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10-03-2008, 11:25 AM   #1
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F/2.8 lens only goes to 4.0

I bought a 28mm F2.8 lens on eBay. I hadn't heard of the mismatches in mounts before and I stupidly bought a lens with the super large flange in the mount and the pin that jams on some cameras. Even more stupidly, I did the surgery instead of returning it. I cut the flange and removed the pin so that it won't jam. Now I find that the shutter can't open to F2.8, it only goes to 4.0. Is it because I didn't remove enough of the flange, or is there another problem?



10-03-2008, 11:29 AM   #2
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look at this site to see if the lens is correctly coded for F2.8

Pentax Lens Modifications for Matrix Metering - Mark Roberts Photography

otherwise it could be a dirty contact
10-03-2008, 03:30 PM   #3
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QuoteOriginally posted by Lowell Goudge Quote
look at this site to see if the lens is correctly coded for F2.8

Pentax Lens Modifications for Matrix Metering - Mark Roberts Photography

otherwise it could be a dirty contact
Thanks for the link, I hadn't heard about this multi-metering issue. The camera asks for the focal length when I turn it on if I power up in CW or Spot.

I can't see any dirt in the contactors. The lens meters correctly but it stops all the way down to f32, which IIRC is beyond spec, so it may be some sort of assembly issue.
10-03-2008, 03:30 PM   #4
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The simplest thing to do is clean the contacts with a pencil eraser and see if it fixes anything.

Test the lens by setting it at A, putting the camera in Av mode, setting the lens to to different apertures and activating the DOF preview lever. You should see the lens stop down at different apertures for each stop.

10-03-2008, 03:48 PM   #5
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In any mode, it will not go wider than f4. I can only use it in A mode. Should I be able to use it with manual apertures? If I try to set apertures using the mechanical ring, the camera flashes F and refuses to operate.

Sorry for all the questions, this is my first MF lens and I seem to have made a poor choice.
10-03-2008, 04:41 PM   #6
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So when you have it off the camera the aperture is fully open right? When you mount it, does it stop down somewhat (look into the lens). What about when you hit the DOF preview button?

What I'm trying to get at is whether it actually won't go below f/4 or whether it just TELLS you it won't go below f/4.
10-03-2008, 05:05 PM   #7
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For the issue you are having with metering in manual using the aperture ring... you need to go into the menus and change the "Using aperture ring" setting to permitted.

"The lens meters correctly but it stops all the way down to f32, which IIRC is beyond spec"

Thats definately NOT beyond spec. It says right there in that photo that it stops down to F32.

Does that series of numbers stop at "4" by any chance?
10-03-2008, 05:58 PM   #8
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QuoteOriginally posted by sewebster Quote
So when you have it off the camera the aperture is fully open right? When you mount it, does it stop down somewhat (look into the lens). What about when you hit the DOF preview button?

What I'm trying to get at is whether it actually won't go below f/4 or whether it just TELLS you it won't go below f/4.
I'll try to determine if it opens fully.

The lens and the camera clearly agree that it's at f4, because it's metering properly. I tried some shots at -1 EV and they're underexposed.

10-03-2008, 06:02 PM   #9
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QuoteOriginally posted by bentax Quote
For the issue you are having with metering in manual using the aperture ring... you need to go into the menus and change the "Using aperture ring" setting to permitted.
I see, thank you.

QuoteOriginally posted by bentax Quote
"The lens meters correctly but it stops all the way down to f32, which IIRC is beyond spec"

Thats definately NOT beyond spec. It says right there in that photo that it stops down to F32.

Does that series of numbers stop at "4" by any chance?
Sorry, I misled you. That's not my lens. It's a handy illustration of the modification I did to my lens. The highest number on my lens is F28, the lowest is 2.8.
10-03-2008, 07:23 PM   #10
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QuoteOriginally posted by Lowell Goudge Quote
look at this site to see if the lens is correctly coded for F2.8

Pentax Lens Modifications for Matrix Metering - Mark Roberts Photography

otherwise it could be a dirty contact
This site works for some lenses, but not for my two Pentax-F primes (F28/2.8 and F50/1.7). These lenses both apparently have the same contact arrangement when I look at the mounts. I suspect they are wired differently internally to agree with the patterns listed, but I'd have to check with an ohmmeter. The OP's lens pattern looks like a lens that goes from f4.5 to f32, or I'm getting something wrong.

