by mladen sever, January 2007

Although I consider myself lucky to have spent the last few weeks using one of the most exotic lens available to DSLR users, I must admit parting ways with the Sony 135 F2.8 [T4.5] STF SAL-135F28 has left me with mixed feelings. This uncommon and underrated lens exists in a world of it’s own, waiting to be discovered by those who are willing to make a sacrifice to behold what I can only describe as the "perfect" bokeh tool.

The history of Minolta cannot be discussed without the phrase "bad marketing" being uttered. It surprises me that Sony is following a similar path in the marketing of the Minolta 135 F2.8 [4.5] . Although it is certainly encouraging to see Sony reintroduce this lens, it is disappointing that Sony hasn’t drummed up any enthusiasm about their most exotic glass. Sadly, the lens remains relatively unknown among professional photographers and Minolta/Sony DSLR users alike. When I described the STF lens to my colleagues (experienced Canon and Nikon shooters) they were stunned to hear that such a "beast" existed, and even more so that it is made by Sony.

Before reading this article I recommend you visiting interesting links I have found on this (or original Minolta) lens. If you are aware of additional links about this lens please let me know.

Unique design features

This lens is designed and made with one goal in mind: To render the out-of-focus areas of an image (widely called bokeh) as smoothly and aesthetically pleasing as possible. To achieve this goal Minolta engineers have used a special "apodization element" near the main aperture.

For some of you, the explanation provided in the product manual may sufficiently explain how this works:

A special "apodization element" is situated near the aperture of the lens optical system. This special optical element is a type of ND filter which gradually becomes thicker (darker) towards the perimeter, thereby reducing the amount of light that passes through around the outer perimeter. The aperture of this lens is indicated and controlled by the T No., which compensates for the reduction imposed by the special apodization element. The T No. can be used as the F No. on a normal lens when the exposure is determined. Defocused images appear as blurred clusters of dots. A lens with well compensated aberration reproduces the image shape accurately, but cannot produce smooth blurs in a defocused area. There may be unpleasant blurs, which for example, make one line appear to be doubled, depending on the way of compensation. This lens adopts a special apodization element that provides a gradually diffused image toward the perimeter without losing the core shape. In other words, it provides soft and natural defocusing without transforming the original shape unnaturally.

If you followed my suggestion and visited the links mentioned above, you already know that this lens has two sets of aperture blades and the manual aperture ring on the lens allows you to select the appropriate aperture diameter. Again, the Sony 135STF lens manual may help you to better understand this two-aperture design:

This lens has two aperture settings; A position and a stepless aperture control. Use the aperture ring to switch between the two settings. T No. is used in both notation and control on the camera and lens. This lens has two apertures; the stepless aperture, manually controlled with the aperture ring, and the automatic aperture (A position), controlled by the camera. To use the stepless aperture setting This setting allows stepless control of the aperture between T/4.5 - T/6.7. Turn the aperture ring to set the desired aperture. This stepless range is recommended when a large aperture is desired.

Set your camera to A mode or M mode when using this setting. (In P mode or scene-selection setting, the settings will be the same as A mode. In S mode, the settings will be the same as M mode.)

A slight click can be felt at the T/4.5, T/5.6, and T/6.7 marks.

The index marks between T/4.5 and T/5.6 represent 1/3 aperture values.

Stop-down metering is used when the stepless aperture is selected.

When shooting, the aperture values set by the aperture ring are used. Those values are not accurately reflected in the values displayed or recorded by the camera. To use the A position This setting (A position) allows the lens to be used the same way as traditional manual-focus lenses. Set the aperture on your camera. In the stepless aperture setting, emphasis is put on the aperture shape, which is rounder than the automatic aperture. For large aperture settings, selecting the stepless aperture is recommended. All exposure modes (P, A, S, M) can be used on the camera. This lens has two apertures; the stepless aperture, manually controlled with the aperture ring, and the automatic aperture (A position), controlled by the camera. In the stepless aperture setting, emphasis is put on the aperture shape, which is rounder than the automatic aperture. For large aperture settings, selecting the stepless aperture is recommended. This lens has two apertures; the stepless aperture, manually controlled with the aperture ring, and the automatic aperture (A position), controlled by the camera. In the stepless aperture setting, emphasis is put on the aperture shape, which is rounder than the automatic aperture. For large aperture settings, selecting the stepless aperture is recommended.

