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06-17-2010, 04:19 PM   #1
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Bit of a tricky one - Can't focus to infinity

OK, so I've got a Pentax K-x with a genuine M42 mount adaptor. I just bought an old Flexar 500m/f.8 lens and I can't focus to infinity. It appears to have a T-mount adaptor on the end.

The lens can be disassembled into three components (four if you count the T mount adaptor ring). I am also now doubting that the tube piece that is connected to the T mount adaptor is original - It does not fit very well in the thread.

Any ideas?


Last edited by maxwolfie; 06-17-2010 at 04:57 PM.
06-17-2010, 05:26 PM   #2
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It sound like the adaptors are register distance. I'd get T-mount to K-mount adaptor or remove all the unnecessary bits. Or you could try standing a bit closer.
06-17-2010, 06:15 PM   #3
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What's "regular distance"?
06-18-2010, 04:03 PM   #4
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QuoteOriginally posted by maxwolfie Quote
What's "regular distance"?
Didn't explain very well (new keyboard) 'register distance' is the distance from the back element of the lens to the focal plane of the camera.

Sounds like the back element of your lens is too far forward.

06-18-2010, 04:08 PM   #5
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QuoteOriginally posted by maxwolfie Quote
OK, so I've got a Pentax K-x with a genuine M42 mount adaptor. I just bought an old Flexar 500m/f.8 lens and I can't focus to infinity. It appears to have a T-mount adaptor on the end.

The lens can be disassembled into three components (four if you count the T mount adaptor ring). I am also now doubting that the tube piece that is connected to the T mount adaptor is original - It does not fit very well in the thread.

Any ideas?
Are you sure the Tube on the end is not some sort of Extension tube?



This lens has a 60mm tube (removable) on the end for closer focusing.

06-20-2010, 02:20 AM   #6
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Here's some pictures of the lens.. It doesn't appear to have an extension tube on it?

I still don't know how the original owner managed to focus on infinity (I am assuming, as per the pictures) but I can't??

Last edited by maxwolfie; 06-20-2010 at 03:10 AM.
06-20-2010, 06:21 AM   #7
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The pictures you show look out of focus to me. Certainly not sharp but that may just be the lens. You said the lens comes apart into 4 pieces. The first thing I would try is replacing the Tmount adapter (after making sure the lens is properly assembled).

RE focus, my question would be can you get a sharp photo at ANY distance?

06-20-2010, 03:48 PM   #8
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Replace the lens with another T mount adaptor of the same type? I have another one exactly the same, and it does the same thing (i.e. can't focus at inf) as you would expect.

When I say out of focus at inf, I mean a completely blurry blob, not just "unsharp"

06-20-2010, 04:17 PM   #9
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QuoteOriginally posted by maxwolfie Quote
OK, so I've got a Pentax K-x with a genuine M42 mount adaptor. I just bought an old Flexar 500m/f.8 lens and I can't focus to infinity. It appears to have a T-mount adaptor on the end.

The lens can be disassembled into three components (four if you count the T mount adaptor ring). I am also now doubting that the tube piece that is connected to the T mount adaptor is original - It does not fit very well in the thread.

Any ideas?
These lenses were usually produced with an original T2-thread at the rear, to adapt them to almost any camera. This T2 thread has the dimension of M42x0.75. Can you measure at least the diameter (which should be 42mm) of the thread? You simply need to find the original rear and then fit a T2-K-mount adapter there.

It is well possible, that somebody fitted (well, forced) an T2-M42 adapter on the original rear and later a T2-K-mount adapter was squeezed onto the M42-adapter. This is possible as T2 and M42 have thew same thread diameter, just a slightly different pitch. If that would be the case (an obsolete T2-M42 adapter mounted), it would easily explain the out of focus images at infinity.

Ben

These old 500/8 were sold under many names and some were very sharp in the center.
06-20-2010, 06:43 PM   #10
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QuoteOriginally posted by Ben_Edict Quote
These lenses were usually produced with an original T2-thread at the rear, to adapt them to almost any camera. This T2 thread has the dimension of M42x0.75. Can you measure at least the diameter (which should be 42mm) of the thread? You simply need to find the original rear and then fit a T2-K-mount adapter there.

It is well possible, that somebody fitted (well, forced) an T2-M42 adapter on the original rear and later a T2-K-mount adapter was squeezed onto the M42-adapter. This is possible as T2 and M42 have thew same thread diameter, just a slightly different pitch. If that would be the case (an obsolete T2-M42 adapter mounted), it would easily explain the out of focus images at infinity.

Ben

These old 500/8 were sold under many names and some were very sharp in the center.

Thanks Ben, width of thread is approx 41mm (close enough to 42mm.) Length of rear tube is approx 89mm - not including thread.

There are no obvious signs of forced fitment - I.e. the black coating on the threads are still very much intact. Perfect even. HOWEVER, the OTHER end of the rear tube (that is, the end that connects right underneath the tripod mount) does NOT fit very well into the next section at all. It will screw on, very poorly though (I can push them together if I apply a tiny bit of force, ignoring the threads altogether), as I believe that the diameters/thread pitches are slightly off. Must be only very marginal though as I can still tighten them together at the end of the thread, enough to hold the weight of my SLR (though I am worried, and will probably use locktite on the thread just to make sure once I've got this issue sorted out).

Weird

I guess this could mean that the rear tube is not original and it could be slightly longer than the original (hence my reasoning for putting the rear tube length of 89mm above)

Here's a pic of all pieces including the rear tube in the middle.


Last edited by maxwolfie; 06-20-2010 at 06:51 PM.
06-20-2010, 08:55 PM   #11
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Max you say you have two lenses that do the same thing. Can you clarify what you are doing to focus at infinity - are you turning the focus ring all the way to the infinity stop - because if you are you are actually focussing "past" infinity. All lenses have built in tolerance to allow different expansion rates of the different materials that lenses are made from. "Infinity" is just short of the stop.
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06-21-2010, 12:07 AM   #12
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Hi Baldrick

I understand that it is possible to in fact focus past infinity. To focus, I am looking through the lens at something very far away, starting at the nearest point and moving through the whole range. The subject becomes "clearer" though it's still a blob at "infinity" - therefore I would assume that I need to focus further in order to reach infinity

I bought a small screwdriver set today to try and fix the lens, but even with the tiny grub screw out of the side of the lens, I can't seem to get it to budge (i.e. I am trying to adjust the bit of glass furthest from the sensor such that it is closer to the sensor)
06-21-2010, 12:53 AM   #13
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Problem solved by moving the furthest away/biggest element towards the sensor approx. 1-2cm. Did some pretty ugly cosmetic damage in the process but oh well, it only cost me $12.50 and it is now functional (I tweaked it so that it will actually focus past infinity a little - Just to make sure I'm not missing anything).

Cheers
06-21-2010, 01:20 AM   #14
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QuoteOriginally posted by maxwolfie Quote
Problem solved by moving the furthest away/biggest element towards the sensor approx. 1-2cm. Did some pretty ugly cosmetic damage in the process but oh well, it only cost me $12.50 and it is now functional (I tweaked it so that it will actually focus past infinity a little - Just to make sure I'm not missing anything).

Cheers
That was indeed some surgery: 1-2cm! That's a lot and has nothing to do with tolerances or a slightly mis-set focusing tube. You may be right, that the 89mm tube is not original, but that should be confirmed by somebody who actually owns one of these lenses.

Ben
06-21-2010, 03:11 AM   #15
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I would think not. The fit of the 89mm tube is horrible - I used some locktite and heaps of duct tape to ensure that my camera does not go walk about (i.e. to the ground!!)
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