Review: Minolta AF 28-70mm f/2.8 G

1. Introduction

The 28-70mm f2.8 lens is part of the Minolta AF G – Series that are Minolta’s high performance class optics designed to give the highest optical quality. Let us see if the lens fulfils these promises.

The 28-70/2.8 has been my standard travel lens together with the 200/2.8. The 2.8 aperture makes it good to use in low light and the zoom range makes it useful for street photography. I have used this lens on several trips to Africa and Asia. 

A summary of my thoughts on the lens follows.  Remember, this is a subjective review and your opinions might be different. This review is based on my experience with one lens only and sample variations might occur. Anyway I hope you find it useful.

 

 

2. Lens data

  • Focal Length: 28-70mm
  • Filter diameter: 72mm
  • Hood Mount: Bayonet type hood.
  • Dimensions: 83mm x 114.5mm (diameter x length)
  • Weight: 850g
  • Aperture:
    • Largest: f/2.8
    • Smallest: f/32
    • Diaphragm Blades: 9 curved aperture blades.
  • Focusing:
    • Method: Internal focusing, non rotating front element.
    • Minimum distance: 0.85m
    • Maximum magnification: 0.09X (at 70mm)
    • One focus hold button
  • Optics:
    • Construction:  16 elements (2 aspherical glass elements)
                             11 groups
    • Angle of view: 75° - 34° 


Wat Si Saket Temple, Vientiane, Laos.

Minolta Dynax 9 with Minolta 28-70mm f/2.8 G
1/20 @f/2.8, Fuji Sensia 100
(Photo © Marcus Karlsen)

3. Appearance and Handling

Constructed with 16 elements in 11 groups, the AF 28-70mm f/2.8 G lens clearly is an advanced optical design even if it dates back to 1993. Two of the elements consist of aspheric glass and the lens has a specially designed flare cutter. The aspheric glass contributes to give the lens a tight control on the  chromatic aberrations. The angle of view varies with the zoom from 75° to 34°. The filter thread size follows the current 72 mm standard for Minolta pro lenses. A very short and almost useless dish-shaped lens hood comes with the lens. Why Minolta has not designed a deeper petal-shaped lens hood is beyond my understanding.

The manual-focusing collar could have been wider and is located at the extreme front end of the lens barrel. The customary golden ring, designating a G construction, is placed just behind the manual-focusing collar. Zooming is by another wider collar located around the mid part of the lens barrel. The lens barrel itself has a smooth finely crinkled-paint black finish. It looks nice and withstands wear better than the white paint on the telephoto lenses. The lens itself is very well built with a metal barrel and mount.

This lens is an internal focusing lens so the front element is not rotating during focusing or zooming. It does have a rotating focus ring near the front of the lens for manual focusing which rotates during auto-focus with camera bodies older than the Dynax 7. While zooming the lens does not extend but stays at the same length.

The lens focuses by moving the lens elements by a shaft. A motor in the camera drives the shaft, making the focusing speed dependent on which camera model the lens is coupled with. I have tried the lens on the Minolta Dynax 700si and the Minolta Dynax 9.

The focusing speed is not very fast and responsive, in fact it is very slow. The lens also makes quite a lot of noise when focusing. This is very disappointing and more should be expected from a G series lens. Near focus occurs at 0.85 m. This is not sufficient for nice tight-composed close-ups and I really wish the near limit be moved a bit closer, say to about 0.4 m. The lens is an internal-focusing design so its total length will not change with the focus setting. Neither will the front element turn making it easy to use a polarizing filter.

This lens would surely benefit from SSM focusing to speed up the focus system and to lower the audible noise. Minolta actually announced a SSM version a couple of years ago but it have still not materialized.

The 28-70 G lens is no real featherweight at 0.85 kg, neither is it very small. However, I have used this lens, together with the AF 200mm f/2.8 Apo G, extensively when travelling as they fit perfectly into a small camera bag like the Tamrac Explorer II. Actually this combination has been my favourite travel kit for street photography, giving excellent optical quality in a relatively small package (at least compared to other manufacturers high end glass).



The close focusing distance of 0.85m is not close enough to make a really tight portrait. This curious young Himba boy was just too close to get him in focus.
Himba village, Kaokoland, Namibia.