Sony Alpha lens book provides some additional info about STF lens

The 135mm F2.8 [T4.5] STF lens (SAL135F28) is a "Smooth Trans Focus" lens that is unique to the alpha System, and incorporates a special apodization optical element that is designed to achieve the most aesthetically pleasing defocusing effect possible. The apodization optical element is created by bonding convex and concave elements together. Although the convex element is made of normal optical glass, the concave element is designed to act as a kind of graduated neutral density filter, with light transmission highest at the center and gradually decreasing towards the periphery. So whereas conventionally designed lenses produce a defocusing pattern of uniformly bright circles, the 135mm F2.8 [T4.5] STF's apodization optical element causes the intensity of defocused point light sources to fade out radially from the center, ensuring ultra-smooth defocusing with no sharply defined edges or visible geometric form. Conventionally designed lenses can also suffer from residual spherical aberration that causes the brightness of the defocusing pattern to vary in different areas of the image. In addition, the quality of foreground and background defocusing can differ widely, with many lenses providing beautiful background defocusing but poor foreground defocusing. Defocusing characteristics can also be degraded by vignetting and uncorrected residual comatic aberration or curvilinear distortion. To avoid such problems and ensure excellent defocusing both in front of and behind the focus point, the 135mm F2.8 [T4.5] STF is designed to the highest optical standards in every detail. Vignetting is suppressed by its generously large diameter and 135mm fixed focal length, and as the MTF (Modulation Transfer Function) curve shown on page 134 shows, contrast and resolu- tion are exceptionally high, with consistently sharp imaging from the center of the lens right out to the periphery. To further maximize its beautiful defocusing, the 135mm F2.8 [T4.5] STF is also equipped with two circu- lar aperture control mechanisms. One is an automatic aperture controlled by the camera, and the other is a manual aperture that offers stepless manual control ranging from T4.5 to T6.7. The lens is marked with both F and T aperture settings for easy reference when shooting, with the T aperture settings reflecting the reduction in actual light transmission effected by the apodization optical element. The inclusion of an apodization optical element ensures that defocusing at points "b" and "c" are equally smooth and natural. With a conventional lens, defocusing at "b" and "c" has a clearly defined circular pattern Aperture efficiency comparison The 135mm F2.8 [T4.5] STF has an exceptionally large diameter (72mm filter size) for a fixed focal length lens, ensuring high aper- ture efficiency with virtually no vignetting. Even with the aperture fully opened, defocusing is superb right up to the periphery of the image. Spherical aberration With almost no spherical aberration, the 135mm F2.8 [T4.5] STF offers image sharpness and clarity comparable to that of the 300mm F2.8 G (SAL300F28G).

Our dyxum member laguire has given us the following explanation of the optics design involved:

There is an apodization filter near the aperture. It's basically a graduated neutral density filter that filters out more light peripherally and is clearer in the center. So instead of having an aperture that is solid with a sharp cut off of the focused or unfocused rays allowed to hit the sensor, it allows a graduated amount of the unfocused rays of light in. In a perfect pinhole situation (as in infinitely small aperture), each point from the object casts only one point on the sensor, therefore you have a perfectly sharp image (ignoring diffraction) at any focal plane. With a larger aperture, only a certain range of focal planes will cast a sufficiently distinct image that will be sharp, everything else will be out of focus and blurred. If you have a really large aperture only one focal plane will be in focus (small depth of focus). Each out of focus light ray (point source) will cast a circle instead of a perfect corresponding point. The larger the aperture and the further away from the plane of focus, the larger that circle. That circle should have a fairly distinct border if the aperture is solid (and the lens elements are theoretically perfect). With the apodiztion filter, think of combining separate images. Lets simplify and say you combine an image with a pinhole, an image with a moderate sized aperture and an image with a large aperture. Lets consider single point that is out of the focus plane. With the pinhole, you get a perfect point still. with the moderate aperture you get a small circle with a distinct border. With the large aperture you get a large circle. If you combine the images you will get a point with 2 concentric circles around it. Now imagine an infinite number of apertures and combine the images so that the larger aperture images (the larger circles) are fainter than the smaller aperture images, you will get a point of light that gradually fades away the further out you go. So instead of a harsh circle bokeh, you get a smooth drop off. think of looking at a cut end of a cylinder head on versus looking down on a bell shaped curve with the peak of the curve brightest. Note that a point that is in the focal plane will be a point with any of the sized apertures, so the objects in the focal plane (within the depth of focus) will be sharp. Only the out of focus areas will be gradually smoothed out. So in essence the apodization filter is like a infinite number of partially transmitting apertures betweenwide open and stopped down. The net effect is that you have a sharp in focus image and a smooth bokeh. You also lose light since that your essentially partially filtering out some of the light to smooth out the out of focus image. And finally autofocus doesn't work because you no longer have the distinct contrast of out of focus areas needed to autofocus.

