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07-03-2011, 06:19 PM   #1
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T-mount adapters and infinity focus?

I have a few lenses that are T-mount and I have a single t-mount adapter I use for all of them.

Before I picked up a 135 and 300 in ys mount (t mount with aperure control), the only t-mount lens I had was a 12mm fisheye which has no focus adjustment so I had no idea that the focus was way off, It just seemed like it was soft on longer distance ojects which is also what I found to be the case on tests done by other people with the same lens.

Once I tried the same t-mount adapter on my 135 and 300 it was apparent that I was only able to get clear focus to around 50-75 feet or so.

I have since sanded the t-mount adapter lens side flange down quite a bit and can not get clear focus (using a split prism focusing screen) to about 1/5 to 1/4 of a mile away on the 135 and 300.

How far away is "infinity" focus usually set at?

07-03-2011, 06:29 PM   #2
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Try a full moon.

I have one of those 12mm lenses too..

Nothing is in complete focus with it and I've made just about every adjustment (with a lathe) to adapters possible.

07-03-2011, 07:01 PM   #3
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Well, I tried my 300 on the little sliver of the moon that is out there now and it was showing that I was able to focus out farther than the moon... so I guess I have inifnity focus down for the 300 and probably the 135 as well since they are both same era sigma lenses.
07-03-2011, 07:09 PM   #4
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Cyclone, if I'm reading your post correctly, it sounds like you are NOT able to get infinity focus on the 135 and 300 lenses with the T adapter??? Correct?? And both those lenses are ys mount??

It sounds like the distance from the rear lens element to the sensor is off. Try this experiment - verify that you CANNOT get infinity focus with one of the lenses using the T mount. Then remove the T mount, set the lens to infinity and simply hold it on the front of your camera. Focused? If so, then using the T mount extends the lens too far from the sensor.

I actually have a ys mount lens also - tomorrow, when it's light out, I'll try the same experiment (I haven't used the lens yet - just got it) and will post my results.

07-04-2011, 12:52 PM   #5
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QuoteOriginally posted by ChipB Quote
Cyclone, if I'm reading your post correctly, it sounds like you are NOT able to get infinity focus on the 135 and 300 lenses with the T adapter??? Correct?? And both those lenses are ys mount??

It sounds like the distance from the rear lens element to the sensor is off. Try this experiment - verify that you CANNOT get infinity focus with one of the lenses using the T mount. Then remove the T mount, set the lens to infinity and simply hold it on the front of your camera. Focused? If so, then using the T mount extends the lens too far from the sensor.

I actually have a ys mount lens also - tomorrow, when it's light out, I'll try the same experiment (I haven't used the lens yet - just got it) and will post my results.
Just tried my ys mount lens, with a T mount and I was able to achieve infinity focus - so, perhaps a different issue.
07-05-2011, 06:38 AM   #6
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QuoteOriginally posted by ChipB Quote
Just tried my ys mount lens, with a T mount and I was able to achieve infinity focus - so, perhaps a different issue.
Can you post a few shots of your t-mount adapter? If you can really get infinity focus on your ys lens with an unmodified t-mount adapter, I am betting that you have a different brand adapter than I do.

Also, before I changed my focusing screen to a split prism one, my camera was indicating that far away stuff was in focus even though it wasn't.

With the split focusing screen I can get way better focus on everything with manual lenses.

I still need to adjust the AF confirm because it seems to be slightly front focusing with AF.
07-05-2011, 09:44 AM   #7
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QuoteOriginally posted by cyclone3d Quote
Can you post a few shots of your t-mount adapter? If you can really get infinity focus on your ys lens with an unmodified t-mount adapter, I am betting that you have a different brand adapter than I do.

Also, before I changed my focusing screen to a split prism one, my camera was indicating that far away stuff was in focus even though it wasn't.

With the split focusing screen I can get way better focus on everything with manual lenses.

