Pentax A 28-80 f3.5 - 4.5 lens on Sony a6500

I'm continuing to search for a half decent walk around lens for my Sony a6500, having tried and discarded the Zeiss and Sony offerings. My latest discovery is a Pentax A 28-80 f3.5-4.5 zoom from the film era. I've had my copy for years, bought second hand it's not in the best of condition, with marks visible on the front element and a general slackness in the zoom helicoid. This is what it looks like on the camera.

Be warned, this is a manual focus lens, if that's not your cup of tea, read no further!

It's no lightweight being of mixed metal and plastic construction. With adapter it weighs 502 g, but this is appreciably lighter than the Tamron 28-80 that I have also tested.

How does it perform, I've not conducted any brick wall tripod tests as yet, but having taken a number of shots with it I am reasonably impressed.

Here's an example. It was a muggy, hazy day with poor visibility, but the image quality is OK

First of all the overall view, the German cruise ship Mein Schiff 3 moored at North Shields terminal.

(Click any image for a larger view)

Now actual pixel crops from the extreme left and right of the image

The edges are far better than either the modern Zeiss or Sony zoom achieved, but the overall image lacks a bit of contrast. I really do need to do some formal testing on a bright sunny day.

I need to compare this lens with the Tamron 28-80 that I have also tested, and with some of my old film camera primes.

Sadly, further shooting has revealed a fatal flaw with this lens, at closer distances and probably due to the ravages of age, it does not always achieve sharp focus in the centre of the field. Take a look at this image of some ironstone rocks and the actual pixel crops from centre, left and right.

The overall picture

Centre crop

Left crop

Right crop

The camera was focused on the centre point, so this should not be a field curvature issue. Further I have more examples showing the same pattern. Maybe the wobbly focusing ring is the culprit? This particular copy of the lens is unusable, but I would like to get a hold of another example as, surely, this can't be a generic fault!

I've since carried out a tripod and wall test. Out of 15 images only one displays this fault, taken at 35mm and f8. Two other shots at that focal length and aperture do not.

Edit 28-8-19 - blurred centre!

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