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11-17-2013, 12:54 PM   #1
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Pentax M lens focus past infinity?

One thing I have noted with various adapters and different digital camera bodies is that most adapters have a slightly shorter flange distance than spec. This allows most lenses to focus to infinity. If the adapter was out of spec longer than flange distance, it would act as an extension tube for shorter focus at the expense of infinity focus.

This is not necessarily a bad thing since it allow a lens to have some variance for focus at infinity to compensate for environmental conditions including temperature of the lens itself or for manufacturing variances. This is especially true for mirror lenses.

I have a SMC M 100mm f2.8 lens that I just received. I note that on the Pentax Adapter Q for K mount lenses, this lens achieves infinity focus before the infinity stop. "It will focus past infinity."

When I mounted the SMC M 100mm with a Novoflex Pentax K to Sony E adapter to a NEX-6 and focused the lens I noted infinity focus was achieved prior to the infinity stop. My experience is that Novoflex adapters are usually exactly on spec and infinity focus is only achieved at the infinity stop for the lens.

It is my expectation that the Pentax Adapter would have the exact spec for flange distance and achieve focus at infinity only at the infinity stop for the lens. I fully expect that "cheap" adapters would not be as precise as a Pentax or Novoflex adapter.

Question: When mounted on a K dSLR or K-01, it is common for the SMC M 100mm to focus past infinity?

Question: Is this an issue of the Pentax adapter or is this an issue of the lens?

Question: Can the SMC M 100mm be serviced to adjust the infinity focus spot?

(I have posted these questions in the Pentax Q forum)

11-17-2013, 04:41 PM   #2
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I don't use any sort of adapters but I can give you the answers to your three questions:

QuoteOriginally posted by GXRUser Quote
Question: When mounted on a K dSLR or K-01, it is common for the SMC M 100mm to focus past infinity?

Question: Is this an issue of the Pentax adapter or is this an issue of the lens?

Question: Can the SMC M 100mm be serviced to adjust the infinity focus spot?

A1) No, it's not common for MF lens to focus past infinity (some AF lenses do). However, keep in mind that M series lenses are quite old and most have had a service history of some sort and it's quite possible that the infinity focus adjustment was not calibrated correctly by the serviceperson.

A2) If the lens focuses properly at infinity mark when used on a camera without an adapter, then the lens is OK and the adapter will be at fault.

A3) Yes, the lens can be serviced to adjust the infinity focus point.
11-19-2013, 11:14 AM   #3
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It is my understanding that some longer lenses (but probably longer than 100mm) deliberately allow focus past infinity. This is to accommodate thermal changes during use.
11-19-2013, 01:28 PM   #4
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QuoteOriginally posted by JimJohnson Quote
It is my understanding that some longer lenses (but probably longer than 100mm) deliberately allow focus past infinity. This is to accommodate thermal changes during use.
That is my understanding as well, especially for catadioptric (mirror) lenses.

I did think that short telephoto lenses (like the SMC M 100mm f2.8) would have the infinity focus stop correlate to the infinity marking on the lens barrel. This is important to me as I want to be able to just turn the focus ring to infinity and have that be in focus.

11-19-2013, 02:22 PM   #5
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To add to the above, if the rear element is loose, it'll be closer to the film plane at the infinity stop than is necessary. Shake it gently and see if anything is loose, and see about tightening things down if they are.

I had an SMC 28mm where the rear element was partially unscrewed, and was focusing past infinity. I thought it was just because it was really beat up, but the rear element ended up coming all the way unscrewed, and then it proceeded to fall into the mirror box, breaking the mirror and ruining the focusing screen. Thankfully, it was only an ME Super, and not one of my DSLRs.

Also, if it was disassembled/reassembled before, shims could be missing or in the wrong place.
11-21-2013, 07:18 AM   #6
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QuoteOriginally posted by GXRUser Quote
I have a SMC M 100mm f2.8 lens that I just received. I note that on the Pentax Adapter Q for K mount lenses, this lens achieves infinity focus before the infinity stop. "It will focus past infinity."

When I mounted the SMC M 100mm with a Novoflex Pentax K to Sony E adapter to a NEX-6 and focused the lens I noted infinity focus was achieved prior to the infinity stop. My experience is that Novoflex adapters are usually exactly on spec and infinity focus is only achieved at the infinity stop for the lens.
Am I correct in understanding that both combinations suffer from the same issue?

What do you mean by "this lens achieves infinity focus before the infinity stop"? Do you mean
  1. that when the (center of the) infinity symbol is aligned with the diamond on the DOF scale, you can still turn the focus ring a little further?
  2. that if you or the camera think that focus at infinity is achieved, you can still turn the ring further before you hit the stop and before the infinity symbol is aligned
    with the diamond?
For (1)
I have the M120/2.8; when turned to the infinity stop, the diamond on the DOF scale is perfectly aligned with the center of the infinity symbol. For this exercise it does not even have to be mounted on a camera. If that is not the case with your M100/2.8, I would have it serviced.

For (2)
That's not 'focus past infinity'; 'focus past infinity' is a function of the lens.

QuoteOriginally posted by GXRUser Quote
Question: When mounted on a K dSLR or K-01, it is common for the SMC M 100mm to focus past infinity?
The M120/2.8 mounted on my K5 does not focus past infinity as described by you.

QuoteOriginally posted by GXRUser Quote
Question: Is this an issue of the Pentax adapter or is this an issue of the lens?
Why do you only mention the pentax adapter and not the novoflex? As far as I understand your post, you have the issue with both.

QuoteOriginally posted by GXRUser Quote
Question: Can the SMC M 100mm be serviced to adjust the infinity focus spot?
I would expect that to be the case. You can contact Eric Henderson ( http://pentaxs.com/ ); he has a good reputation here on the forum.
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