect it will be good. Stopping down one stop gave a barely measureable increase in resolution, which is a testament to how good the performance wide open is. Looking very closely at the edge of the image I could see just a hint of chromatic aberration, but it was only about 1 pixel wide and that's excellent performance.

The lack of vignetting is probably due to the use of oversized optics. The 135 STF takes 72mm filters, while many other 135/2.8 lenses only need a 55mm filter.

Alternatives

Sony has two 135mm prime lenses, the Sony 135mm f/2.8, (compare prices) (review), and the Sony 135mm f/1.8 Carl Zeiss, (compare prices). The advantage of the STF lens is the smooth out of focus backgrounds it can create. However, the faster speed (f/1.8 is about 2.6 stops faster then T4.5) and autofocus abilities of the 135/1.8 light be of more use to a majority of users than STF.

Sony doesn't make a lower-cost conventional 135/2.8 lens, but they do have an autofocus 100mm f/2.8 Macro, which on an APS DSLR may be a more practical focal length for portraits and the price ($565) is more reasonable. Sigma has a similar lens (Sigma 105mm f/2.8 EX DG Macro) for around $400.

There was a Minolta AF 135 f/2.8 but it hasn't been carried over into the Sony lens lineup. You may find one used and if you do you can probably expect to pay around $250-300 for it. By all accounts it's a very good lens and is much smaller (55mm filters) and lighter then the 135 STF, as well as being much cheaper and has autofocus! Of course it doesn't have the blur quality of the STF.

There's also a very nice Carl Zeiss Planar T* 85mm f/1.4, which would make an excellent portrait lens. However, the price is still pretty steep at around $1300.

Conclusion

The Sony 135 f/2.8 [T4.5] STF SAL-135F28 is a unique lens, which produces a very soft and smooth background blur and for some people that might be enough to justify purchase of the lens. However, it has some downsides. As a portrait lens it's a bit long on APS DSLRs (and at the time of writing Sony does not have a full frame DSLR). That together with the lack of autofocus, the lack of any electronic focus indication when manually focused, and the relatively high price of $1200 may make the the lens less desirable to many. Technically, I can't really fault the lens, but I'm not sure it's all that practical for most Sony DSLR users.

Where to Buy

The 135/2.8[T4.5] STF is available from amazon.com. It's normally in stock and overnight shipping is available.

135/2.8 [T4.5] STF Specifications

Focal length 135mm (202.5mm full frame equivalent when used on APS-C DSLR)
Filter Size 72mm
f-stop range f/2.8 [T4.5] - f/32
Aperture 9/10 blades (circular)
Focusing floating elements
Minimum Focus distance 870mm
Maximum magnification 1:4 (0.25x)
Groups/Elements 6/8
Length 99mm (minimum)
Weight 730g

Further Reading

More

Sony 135/2.8[T4.5] STF Sample Images

Sony Alpha A700 (review), f/2.8[T4.5], 1/200s, ISO 200.

Sony Alpha A700, f/2.8[T4.5], 1/500s, ISO 200.

Sony Alpha A700, f/2.8[T4.5], 1/640s, ISO 200.

Sony Alpha A700, f/2.8[T4.5], 1/1000s, ISO 200.


Original text and images ©2008 Bob Atkins

Article created May 2008

Readers' Comments


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Lex Jenkins , December 01, 2009; 12:13 A.M.

For years I've been advising bokeh-obsessed n00bs to find the bokeh insertion valves on their lenses and refill them with fresh bokeh. Little did I suspect that any lens maker would take it seriously. The apocalypse is nigh.


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