Why did my old MF lens just stop focusing?

Sagitta · 03-31-2017, 11:42 AM

A bit baffled on something, and hoping someone may be able to toss some advice my way as to how to approach fixing this.

I have an old MF 300mm internal focus lens which worked perfectly fine when I shelved it a few months back. I just went to use it, and discovered it no longer focuses to infinity (or anywhere even close to infinity). The lens will allow me about half the range that it should when trying to focus (basically MFD up to about 30 feet if looking at the focus ring). The lens of course goes up to somewhere north of 200 feet before hitting infinity.

It seems like something is causing a hard stop while focusing the thing, and it has me baffled as as far as I know the lens hasn't been abused (no drops, crashes etc) since I put it away last.

Again, the lens is all-manual (its ~vintage!~), and is a Sears rebrand of the Tokina 300mm f/5.6. The thing was a dog to find (not many out there in the wild) and is one of my favorite lenses, so having it go down has me more annoyed than anything else.

Is this something I can fix? What the heck actually happened? Am I going to have to start interrogating my wife and kids because it was probably dropped at some point when I wasn't looking (FWIW, it shows no outward signs of damage).

EDIT: I just discovered some damage of some sort - the aperture lever is loose. I can physically push and pull it into the mount, and its all 'wobbly' now. I know it wasn't like that when I shelved the thing!

MarkJerling · 03-31-2017, 01:37 PM

Contact user dcshooter - He may have advice for you.

**UncleVanya** · 03-31-2017, 02:31 PM

@dcshooter he needs some advice and I'm hoping you can give it.

**.a.t.** · 03-31-2017, 03:13 PM

Originally posted by Sagitta

. . .
EDIT: I just discovered some damage of some sort - the aperture lever is loose. I can physically push and pull it into the mount, and its all 'wobbly' now. I know it wasn't like that when I shelved the thing!

Until someone more experienced than I chimes in, I'll speculate a bit:

Before I reached your edit, I was wondering if the aperture was functioning normally? I had a similar situation with a Pentax 135mm lens. The focus ring was suddenly frozen. And the aperture also suddenly wouldn't stop down at all.
It turned out that an internal pin which was part of the aperture mechanism had broken and lodged in the focus helical gear. By removing the mount I was able to see the problem and get the broken piece out. The lens could then focus, but the aperture wouldn't work without the pin that broke.

If you have a set of JIS screwdrivers (link), I'd suggest removing the mount and taking a look around.
To make re-installation easier, note the arrangement of the levers, etc. as you lift the mount off. And be careful not to remove the aperture ring in the process or you might lose the ball bearing that gives the ring its click stops.

I have a Vivitar 300mm f/5.6 lens which is, I believe, about the same as your lens (yours is K mount, right?). Except mine has a screw mount. So the innards of your lens will probably be different from mine, but here's a picture of my lens with its mount removed:

(click the image for a larger size)

Last edited by .a.t.; 03-31-2017 at 04:58 PM. Reason: another clarification

edmundrt · 03-31-2017, 05:58 PM

If it was made by a company like Tokina and sold as rebrands, it may be something like a T4 or TX interchangeable mount. If this has come a little loose, it may cause the problems you've been describing.
Picture of the mount could be useful, someone more expert than me might be able to help!

**niceshot** · 03-31-2017, 06:19 PM

your focus barrel or the sleeve around the helical has slipped and is out of adjustment.Should be and easy fix. Roll off the rubber grip on the focus sleeve and you should find three flea sized screws. Back them out a little and adjust your focus. Good luck Oh one other thing do not use a lot of force on the helical if you do things will go awry

Sagitta · 03-31-2017, 09:34 PM

Originally posted by .a.t.

Until someone more experienced than I chimes in, I'll speculate a bit:

Before I reached your edit, I was wondering if the aperture was functioning normally? I had a similar situation with a Pentax 135mm lens. The focus ring was suddenly frozen. And the aperture also suddenly wouldn't stop down at all.
It turned out that an internal pin which was part of the aperture mechanism had broken and lodged in the focus helical gear. By removing the mount I was able to see the problem and get the broken piece out. The lens could then focus, but the aperture wouldn't work without the pin that broke.

If you have a set of JIS screwdrivers (link), I'd suggest removing the mount and taking a look around.
To make re-installation easier, note the arrangement of the levers, etc. as you lift the mount off. And be careful not to remove the aperture ring in the process or you might lose the ball bearing that gives the ring its click stops.

I have a Vivitar 300mm f/5.6 lens which is, I believe, about the same as your lens (yours is K mount, right?). Except mine has a screw mount. So the innards of your lens will probably be different from mine, but here's a picture of my lens with its mount removed:

(click the image for a larger size)

I paid 20 bucks for the lens, I'm not TOO upset. I'm more irritated about the time it took to find it where someone wasn't asking LOL prices for the thing.

Now that I know it was the lens and not the camera I feel better.

Sagitta · 04-04-2017, 07:20 PM

Originally posted by dcshooter

My ears are burning!

