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SMC Pentax 28mm 3.5 not reaching infinity focus
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PostPosted: Thu Jan 24, 2019 9:55 am    Post subject: SMC Pentax 28mm 3.5 not reaching infinity focus Reply with quote

Hi,

yesterday i tried to make some test shots with my Pentax 28mm on a Sony A7. On short distance and up to 2m everything is fine. When i tried to make a pic at infinity the sharpness stops at around 3m.
The focus ring shows infinity and so i think that the lens needs an infinity focus adjustment. Does anybody have pics or a DIY plan how this can be done or should i try to find a repair shop for this task?


PostPosted: Thu Jan 24, 2019 4:32 pm    Post subject: Reply with quote

It could be that the lens needs adjustment. But it could also be the adapter - if it's too thick, you won't reach infinity focus. This problem will be more pronounced the wider the lens. Did you use any other lens on the adapter to check?

In any case, more specific info on the lens (max. aperture, full name) would be helpful! Smile


PostPosted: Thu Jan 24, 2019 4:41 pm    Post subject: Reply with quote

Adapter issues are quite common but would normally ensure focus beyond infinity.
If you have access to a micrometer or measuring callipers the adapter is easy to check out. It should be 27.46mm from the rear plate that mounts on the camera to the front plate the lens mounts on.


PostPosted: Sat Jan 26, 2019 3:08 pm    Post subject: Reply with quote

With glass like that the first move should be putting it on a native body.


PostPosted: Sat Jan 26, 2019 10:03 pm    Post subject: Reply with quote

Gardener wrote:
With glass like that the first move should be putting it on a native body.


Maybe for you... Smile VLR has asked if the adapter has been used successfully with another lens. If yes what use is putting on native body?


PostPosted: Mon Jan 28, 2019 9:13 am    Post subject: Reply with quote

visualopsins wrote:
Gardener wrote:
With glass like that the first move should be putting it on a native body.


Maybe for you... Smile VLR has asked if the adapter has been used successfully with another lens. If yes what use is putting on native body?


If the other lens has a longer focal length any issues with the adapter will be reduced. So just testing the adapter with another lens isn't necessarily informative. (Sometimes adapters can be modified for faulty lenses too - but I'd expect that to be pretty unusual)


PostPosted: Mon Jan 28, 2019 5:41 pm    Post subject: Reply with quote

DConvert wrote:
visualopsins wrote:
Gardener wrote:
With glass like that the first move should be putting it on a native body.


Maybe for you... Smile VLR has asked if the adapter has been used successfully with another lens. If yes what use is putting on native body?


If the other lens has a longer focal length any issues with the adapter will be reduced. So just testing the adapter with another lens isn't necessarily informative. (Sometimes adapters can be modified for faulty lenses too - but I'd expect that to be pretty unusual)


True, however with 28mm only focusing to 3m, an adapter error would be large enough to affect 50mm lens and likely 135mm lens, which are probably the most common other lenses tried, if any.


PostPosted: Tue Jan 29, 2019 7:56 am    Post subject: Reply with quote

This page may help you. Sorry it's written in Japanese.

[ gryffine.blog100.fc2.com/blog-entry-6.html ]

See the fourth pic.
Loosen three screws beside lens unit.
Next slightly turn a focus ring and tighten the screws.
Test infinity.