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05-26-2009, 01:00 PM   #1
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Chris Stearns's Avatar

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300mm f4 m42 focuses past inf. on k10

I recently picked up a SMC 300mm f4 m42 lens that focuses on objects at infinity before the inf. marker on the barrel. If I keep going 'till the marker it goes blurry. According to an engraving on the mount, it was CLA'd in '06.

What I am wondering is that maybe it was adjusted to work with one of the ebay m42 adapters that have the 1mm shim that makes inf. focusing impossible.

Other than that the lens works beautifully. I just have to pay special attention when fosusing on distant objects.

Any thoughts?

05-26-2009, 01:09 PM   #2
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I don't know the internal focusing arrangement and what adjustments are possible, but perhaps (and this is a guess) someone had infinity focus adjusted for a flanged adaptor?

If so, you could purchase a flanged adaptor and mount it perminantly on the lens. the advantage is that you would insure that you had focus confirmation, and the flanged adaptors use the lens locking pin on the camera body
05-26-2009, 05:23 PM   #3
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Good morning Chris, this is not a fault in your lens. Every lens of all makes for different brands of all the cameras I have owned focuses "past" infinity. There is a small tolerance in manufacture of lenses plus an allowance for expansion of the different materials used in the lens construction.
Simmo
05-27-2009, 10:10 PM   #4
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I tried adjusting a Takumar 50mm for infinity and a flanged adapter but there wasn't enough adjustment in the lens. Other lenses probably have a greater range so your theory is possible.

The adjustment is not that hard on smaller primes with the right tools. I think it's practical to do it yourself on those lenses. On a 300mm, it may be more difficult or the lens might be built differently. I haven't ever owned one so I can't tell if different tools are needed.

The cheapest option is to buy a flanged adapter. A camera repair place should be able to give you an accurate estimate on a simple repair like this.

05-27-2009, 10:47 PM   #5
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Or...the simple answer may be that it was miss-assembled after the CLA. This can happen if you miss engaging the helical focus thread by one notch in some lenses - I'm not sure about this specific lens though. The real down side is you loose an equivalent amount in the near focus range when you do that.

As I recall the flange on the "bad" adapters is about 0.8mm. Slip a spacer (a strip cut from the back of a pad of paper or a match book cover is about right) between the lens and mount. If that solves your focus problem, one of the flanged adapters should correct the issue.

The corollary of course is that you can gain some close-focus range with a tiny bit of extra spacer which can be useful for backyard birding. Like a mini extension ring. I intentionally mod'ed a Tamron-F 300/5.6
Oly > PK mount that way for close work - it now only focuses to only about 160 feet max.

H2
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