Now that you all understand how the lens works I will summarize the three distinctive characteristics of this lens in my own words, just to make the article a bit longer :)

First it's a manual focus lens ONLY but it's made in a-mount. No auto focus with this lens folks! (more on this later)

Second, unlike all other lenses (Minolta or otherwise) we have two apertures. A stepless 10-blade aperture in conjunction with the apodization element gives this lens the ability to produce smooth and pleasing bokeh.

Third, in terms of depth of field, this lens acts like any other lens with a neutral density filter mounted on the front. Shooting wide-open, the depth of field will be the same as a "true" f2.8 lens however the light reaching the sensor will be equivalent to a lens at f4.5. The construction of the apodization element is "consuming" 1.5 stops of light which makes an effective maximum "speed" or "brightness" of f4.5, which is expressed in the optical world as T4.5. The stepless aperture is controlled by the aperture ring (not by the camera) and ranges from f4.5 to f6.7 only.

The second 9-bade aperture is what we may call the standard aperture as used on most other lenses. It is positioned behind the steeples aperture (closer to the sensor) and is controlled in-camera.

Selecting the "A" position on the aperture ring keeps the stepless aperture wide-open, and transfers control of the standard aperture to the camera. In terms of aperture, the lens then functions like any other, with standard controls like the exposure compensation dials at our disposal. Of course, despite the ability to use an automatic aperture, it remains manual-focus only. Since the stepless aperture only closes down to f6.7, the standard aperture is built into the lens to increase flexibility, and let the user stop down the lens (all the way to f32) for different shooting conditions. Both aperture openings cannot be used at the same time (as the image on the left may suggest). When one aperture is used the second is fully open.

Build and handling



The lens is extending during focusing,

but the front element does not rotate Aside from the distinctive features described above, the 135 STF is still "just a lens" and such subject to the same scrutiny as the rest of the Minolta/Sony lineup. As stated the excellent review by Magnus Wedberg, this lens has excellent build quality. The all-metal construction certainly leaves me confident that it will stand the test of time. It doesn't have the unique feeling of some of the old Pentax Takumar lenses (which are in many ways my personal favourites in terms of build quality) but it's certainly among best-built Minolta (Sony) lenses I had the chance to use.

The focus ring is adequate; wide enough with even stiffness across focus range, making the mechanics of manual focusing very easy. Assuring the image actually is in focus isn’t so easy with an APS-C sized viewfinder, but again, more on this later. Focus distance markers are very visible but a depth of field scale isn't provided (not a problem at this focal range).



Sony AF 135 F3.5 STF compared to

Minolta AF 100 F2 and Minolta 50 F1.4 RS Although exceptional in the Minolta/Sony world, am happy to report that the lens hood is actually very good! To be picky, I wish it was petal-shaped and I still can't understand why it is so difficult to provide us with hoods specifically designed for APS-C cameras. But functionally, it is hard to fault the hood on the STF.

The focus mechanism the STF is not an internal design, so the barrel does extend while focusing. To my surprise there is noticeable lens creep with the lens pointed up or down. This certainly isn't something I was expecting in a lens of this class and build-quality. Since the copy of the lens I tested had not had much use, it raises the question of whether lens creep will worsen over time. It is worth noting that the lens creep has been reported in the Minolta version of this lens which suggests that Sony has not made any changes at all to the actual lens design.

For a 135mm lens, the STF has very useful minimum focus distance of 870mm (0.25x maximum magnification ). I have also found that the viewfinder image isn't much darker with the STF on the 7D than with my Minolta 100 F2. The difference is there, but quite minor.

As the subject of the viewfinder arises, so does my main (and not so minor) complaint about using the STF on a digital SLR.

Focus or not to focus

Since this is a manual focus lens only, with DOF equivalent to 135mm F2.8 lens, focusing accuracy is of great importance. With introduction of AF cameras and lenses, camera manufacturers haven’t focussed on providing focussing screens optimized for manual focusing. Cameras come with brighter, clear viewfinders instead of darker matt and split-prism screens, and many of the time are happy this way. The introduction of the APS-C DSLR cameras has decreased viewfinder size, once again reducing the ability to manual focus properly. This is largely irrelevant if we are using autofocus (which most of us do with AF lenses, don't we ?), but with lenses like 135mm STF, our viewfinders are a serious limitation of a manual-focus-only design.

