Handling the 135 STF

The lens has two main working areas on its aperture ring, "A" (automatic control of the aperture) and manual stepless aperture control between T4.5 and T6.7.

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.

Operation of the 135 STF

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.

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-blade 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. When one aperture is used the second is fully open.

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.

stepless aperture ring

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.

Focusing

The focusing ring is wide, with a rubber grip area. Focusing is manual only, and the camera does not attempt to use its focusing motor even if in AF mode. The AF indicator does not work at all, however, but that does not matter because focusing in the viewfinder is good. It's not hard to see very small changes in focal point ("micro focus differences").

Why Manual Focus not Auto Focus?

There is a common mis-perception that the 135 STF does not use autofocus because the lens is too slow (having an effective aperture is 4.5 but a geometric aperture of 2.8). Another prevalent myth is that the mechanics of the aperture blades cannot cope with the speed of autofocus.

In fact, the reason is associated with the internal working of the autofocus drive of the camera. Auto-focus phase detection sensors are CCD line sensors sitting behind split prisms. When light is focused by the lens, an activation pattern is produced by the sensors, depending on how well focussed the image is. The camera adjusts the focussing of the to achieve a standard correctly focused activation pattern. However, the apodization filter of the 135 STF, creates a smooth transition from in-focus parts of the image to the out of focus parts, and therefore interferes with the AF drive adjusting the focus to achieve the correct activation pattern. Not only does this prevent the camera's autofocus from operating, it also prevents focus confirmation.

For further information on the relationship between AF operation and the STF lens' apodisation filter, I recommend Michael Hohner's Minolta FAQ.

Using Manual Focus on DSLRs

The STF is very usable on the KM 7D and 5D, the Sony a700, and I assume also on the a100, a200, a300 and a350. I think it's fair to suggest that the larger viewfinders of the 7D and a700 are advantageous.

However, on an APS-C sensor, as used on each of these cameras, the STF is an effective 200mm on an APS-C camera (in fact 202 mm), with an effective aperture of 2.8, focus precision is unforgiving! The anticipated Full-frame DSLRs (starting with the forthcoming Sony 'Flagship model', widely assumed to be launched under the name a900) are likely to fare better, particularly as the optical viewfinders of DSLRs continue to improve.

The focusing ring is not oil-damped as old style MF lenses, it just moves with the natural resistance from the groups of heavy glass.

Meanwhile, users who find manual focus difficult with the STF have the option of fitting split screens, or using magnifiers or angle viewers. I've not fitted a split screen, but I have used both the Vn Angle Finder and Magnifier, and found them helpful in certain conditions. More often than not I find them unneccessary.

Lens Accessories

Filters

The 135 STF has a 72mm front filter thread, and can be used with a range of screw-on or cokin-style filters.

Teleconverters

The lens is one of the select few that can be used with the Minolta's matched APO converters. This, while being nice on principle, is of no practical value -- a 189/6.3 (with 1.4x TC) or 270/9 (with 2x TC) isn't all that great - particularly with APS-C digital SLRS and their 1.52x 'crop factor'!

Extension Tubes

Extension tubes can be used with the 135 STF to use the lens fr dedicated macro photography. I've done this several times, and the results are very sharp. However, the very narrow depth of field achieved with extension tubes means that the smooth trans focus advantage is largely lost when using the lens in this way.

© Michael Fotheringham