Minolta Dynax 9 with Minolta 28-70mm f/2.8 G lens
(Photo © Marcus Karlsen)

4. Optical Performance

Aspheric Design is different from the smooth continuous arc of a normal lens element surface, the curvature of an aspherical lens element changes shape across its surface. This irregular surface helps to correct spherical aberration and coma, an aberration that causes off-axis object points to appear as short "comet-like" images on film. It also reduces sagital flare. The inclusion of aspheric elements in the lens design enables designers to reduce the amount of elements in the design. This in turn reduces the amount of flare producing surfaces, two per element, which increases the contrast and "snap" of the final image. The reduction in elements also makes the lens smaller and lighter. 

The tables below summarize my findings when shooting a resolution chart with the lens to test its optical quality. The ratings are as follows:

     *      Very poor 
    **     Poor 
   ***    Ok 
  ****   Good 
 *****  Very Good 


28-70mm f/2.8 G @ 28mm

f-stop

Sharpness centre

Sharpness middle

Sharpness corner

Contrast

Distortion

Vignetting

Colour

2.8

****

***

**

***

slight barrel

some

neutral

4

*****

****

**

***

slight barrel

slight

neutral

5.6

*****

****

***

****

slight barrel

slight

neutral

8

*****

*****

****

****

slight barrel

no

neutral

16

*****

*****

*****

*****

slight barrel

no

neutral

22

*****

*****

*****

*****

slight barrel

no

neutral


28-70mm f/2.8 G @ 50mm

f-stop

Sharpness centre

Sharpness middle

Sharpness corner

Contrast

Distortion

Vignetting

Colour

2.8

*****

*****

****

*****

no

no

neutral

4

*****

*****

*****

*****

no

no

neutral

5.6

*****

*****

*****

*****

no

no

neutral

8

*****

*****

*****

*****

no

no

neutral

16

*****

*****

*****

*****

no

no

neutral

22

*****

*****

*****

*****

no

no

neutral


28-70mm f/2.8 G @ 70mm

f-stop

Sharpness centre

Sharpness middle

Sharpness corner

Contrast

Distortion

Vignetting

Colour

2.8

*****

****

****

*****

slight pillow

no

neutral

4

*****

*****

*****

*****

slight pillow

no

neutral

5.6

*****

*****

*****

***

slight pillow

no

neutral

8

*****

*****

*****

*****

slight pillow

no

neutral

16

*****

*****

*****

*****

slight pillow

no

neutral

22

*****

*****

*****

****

slight pillow

no

neutral

 

Overall the AF 28-70mm f/2.8 G lens delivers a very good image quality. At 28mm it should be stopped down to f/5.6 or f/8 for the best performance all the way out to the corners. At 50mm and 70mm it is very good even wide open.

Illumination was even across the entire frame at f/2.8, so corner light fall-off is absolutely negligible except wide open at the 28mm zoom setting. Geometric distortion is negligible, only a very slight hint of barrel distortion at 28mm and of pincushion distortion at 70mm.

The graphs below are the Minolta MTF graphs for the lens.

The graph 
The graph shows MTF in percent for the two line frequencies of 10 lp/mm and 30 lp/mm, from the centre of the image (shown at left) all the way to the corner (shown at right). The bold lines represent sagital MTF (lp/mm aligned like the spokes in a wheel). The thin lines represent tangential MTF (lp/mm arranged like the rim of a wheel, at right angles to sagital lines). On the scale at the bottom 0 represents the centre of the image (on axis), 3 represents 3 mm from the centre, and 21 represents 21 mm from the centre, or the very corner of a 35 mm-film image. Separate lines show results at f8 and full aperture. This is Minoltas own MTF graph for this lens.

5. When the image is out of focus

The word Bokeh is of Japanese origin and relates to the fashion in which the out-of-focus areas of the image are rendered. A sharply focused subject set against a pleasingly silky smooth background characterizes a good bokeh. The transition from in-focus to out-of-focus should occur gradually. A large number of aperture blades give a more circular opening when the lens is stopped down, but this in itself is not sufficient to give a good bokeh. Another feature of the lens that affects bokeh is the degree of spherical aberration correction. Spherical aberration is when the rays of light from the middle and from the outside edges of a lens do not focus to exactly the same point.