A quick look at a DOF calculator will shows total depth of field of less than 2cm when focusing this lens, wide open at subject distance of 2m (even less on closer subjects). Missing focus by a few centimetres isn't difficult with the viewfinders found on an APS-C DSLR! To reiterate: Aside from being a manual focus design, the problems focusing the STF are not due to the lens itself. In fact, manual focus with the STF is easier than with most other lenses I have used on APS-C Cameras. Rather, I see it as a very good reason to invest in full frame Sony when they become available :)

Why, you may ask, can’t we use the AF/MF confirmation signal in the viewfinder to confirm focus on our subjects? Unfortunately, this lens cannot take advantage of the focus confirmation used in our cameras.

The lens is completely disconnected from the camera AF mechanism and doesn't forward any info about subject distance (Dalifer utility is reporting subject distance of infinity), so it's practically as we are using m42 lenses with and adapter. I'm not so sure that this can't be improved by adding info feedback from the lens to the camera and make focus confirmation signal work. I wish Sony did something in this regard since focus confirmation signal may actually help a lot here.

This leaves focusing accuracy exclusively to our superior vision, or in my case, completely up to chance (do you feel lucky, punks?)

This is my biggest gripe about this lens, and there's not much that we can do about it. We can invest in alternative focusing screens (like this one) or we can (with "a bit" of help from Sony) wait for a full frame body with larger viewfinder more adequate for manual focusing.

Performance

Now this is the part of the review where you would normally find a dozen or so pages of charts, diagrams, comparisons and examples. Well, I’m sorry to disappoint, but empirical testing is not my strong suit. What I can offer is my own impressions of the STF after having shot with it for a period of 3 weeks in December 2006 (with hardly a week of good light :( ).

First, I seriously doubt there are many (if any) unsatisfied owners of this lens. In almost all respects, this lens performance is just superb. As can be expected from the research and development that must have gone into this lens, the performance is great even wide open. In fact, 99% percent of my images were shot wide open since this is where the STF really shines.

Sharpness is excellent and once you obtain proper focus, there is little left to be desired. The Minolta 100mm f2 (an older lens design that is renowned for its sharpness) resolves slightly more detail but I have no doubt that the image quality obtained with STF is limited only by the 6mp sensor of my 7D.

Converted with ACR 3.6 at default setting; nothing more. 1/200; F4.5; ISO 200. Saved for web with high quality setting (60%)

Converted with ACR 3.6 at default setting; nothing more. 1/1000; F4.5; ISO 200. Saved for web with high quality setting (60%)

I must admit that I was a bit scared the first day I used the lens as I noticed significant purple fringing... something I didn't expect. Discussing the matter on the Dyxum forums, other members were surprised by these results as well. Stopping down the aperture does solve the problem, but is hardly a valid solution since the STF is designed to work best at larger apertures. I paid special attention to the issue throughout the rest of my testing, and although I have concluded that this isn't as severe as I first thought, there are certainly lenses with better control of purple fringing.

I returned to my comparison of the Sony 135mm STF with my favourite Minolta lens 100/2, a lens which is reputed to be far inferior to new designs in terms of purple fringing. The quick and dirty test demonstrated that "PF-wise" STF lens is far from being perfect while the old and long-since discontinued 100/2 has coped very well against in these harsh test conditions. I made several images of this target and what I'm presenting here is an "average" result from each lens.

Between the relatively informal test results and my day-to-day experience shooting with the lens, I concluded purple-fringing is more prevalent than one may expect, but should only affect a fraction of images shot in the real world. Note too that variables such as scene contrast and details, and the properties of the sensor and camera used will affect the purple fringing phenomenon.

Purple fringing aside, colours and contrast are very pleasing and close to my personal taste. In combination with the excellent jpeg output from the 7D, images straight from the camera are very pleasing. Images being prepared for presentation in a gallery may warrant slight tweaking (if at all), while raw output can obviously be adjusted to your personal taste in terms of sharpness, colour, and so on.

I didn't spent too much time watching for vignetting or distortion; I didn't find anything "suspicious" and as far I'm concerned this lens is functions perfectly in this regard.