I still need to adjust the AF confirm because it seems to be slightly front focusing with AF.
Cyclone, here's some photos of the lens and the mount. I have several T-Mount adapters and they all appear to be the same thickness. In this instance, the one I used has an M42 mount so I simply put a Pentax M42 to K-mount adapter on it. Seems to work fine.

Last edited by ChipB; 01-13-2014 at 02:49 PM.
07-05-2011, 05:46 PM   #8
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That is actually a ys - m42 adapter.

It is not near as tall as a regular t-mount adapter AND it has the aperture control pin in it.

07-05-2011, 06:25 PM   #9
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QuoteOriginally posted by cyclone3d Quote
That is actually a ys - m42 adapter.

It is not near as tall as a regular t-mount adapter AND it has the aperture control pin in it.
I have 4 different T-Mount adapters - including the one on the YS lens mount - each one screws onto each T-Mount lens I have. Two are M42 adapters I use with a M42 to K-Mount adapter - the other two are T-Mount to K-Mount adapters. Three of the four are identical in height - one is just a bit shorter (NOT the YS one - a real cheapo from overseas). The YS one is the same height as the other two.

In the photo of the four in a row with the ruler on top, the YS adapter is the one on the far right - the cheapo is next to it. The other photo is just to show the adapters - the YS adapter is the left-most.

Last edited by ChipB; 01-13-2014 at 02:49 PM.
07-05-2011, 06:53 PM   #10
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My experience is on some early YS lenses, T2 adapter is too thick, or, T2 will not fully screw onto the lens. I have 4 versions of a YS lens -- the later YS adapters (and T2 adapters) do not fully screw on, cannot reach infinity. (Yes I will be documenting with photos et al later...)


Heh, the moon is not the best infinity target. Somebody around here actually plugged the numbers into a DOF calculator, to prove it. I used to think a bright star was best, but these methods are MUCH better, more accurate, by far (pun intended):

Focusing Methods for Astrophotography
07-05-2011, 09:19 PM   #11
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QuoteOriginally posted by ChipB Quote
Cyclone, here's some photos of the lens and the mount. I have several T-Mount adapters and they all appear to be the same thickness. In this instance, the one I used has an M42 mount so I simply put a Pentax M42 to K-mount adapter on it. Seems to work fine.
Well, after looking at your measurement and comparing it to my modified adapter, it looks like mine is right about the same thickness as yours.

Maybe the t-mount to k-mount adapter I bought is just a horribly too thick one. It is probably about 1/8" or more thinner then when I bought it.

I would sand/lap it down some, test it, and repeat. It probably took over an hour or more with my electric sander with 100 grit paper to get it where it is... and I had sanded it a little bit right after I bought it.
07-05-2011, 09:25 PM   #12
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I've tried a couple of adapters on my K20D. The extra lens distance has caused back focusing with both, so infinity is beyond infinity. I'm not sure if that's mathematically possible but with focus rings it is.
07-06-2011, 08:31 AM   #13
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QuoteOriginally posted by mysticcowboy Quote
I've tried a couple of adapters on my K20D. The extra lens distance has caused back focusing with both, so infinity is beyond infinity. I'm not sure if that's mathematically possible but with focus rings it is.
If the adapter is too thick, it will cause you to lose infinity unless the lens focused enough past infinity by default. It will also cause you to be able to focus closer than normal. (rear element further away from the sensor)

If the adapter is too thin it will cause you to lose some close focusing while at the same time causing the lens to be able focus past infinity or farther past infinity. (rear element closer to the sensor)
07-06-2011, 01:51 PM   #14
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I replaced the B-mount adapter on a Vivitar 500mm with a T-mount. I had a non-focus at infinity, but figured out I did not remove all the mounting assembly from the lens. Once I figured that out, the focus worked. Incidentally, last weekend I got the chance to play with it and I think the mount shifted while carrying the lens around. I'll have to check it tonight if I get the chance.
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