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02-11-2018, 04:18 PM   #1
Pentaxian




Join Date: Apr 2016
Posts: 505
Pentax F 50mm f/1.4 - Auto aperture show maximum of f/4.0

Hi guys,

I've got a lens here that is showing an aperture value of f/4.0. I've had a look at this website Features and Operation of the Ka Mount and it seems to hint that there is a contact problem with the lens. I think it's a bit deeper of an issue since I've noticed signs of oxidation damage on the mount and internals when opened. I'd like to know whether a lens like this is salvageable from this situation. Read the details below if you're interested.

Details:
Lens did not autofocus when first obtained, fungus and dirt throughout. Mount had green oxidation and a patch of chrome removed with rust behind it. Two screws closest to the contacts turn freely due to previous repair attempt (it seems). Lens focus ring didn't turn freely, neither did the aperture ring. Lens disassembled and optics cleaned (to 96% original state). Focus helicoid cleaned using alcohol and re-greased. Aperture mechanism opened up and cleaned with alcohol. Focusing position forks (the fragile 4 pin ones) cleaned as they were heavily covered with blue-green oxide layer. Lens reassembled and focusing now working. Max aperture showing as f/4.0 unless the lens is flexed while on camera, in which case it flickers between -- and f/4.0. Aperture value of f/1.4 comes up for a split second before f/4.0 holds. Stopped down aperture value of f/22 shows up as required. Lens overexposes by 3 stops if shooting an image at f/4.0, as the camera meters for f/4.0 but lens is shooting wide open.

Thanks in advance!

02-12-2018, 01:26 AM - 1 Like   #2
Pentaxian




Join Date: Jun 2013
Location: Newcastle
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Q: What camera body are you using?

I ask this question as I coupled by K3ii to my F*600mm on the weekend and had F-- feature. Perplexed, I re-coupled the lens to my K-1 and found no trouble.
02-12-2018, 05:04 AM   #3
Pentaxian




Join Date: Apr 2016
Posts: 505
Original Poster
QuoteOriginally posted by Wild Mark Quote
Q: What camera body are you using?

I ask this question as I coupled by K3ii to my F*600mm on the weekend and had F-- feature. Perplexed, I re-coupled the lens to my K-1 and found no trouble.
I've used the lens on both a KP and a K-1. All other lenses operate as normal so it's almost definitely a lens problem. The main culprit is probably the aperture flex board (if there is one) or possible corrosion of wiring in the lens. I could probably open the lens up even more to expose the wiring but I don't see the point unless someone else has had experience of it previously.

It's a really weird situation in that I've got an autofocus 50mm f/1.4 with only manual aperture. A bit like taking a K 50mm and putting a screwdrive unit in it.
02-12-2018, 05:33 AM   #4
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Just to make sure:
- the aperture ring is set to "A".
- the largest opening you can set on the camera is f4.
Is that right?

02-12-2018, 10:38 AM   #5
Pentaxian




Join Date: Apr 2016
Posts: 505
Original Poster
QuoteOriginally posted by blende8 Quote
Just to make sure:
- the aperture ring is set to "A".
- the largest opening you can set on the camera is f4.
Is that right?
Correct as of yesterday... I tried the lens today and I'm getting correct aperture behaviour on my K-1, with rough/slow focusing.

On my KP, the focusing is better but the higher apertures do not stop down properly. Even stranger behaviour...
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