Defocusing blur .. or call it boke(h)

Now we already know which lens is indisputably the best in terms of bokeh....don’t we?

Yes we do.

Unlike some other lenses with almost "mythical" properties, I can hardly imagine anyone (experienced enough to judge) disputing the ability of this lens to beautifully render of the out-of-focus areas of an image. Some are even of the opinion that 135 STF lens exaggerates subjects by rendering the defocusing effect too perfectly, and thus "unnaturally". I certainly understand where this opinion is coming from! If you think your lens produces beautiful bokeh but you haven’t used the STF, you can remain blissful. Experience with the STF will shed a whole new light on how defocused areas can be "painted" while simultaneously changing your opinion about the "nice bokeh" of your other lenses.

Before I throw few sample images I recommend you first to visit and read (here and here) where the word boke(h) comes from and what actually this means. For far more easier understandings of the subject please read this.

Since we now know what to watch for let's see some samples.

Yep, Canon lens in the foreground ;) and a typical Christmas tree with bright decoration in the background (jpg compression did "ruined" fine gradation here)

Again a small Christmas tree decoration detail on the left and aluminium foil on the right.

These samples clearly demonstrate how well Minolta engineers have done their job. Of course, different situations will produce different results and will always be times where bokeh isn’t pleasing, but you can see the kind of results this lens can produce.

Now after seeing the bokeh produced with the STF, we can continue with some comparisons images with other lenses in various situations. The following are a mix from various sources, some taken with Sony STF and some with Minolta STF lens (again, they are optically identical).

As a start, let's keep things simple: crops are of a highlight taken with the Minolta 135/2.8 and Minolta STF 135

Images taken with Minolta 135/2.8 left and 135 STF right (thanks Bernard (ab012) !).

The difference is quite evident.

Now a more challenging test; Minolta 135 STF compared to both Minolta 135/2.8 and Minolta 85/1.4 (original)

135STF@4.5 135/2.8@2.8 85/1.4@1.4 85/1.4@2.8 My gratitude goes to Will Burke (wblc) for giving me these

Minolta 85/1.4 doesn't look as smooth as we’re used to, does it! Don't worry; any lens in this comparison will be hard pressed to keep up with 135 STF. These images demonstrate what the lens is designed to do, and that's why it is so unique and special.

Time for a zoom :)

The Minolta AF 80-200 G HS APO, despite being a zoom isn't bad bokeh-wise. It is not perfect (as you will see) but is still considered by many to be a solid performer in rendering defocused light, and that is why I did decided to test it against 135 STF.

And finally, here is a series of shots with aperture stopped down.

135STF@F4.5 135STF@F5 135STF@F5.6 135STF@F6.7 My gratitude goes to Will Burke (wblc) for giving me these

Final words

There's no doubt Sony made the right move in rebadging this lens when the brilliant design faced the possibility of being discontinued. The introduction of the Sony 135 F2.8 [T4.5] STF and new Zeiss designed lenses certainly gives us hope that we may see a full frame body in the near future, since as this lens is crying for a big viewfinder.

Of course the STF is very usable on current 7D, 5D a100 cameras too (with the 7D in advantage here for its bigger viewfinder) but I had pretty high percentage of out-of-focus images. Although I am referring to very slight focusing errors, they were significant enough to ruin the photo completely. I guess I'm not the only one with this gripe. Focusing accuracy with this lens is so important that I would suggest to anyone who is considering purchasing the STF to buy a split focusing screen as requirement for successful shooting. I will take this one step further and suggest that a replacement focusing screen should be part of the lens package, at least until we have an option to buy a full frame DSLR !

This isn't a cheap lens; and for most dyxum members it's simply out of reach. With the introduction of Carl Zeiss - Sonnar T* 135 F1.8 (I hope to have it reviewed soon) choosing this lens is even harder if we aren't rich enough to buy them all. So, is this lens worth serious consideration?

Simply put, Yes, but not to all of us.

As with all "specialized" lenses the price isn't an issue for those who need this kind of characteristic.

Excellent build quality, resolution, color and contrast it's making this lens pretty competitive; and in return for not being an AF lens, you get bokeh performance that is simply outstanding.

No it's not a lens you will carry alone in your bag, but once you start using it properly you will end watching your prints (or screen) for many times wondering how you did consider your other lenses as great bokeh performers. This lens is bringing kind of curse; isn't perfect, has few shortcomings and isn't cheap but it's a wonderful "light-painter" worth mythical status.

I already miss it !

Rating 5

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