Pentax 28-70 mm f4 AL on Sony a6500

Continuing in my quest to find a decent general purpose zoom to use with my Sony a6500 camera, I've just taken delivery of a used Pentax 28-70 mm f4 AL lens.

This is a light and compact lens. In common with other early autofocus lenses from Pentax the focusing ring is a bit loose and vague, but it's not diffcult to achieve accurate manual focus.

Working at f8, my initial impressions are reasonably favourable. From 50 to 70 mm it appears very sharp across the frame. Below that the edges fall away a bit, with 28mm being the weakest focal length, but central definition remains good throughout. I have a feeling that this may be due to a field curvature issue, as better results are obtained by focusing near the edge of the frame rather than in the centre.

I need to do more testing and use the lens for some practical photography, but this one may well be a keeper!

Further testing has confirmed my fear that. at 28mm, the edges are very soft. A pity because at the centre at 28mm the image is crisp while at longer focal lengths, the lens produces clear contrasty results across the frame. Here's an example of the problem at 28mm, first the overall view and then actual pixel crops at the left, centre and right hand side of the frame. Click on any image for an enlarged view.

Left hand crop 900x600 pixels

Centre

Right hand crop

As always with older lenses it's not possible to know if this is a generic fault of the design, or a one off problem. Sadly this one is another being returned to the vendor.

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