M*300 not focusing to infinity

**Mikesul** · 02-18-2015, 12:22 PM

I just received a Pentax SMC M*300 lens. It seems fine except the the focus ring will not turn beyond the 40m mark. It moves smoothly below that point. Any ideas of if this can be fixed? I really do not want to return it but it is not usable like this. Thanks for any help.

stevebrot · 02-18-2015, 01:37 PM

Originally posted by Mikesul

Any ideas of if this can be fixed? I really do not want to return it but it is not usable like this. Thanks for any help.

It depends on what is causing the problem. If the inner workings of the focus helicoid are fractured, the lens is toast. If it is just a matter of lack of lubrication or botched service attempt, there may be hope.

Steve

**Mikesul** · 02-18-2015, 01:52 PM

Any idea how I can test it? The stop is pretty firm and unyielding.

stevebrot · 02-18-2015, 03:22 PM

Originally posted by Mikesul

Any idea how I can test it? The stop is pretty firm and unyielding.

Does it turn past the near focus mark in the near direction? If so, a previous owner may have modified to allow closer focus.

Steve

**Mikesul** · 02-18-2015, 05:22 PM

I had to go back to work after the first message but have now mounted it on the camera again and this time it turned smoothly to just past the infinity mount. However, continued use caused it to bind briefly a couple of times. Most of the time it moves smoothly but I am a bit concerned. I am hopeful that it turns out to be a piece of grit which works itself out. I am very appreciative of the advice offered so far.

**Mikesul** · 02-18-2015, 07:08 PM

Update. I was able to about 40 pictures after it started working again but now it is stuck on the near focus end. The seller has agreed to take it back do I guess that is what I must do unless someone suggests a fix by morning.

tvdtvdtvd · 02-18-2015, 07:38 PM

The fix would be to open up the lens and see if it can be repaired. Best guess is either grit in
the helical or a loose internal screw that is causing something to bind.

Do you have confidence in opening the lens yourself? The M*300mm is not in the repair database,
however M series primes are not overly complicated and I'm sure you could manage a repair
using the other examples at a guide. If you do decide to keep the lens, or have someone else
repair it, be sure to get a healthy refund from the seller.

**southlander** · 02-18-2015, 09:41 PM

The M*300 is quite different in construction to the shorter focal length M series. The guides I have seen online generally relate to a M 50mm. From my own experience the M 20, 28, and 35 follow the M 50 construction and you can follow the 50mm guides. The M 85 may be the same but I haven't taken a screwdriver to it. The longer M's have a different construction, particularly getting in from the front of the lens, and the M*300 has internal focus as well which other M's don't. I suggest the guides for short lenses will not help that much with the M*300 and you would pretty well be on your own voyage of discovery. Stuff it up and it would be hard to get your refund from the vendor. If you accept the challenge and find a failing/worn component is causing the issue, you won't find a replacement part either.

I'd be inclined to ship it back and try again with another lens. They do pop up from time to time.

Damn sharp glass if you get a good one and use it well.

02-18-2015, 12:22 PM	#1
Mikesul Loyal Site Supporter Join Date: Sep 2009 Location: Phoenix, AZ Posts: 7,594	*M300 not focusing to infinity** I just received a Pentax SMC M*300 lens. It seems fine except the the focus ring will not turn beyond the 40m mark. It moves smoothly below that point. Any ideas of if this can be fixed? I really do not want to return it but it is not usable like this. Thanks for any help.

02-18-2015, 01:52 PM	#3
Mikesul Loyal Site Supporter Join Date: Sep 2009 Location: Phoenix, AZ Posts: 7,594 Original Poster	Any idea how I can test it? The stop is pretty firm and unyielding. Last edited by Mikesul; 02-18-2015 at 02:54 PM.

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02-18-2015, 01:37 PM	#2
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by Mikesul Any ideas of if this can be fixed? I really do not want to return it but it is not usable like this. Thanks for any help. It depends on what is causing the problem. If the inner workings of the focus helicoid are fractured, the lens is toast. If it is just a matter of lack of lubrication or botched service attempt, there may be hope. Steve

02-18-2015, 03:22 PM	#4
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by Mikesul Any idea how I can test it? The stop is pretty firm and unyielding. Does it turn past the near focus mark in the near direction? If so, a previous owner may have modified to allow closer focus. Steve

02-18-2015, 05:22 PM	#5
Mikesul Loyal Site Supporter Join Date: Sep 2009 Location: Phoenix, AZ Posts: 7,594 Original Poster	I had to go back to work after the first message but have now mounted it on the camera again and this time it turned smoothly to just past the infinity mount. However, continued use caused it to bind briefly a couple of times. Most of the time it moves smoothly but I am a bit concerned. I am hopeful that it turns out to be a piece of grit which works itself out. I am very appreciative of the advice offered so far.

02-18-2015, 07:08 PM	#6
Mikesul Loyal Site Supporter Join Date: Sep 2009 Location: Phoenix, AZ Posts: 7,594 Original Poster	Update. I was able to about 40 pictures after it started working again but now it is stuck on the near focus end. The seller has agreed to take it back do I guess that is what I must do unless someone suggests a fix by morning.

02-18-2015, 07:38 PM	#7
tvdtvdtvd Veteran Member Join Date: Jan 2010 Posts: 1,665	The fix would be to open up the lens and see if it can be repaired. Best guess is either grit in the helical or a loose internal screw that is causing something to bind. Do you have confidence in opening the lens yourself? The M*300mm is not in the repair database, however M series primes are not overly complicated and I'm sure you could manage a repair using the other examples at a guide. If you do decide to keep the lens, or have someone else repair it, be sure to get a healthy refund from the seller.

02-18-2015, 09:41 PM	#8
southlander Site Supporter Join Date: May 2012 Location: Adelaide Photos: Gallery \| Albums Posts: 1,364	The M300 is quite different in construction to the shorter focal length M series. The guides I have seen online generally relate to a M 50mm. From my own experience the M 20, 28, and 35 follow the M 50 construction and you can follow the 50mm guides. The M 85 may be the same but I haven't taken a screwdriver to it. The longer M's have a different construction, particularly getting in from the front of the lens, and the M300 has internal focus as well which other M's don't. I suggest the guides for short lenses will not help that much with the M*300 and you would pretty well be on your own voyage of discovery. Stuff it up and it would be hard to get your refund from the vendor. If you accept the challenge and find a failing/worn component is causing the issue, you won't find a replacement part either. I'd be inclined to ship it back and try again with another lens. They do pop up from time to time. Damn sharp glass if you get a good one and use it well.