Back to the subject at hand, you should be able to see if your 16-45 set at 28mm and f4 has the same exposure as the prime lens at f4. You should definitely try using the DOF preview lever as I suggested above, in Av mode, let's say set at f8 on the camera, with the lens aperture ring set to A. Then move the aperture ring off of A to f8, switch to M mode and try the DOF preview again. The lens should stop down the same amount both times.

Last edited by Just1MoreDave; 10-03-2008 at 07:43 PM.
10-03-2008, 08:40 PM   #11
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QuoteOriginally posted by audiobomber Quote
The lens meters correctly but it stops all the way down to f32, which IIRC is beyond spec, so it may be some sort of assembly issue.
As sewebster already said, if it meters correctly don't worry about it, it just reads the lens coding wrong, but f4 is still wide open (f2.8). The same thing happens with some tamron lenses. If cleaning doesnot help you can also try to wiggle the lens a bit after mounting, maybe it is a tiny bit misaligned.
10-03-2008, 09:21 PM   #12
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I think everyone probably missed an important point here...
This is not a Pentax A-mount lens!
The original post indicates this is a Ricoh R-K mount lens that has been modified. This mount generally lacks the A-mount contacts. As such, it should be treated as a standard K-mount with the aperture ring off the A setting, in M mode with green-button metering.

Here is another thread by the OP about (I think) the same lens:

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/38431-k-mount-wont-mount.html

Steve

Edit: I looked at the picture of the lens flange on the other thread and it looks like the mount was intended to do double duty as both R-K and Pentax-A. It may be that the Pentax A-mount features are poorly implemented, hence the OP's problems.

Last edited by stevebrot; 10-03-2008 at 09:38 PM.
10-03-2008, 09:25 PM   #13
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QuoteOriginally posted by Just1MoreDave Quote
The OP's lens pattern looks like a lens that goes from f4.5 to f32, or I'm getting something wrong.

Back to the subject at hand, you should be able to see if your 16-45 set at 28mm and f4 has the same exposure as the prime lens at f4. You should definitely try using the DOF preview lever as I suggested above, in Av mode, let's say set at f8 on the camera, with the lens aperture ring set to A. Then move the aperture ring off of A to f8, switch to M mode and try the DOF preview again. The lens should stop down the same amount both times.
Cross-checking with the 16-45 helped. I confirmed that in A mode, when the camera says f4, it is f4. It metered 1/13s with both the 16-45 and 28mm. I then tried the aperture ring set to 2.8. The camera didn't know what the f-stop was (not reporting it), but it metered for f2.8 (1/30s). So I can only get down to f4 in auto aperture mode, and 2.8 with the aperture ring. I'm satisfied with that. I'm not taking the lens apart any more. I'm lucky I haven't screwed it up with all this messing around.

Thanks to everyone who replied. It's time to stop screwing around and take some photos.
10-03-2008, 09:26 PM   #14
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QuoteOriginally posted by audiobomber Quote
Cross-checking with the 16-45 helped. I confirmed that in A mode, when the camera says f4, it is f4. It metered 1/13s with both the 16-45 and 28mm. I then tried the aperture ring set to 2.8. The camera didn't know what the f-stop was (not reporting it), but it metered for f2.8 (1/30s). So I can only get down to f4 in auto aperture mode, and 2.8 with the aperture ring. I'm satisfied with that. I'm not taking the lens apart any more. I'm lucky I haven't screwed it up with all this messing around.

Thanks to everyone who replied. It's time to stop screwing around and take some photos.
Dan,
See my note above.

Steve
10-03-2008, 09:29 PM   #15
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QuoteOriginally posted by stevebrot Quote
I think everyone probably missed an important point here...
This is not a Pentax A-mount lens!
The original post indicates this is a Ricoh KP mount lens that has been modified. It has no A-mount contacts. As such, it should be treated as a standard K-mount with the aperture ring off the A setting, in M mode with green-button metering.

Steve
It does have A-mount contacts. I can set the aperture electronically and it meters automatically. It just won't open up to F2.8 in auto mode. I'm 90% certain it's a little misalignment that causes the electronic aperture control to cycle from f4-f32 instead of f2.8-f28.

Anyway, I have manual control of the aperture and auto metering, and that's all I require.

Last edited by audiobomber; 09-12-2012 at 10:56 AM.
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