Here's my guess as to what it is. When you turn the aperture ring, it moves a cam with a peg on it against an armature that pushes a little lever that determines how far open or closed the iris is. when you are in auto mode, it is spring loaded, with the pin moving the arm until it stops against the lever. Moving the switch to M pushes a stop in place so it's always as if the pin is pressed in (the mechanism used varies from lens to lens).

If the mount is simply held on by screws, you should be able to just remove them and pull the mount off and see what is happening pretty easily.

My theory is that something is wrong with the aperture arm. Either it's jumped the peg, come loose from the lever it moves, or one of the springs is busted or a screw holding the spring mechanism is loose., allowing the armature to just flop around. The reason the lens is bottoming out before infinity is that the rear element is hitting the now loose armature on its way back. The reason the A/M switch is all wobbly is that there is no spring pressure from the armature pushing against the stop it controls.

Sounds about right, except theres no A setting in the picture here - the lens is straight up manual. Whats wobbly is the actual aperture lever sticking out of the mount itself. Its come loose.

I might take it apart if I can get a set of decent jewelers drivers some day. Not so much in hopes of saving it as just out of nosiness at this point.

**niceshot** · 04-04-2017, 07:36 PM

Originally posted by Sagitta

Sounds about right, except theres no A setting in the picture here - the lens is straight up manual. Whats wobbly is the actual aperture lever sticking out of the mount itself. Its come loose.

I might take it apart if I can get a set of decent jewelers drivers some day. Not so much in hopes of saving it as just out of nosiness at this point.

i had a similar problem with a ROKINON 135MM 2.8 Lens. What happened was the focus barrel spun and It went well past Infinity and unbeknownst to me I drove the aperture control lever into the mount and blew up the aperture .Well that was the first Lens I rebuilt entirely .Now I have a few hundred under my belt, but then it was a real challenge, now it would be a breeze. JIS screw drivers ,and a lens spanner and high powered glasses is what you need. Oh and lots of patience. Don't try to focus it any more until you fix it.

03-31-2017, 11:42 AM	#1
Sagitta Veteran Member Join Date: Feb 2011 Location: Maine Photos: Gallery Posts: 3,081	Why did my old MF lens just stop focusing? A bit baffled on something, and hoping someone may be able to toss some advice my way as to how to approach fixing this. I have an old MF 300mm internal focus lens which worked perfectly fine when I shelved it a few months back. I just went to use it, and discovered it no longer focuses to infinity (or anywhere even close to infinity). The lens will allow me about half the range that it should when trying to focus (basically MFD up to about 30 feet if looking at the focus ring). The lens of course goes up to somewhere north of 200 feet before hitting infinity. It seems like something is causing a hard stop while focusing the thing, and it has me baffled as as far as I know the lens hasn't been abused (no drops, crashes etc) since I put it away last. Again, the lens is all-manual (its ~vintage!~), and is a Sears rebrand of the Tokina 300mm f/5.6. The thing was a dog to find (not many out there in the wild) and is one of my favorite lenses, so having it go down has me more annoyed than anything else. Is this something I can fix? What the heck actually happened? Am I going to have to start interrogating my wife and kids because it was probably dropped at some point when I wasn't looking (FWIW, it shows no outward signs of damage). EDIT: I just discovered some damage of some sort - the aperture lever is loose. I can physically push and pull it into the mount, and its all 'wobbly' now. I know it wasn't like that when I shelved the thing!

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03-31-2017, 01:37 PM	#2
MarkJerling Moderator Site Supporter Join Date: May 2012 Location: Wairarapa, New Zealand Photos: Gallery \| Albums Posts: 20,406	Contact user dcshooter - He may have advice for you.

03-31-2017, 02:31 PM	#3
UncleVanya Loyal Site Supporter Join Date: Jul 2014 Photos: Gallery \| Albums Posts: 28,404	@dcshooter he needs some advice and I'm hoping you can give it.

03-31-2017, 03:13 PM	#4
.a.t. Site Supporter Join Date: Mar 2011 Location: yesterday Photos: Gallery Posts: 1,261	Originally posted by Sagitta . . . EDIT: I just discovered some damage of some sort - the aperture lever is loose. I can physically push and pull it into the mount, and its all 'wobbly' now. I know it wasn't like that when I shelved the thing! Until someone more experienced than I chimes in, I'll speculate a bit: Before I reached your edit, I was wondering if the aperture was functioning normally? I had a similar situation with a Pentax 135mm lens. The focus ring was suddenly frozen. And the aperture also suddenly wouldn't stop down at all. It turned out that an internal pin which was part of the aperture mechanism had broken and lodged in the focus helical gear. By removing the mount I was able to see the problem and get the broken piece out. The lens could then focus, but the aperture wouldn't work without the pin that broke. If you have a set of JIS screwdrivers (link), I'd suggest removing the mount and taking a look around. To make re-installation easier, note the arrangement of the levers, etc. as you lift the mount off. And be careful not to remove the aperture ring in the process or you might lose the ball bearing that gives the ring its click stops. I have a Vivitar 300mm f/5.6 lens which is, I believe, about the same as your lens (yours is K mount, right?). Except mine has a screw mount. So the innards of your lens will probably be different from mine, but here's a picture of my lens with its mount removed: (click the image for a larger size) Last edited by .a.t.; 03-31-2017 at 04:58 PM. Reason: another clarification