The 28-70mm f/2.8 G shows a very pleasing image rendition. The background blurring attained by the lens is creamy and silky smooth, entirely up to the bokeh of the very best performers.


Royal Palace, Phnom Penh, Cambodia.
Minolta Dynax 9 with Minolta 28-70mm f/2.8 G lens.
(Photo © Marcus Karlsen)
The image to the left is taken on my trip to Cambodia. Look at the smooth transaction between the highlights and the darker areas in the out-of-focus areas of the image. Makes for a very pleasing image. The small size of the image on this web page does not make it justice, but on full screen or printed out it makes the image stand out.

6. Flare and ghosting

What causes flare

Zoom lenses have always been susceptible to excessive ghosting, often combined with a moderate resistance to flare as well. Despite sophisticated multi-coating treatment of the glass surfaces, unwanted internal reflections are prone to occur.

Since light reflects off glass surfaces, lens flare usually increases with the number of glass elements in the lens. The problem is compounded with backlit subjects. 

The photographer should play an active role in controlling lens flare. The absence of a lens hood contributes to lens flare. Consequently, you should always use a lens hood.

The 28-70mm F2.8 G lens has, in addition to Minoltas own excellent multi-coating, a specially designed flare-cutter that works as a second aperture. Still you have to be very careful when including the sun in your picture, as ghosting is likely to occur.

7. Summary and Conclusions

This lens has the excellent optics that is expected from a Minolta G lens. However, the auto focus and close focusing distance is not as good as they should be on a lens like this. Despite this I highly recommend this lens. For a lens like this, high-speed auto focus is not of such a high importance as on telephoto lenses. Still I have lost a few shoots because of this and the rather long close focusing distance.

Pros 
   o Excellent optical performance 
   o Very good bokeh 
   o Very good build quality
   o Image stabilizer with the Dynax 7 Digital (built into the body)

Cons 
   o Auto focus is noisy and slow
   o Close focusing distance of 0.85m is to long
   o Useless hood

8. Magazine lens tests

  • German magazine Color Foto test results at 28mm, 50mm and 70mm
    18/19/19 out of 30 on sharpness
    23/25/25 out of 30 on contrast
    20/17/20 out of 20 on centering
    2/9/6 out of 10 on distortion
    6/8/7 out of 10 on vignetting
    That makes it 75 out of 100 total and that is 2 point more than the Sigma EX 24-70mm f/2.8 Asp DG DF but 2 points less than the Nikon AF-S Zoom-Nikkor 28-70mm f/2.8 D IF ED and 5 points less than the Canon EF 24-70mm f/2.8 L USM.
  • German magazine Color Foto old test method
    76.4 out of 100
  • French magazine Chasseur d’Image.
    4 out of 5 in optical quality 
    3 out of 5 in value for money 
    4 out of 5 in general impression
  • French magazine Chasseur d'Images test results on the Dynax/Maxxum 5D
    Vignetting 3/5
    Chromatic abberation 4/5
    Distortion 3/5
    Sharpness 4/5
    Total score 4/5
  • German magazine Fotomagazin
    9.2 out of 10 , which is slightly better than its competitors.
  • Swedish magazine Aktuell Fotografi 
    3.9 out of 5
  • Photodo test result
    Grade: 3,7
         Average Weighted MTF: 0,78 
         Effective focal length: 29-68 mm
         Weighted MTF for 28 mm: f2,8 0,75, f4 0,78, f8 0,79
         Weighted MTF for 40 mm: f2,8 0,76, f4 0,80, f8 0,83
         Weighted MTF for 70 mm: f2,8 0,70, f4 0,73, f8 0,76
         Weighted MTF 10 lp/mm: 0,88
         Weighted MTF 20 lp/mm: 0,74
         Weighted MTF 40 lp/mm: 0,49
  • Italian Tutti Fotografi test result
    Distortion 28 mm: 4% barrel
    Vignetting 28mm: 1/2 stop
    Distortion 50 mm: 0% 
    Vignetting 50mm: 1/2 stop
    Distortion 70 mm: 1.5% pincushion
    Vignetting 70mm: 2/3 stop

 

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