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).
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
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.
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.
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.
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.
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.