03-31-2017, 05:58 PM	#5
edmundrt Veteran Member Join Date: Jul 2014 Location: Nagoya Posts: 577	If it was made by a company like Tokina and sold as rebrands, it may be something like a T4 or TX interchangeable mount. If this has come a little loose, it may cause the problems you've been describing. Picture of the mount could be useful, someone more expert than me might be able to help!

03-31-2017, 06:19 PM	#6
niceshot Site Supporter Join Date: Mar 2014 Location: NY Photos: Gallery Posts: 1,564	your focus barrel or the sleeve around the helical has slipped and is out of adjustment.Should be and easy fix. Roll off the rubber grip on the focus sleeve and you should find three flea sized screws. Back them out a little and adjust your focus. Good luck Oh one other thing do not use a lot of force on the helical if you do things will go awry

03-31-2017, 09:34 PM	#7
Sagitta Veteran Member Join Date: Feb 2011 Location: Maine Photos: Gallery Posts: 3,081 Original Poster	Originally posted by .a.t. Until someone more experienced than I chimes in, I'll speculate a bit: Before I reached your edit, I was wondering if the aperture was functioning normally? I had a similar situation with a Pentax 135mm lens. The focus ring was suddenly frozen. And the aperture also suddenly wouldn't stop down at all. It turned out that an internal pin which was part of the aperture mechanism had broken and lodged in the focus helical gear. By removing the mount I was able to see the problem and get the broken piece out. The lens could then focus, but the aperture wouldn't work without the pin that broke. If you have a set of JIS screwdrivers (link), I'd suggest removing the mount and taking a look around. To make re-installation easier, note the arrangement of the levers, etc. as you lift the mount off. And be careful not to remove the aperture ring in the process or you might lose the ball bearing that gives the ring its click stops. I have a Vivitar 300mm f/5.6 lens which is, I believe, about the same as your lens (yours is K mount, right?). Except mine has a screw mount. So the innards of your lens will probably be different from mine, but here's a picture of my lens with its mount removed: (click the image for a larger size) I paid 20 bucks for the lens, I'm not TOO upset. I'm more irritated about the time it took to find it where someone wasn't asking LOL prices for the thing. Now that I know it was the lens and not the camera I feel better.

04-04-2017, 07:20 PM	#8
Sagitta Veteran Member Join Date: Feb 2011 Location: Maine Photos: Gallery Posts: 3,081 Original Poster	Originally posted by dcshooter My ears are burning! Here's my guess as to what it is. When you turn the aperture ring, it moves a cam with a peg on it against an armature that pushes a little lever that determines how far open or closed the iris is. when you are in auto mode, it is spring loaded, with the pin moving the arm until it stops against the lever. Moving the switch to M pushes a stop in place so it's always as if the pin is pressed in (the mechanism used varies from lens to lens). If the mount is simply held on by screws, you should be able to just remove them and pull the mount off and see what is happening pretty easily. My theory is that something is wrong with the aperture arm. Either it's jumped the peg, come loose from the lever it moves, or one of the springs is busted or a screw holding the spring mechanism is loose., allowing the armature to just flop around. The reason the lens is bottoming out before infinity is that the rear element is hitting the now loose armature on its way back. The reason the A/M switch is all wobbly is that there is no spring pressure from the armature pushing against the stop it controls. Sounds about right, except theres no A setting in the picture here - the lens is straight up manual. Whats wobbly is the actual aperture lever sticking out of the mount itself. Its come loose. I might take it apart if I can get a set of decent jewelers drivers some day. Not so much in hopes of saving it as just out of nosiness at this point.

04-04-2017, 07:36 PM	#9
niceshot Site Supporter Join Date: Mar 2014 Location: NY Photos: Gallery Posts: 1,564	Originally posted by Sagitta Sounds about right, except theres no A setting in the picture here - the lens is straight up manual. Whats wobbly is the actual aperture lever sticking out of the mount itself. Its come loose. I might take it apart if I can get a set of decent jewelers drivers some day. Not so much in hopes of saving it as just out of nosiness at this point. i had a similar problem with a ROKINON 135MM 2.8 Lens. What happened was the focus barrel spun and It went well past Infinity and unbeknownst to me I drove the aperture control lever into the mount and blew up the aperture .Well that was the first Lens I rebuilt entirely .Now I have a few hundred under my belt, but then it was a real challenge, now it would be a breeze. JIS screw drivers ,and a lens spanner and high powered glasses is what you need. Oh and lots of patience. Don't try to focus it any more until you fix it.