This lens was Minolta's first professional "standard" zoom for the new AF SLR system, introduced in 1985. While the lens obviously was designed for films and the professional Minolta 9000, it is a gem also on the new 24MP full frame Sony alpha 900 DSLR.

I have checked three copies of the 28-135mm, all with the same (great) result. At any focal length the 28-135mm has the same detail resolution as the best corresponding Minolta AF prime (recently tested with Minolta AF 2.8/28mm, 2/35mm, 2/100mm and 2.8/135mm). Distortion, however, often is clearly visible - especially when using it for architecture.

In the full frame corners its detail resolution equals or even surpasses the Zeiss 2.8/24-70mm (both lenses at f8):

Test images comparing the AF 4-4.5/28-135mm to the AF 2.8/28-70mm G and the Zeiss Vario Sonnar 2.8/24-70mm at f=70mm.

Its contrast wide open, however, is quite low (16 elements in 13 groups and coatings from 1985). Contrast-wise the Zeiss is much better, in the center it reaches the same resolution at f2.8 as the Minolta 28-135mm at f8. In addition the Minolta is a bit sensitive to flare / backlight situations since Minolta has delivered the lens without any lens hood (on APS-C DSLRs the Hama Telematic S, 72mm works perfect). Due to "rear focusing" AF is very fast, but the minimal focus distance of 1.5m is quite limiting.

Sometimes the AF doesn't focus exactly. Using the AF 4-4.5/28-135mm this can lead to strange looking de-focusing, as if the lens would perform very poorly. I did extensive tests with the 28-135mm (against 2.8/28mm, 2/35mm, 2/100mm, 2.8/100mm, 2.8/135mm and 2.8/135mm STF), and I was observing this phenomenon again and again. I suspect that at least a few of these "bad" copies out there were not exactly focused - as simply as that. I will publish a few such "crazy" looking crops from my (de-focused) lens soon.

Even though the building quality of the lens seems to be very good (metal), some of the inner parts may be worn out after long professional use, and the image quality and/or AF accuracy may suffer. To allow perfect "infinity" focusing on Sony DSLRs I had to remove some internal metal shims (it seems to be a quite common problem not only with this lens, but also other Minolta AF lenses). Due to several requests I have documented the process; see the images below:

The procedure itself is quite simple. The only thing to be aware of is NOT to damage the rather sensitive electric contacts between ROM and bayonet. Lenses such as the Minolta AF 3.5-4.5/28-85mm, AF 3.5-4.5/35-105mm, AF 2.8/20mm and AF 2.8/24mm follow the same principle. Good luck ;)

If only the focus ring needs to be re-adjusted, you may follow the procedure of Pete Ganzel cited in this post at dyxum

Pete Ganzel: Disassembly of Minolta AF 4-4.5/28-135mm

The interior of the 28-135mm is rather complex, and everything - including the zoom curves - is completely made from metal: A true masterpiece of lens engineering! as one can see easily, the zoom curves are not even - resulting in a slightly changing resistance of the zoom ring during zooming (at about f40mm). This is completely normal!