I think I may have identified the issue. The aperture arm is very stiff and has no spring

Hi.
I've recently bought a K70 secondhand from Wex and over the weekend received a Pentax 17-70 SDM lens I bought on ebay. I've found an odd problem in that photos with this lens are constantly overexposed by in excess of a stop. This happens in all metering modes. The other 2 pentax lenses I own do not have this issue and exposure compensation corrects it which indicates that the aperture is shutting down, just not enough. Unfortunately I don't have any other bodies to test the lens on, though I've owned lots of Pentax kit in the past. I've cleaned all the contacts off and upgraded the firmware to the latest version but to no avail. Anyone have any ideas? To me it looks like a lens issue but its difficult to be certain.

The problem is with a lot of these reviews is that they are some time in the past. There weren't any 36mp full frame cameras around in 2012 and the majority of cameras would have been approx 12-16 mp apart from the £5000 pro jobs. So excellent is in relation to those cameras . The Samyang is mushy when you drill down into the images. The problem I have with the Irixes is that when I download the sample images from the reviews they don't look particularly good, then theres always the question of whether excellence is an absolute term or relative to price point, which is never made that clear in the reviews. Seriously thinking of dumping pentax because of havng to hunt around for old film lenses which are of variable quality plus the appalling focusing. Did a colour run for my sons school and had a number of back of head shots because the camera refused to lock focus as I followed them round.

What experiences have people had with the 11mm and 15mm Irixes. I've previously bought the Samyang 14mm and returned it because its resolution isn't good enough for a K1. I'm also wondering whether anyone has got hold of the almost impossible to find Laowa 12mm? I've got the 20mm F4 Pentax which is great as long as you're not in too much sunlight and the Sigma 35mm 1.4 which has fantastic sharpness and resolution but I'd like to add an even wider lens because some internal shots still need extra space.

Calumet sell the IRIX, they were selling them at the Photography show. I would think from the photo's and reviews that the IRIX is a better bet, its certainly better built. I ordered the Samyang 14mm and sent it back since I didn't think its resolution could handle 36 mp very well, leaves tended to merge together on closer inspection to create a smudged image.

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Sorry the GPS and the 12-24mm have both now been sold