Pentax-L 43mm f1.9 - Focus Problem

[Chris Bail]

I recently bought a nice user Pentax-L 43mm f1.9 from another RFF'er (little bit of paint loss on the barrel, really nice glass, smooth focus and aperture selection) but I've noticed a problem with it on my M8, and I'm wondering if anyone here can help me sort this out exactly.

I will also say that the seller offered to take it back, but considering the length of time it took to get to me, the relative scarcity, and the fact that I picked up a nice user lens at a nice, user price (and don't want to have to start looking again) left me wondering if anyone here can help me figure out what I need to do with the lens to get it working properly.

As soon as the lens was out of the box, I was shooting pictures with it, and I noticed right away that it seemed to be back focusing, so I did some further test shots and investigation. I noticed when testing it at infinity that the RF patch doesn't quite line up. It stops just short of marrying the two images at infinity. It's not terrible, but it's noticeable. (It needs just a smidge more throw in the focus to make the images in the RF/VF line up) The images at infinity seem adequately sharp and in focus. At least as sharp as my CV 35mm and Canon 50's

After realizing that the images weren't lining up at infinity, I started taking some more test pictures, and intentionally focused just a smidge closer than I wanted to, and my subsequent test shots showed that my sharpness at the point of focus was really good to my eye (for as inaccurate and unscientifically as I was in my "focus correction")

I don't have any similar issues on any other lenses (even the Canon 50/1.2 is sharp at the point of focus when the RF images are aligned) and all of my other lenses have RF images that seem to line up properly at infinity. (Summicron 40, CV 35/1.2, Canon 50/1.2, Canon 50/1.4.)

This leads me to believe that the lens elements are in the right place, (since it is focusing properly at infinity, and everywhere in between, as long as I throw the focus a smidge closer than the RF patch indicates), and everything is working as it should, with the exception of the focusing ring on the back of the lens not quite interacting with the RF cam right.

Does this make sense to those of you that are more experienced with RF lenses? If so, is there a way to fix this problem easily?

The lens doesn't seem to need a CLA, so I'm not inclined to ship it off for repair right away, and I don't mind doing a simple repair myself if it's a matter of loosening a couple screws, making an adjustment and putting it back together again. I'm just not 100% sure where to start.

I'm happy to hear any helpful advice that anyone might have. For now, I'm planning on using it as is and compensating my focus.

I've attached some samples. All were shot wide open, with the first few at close to minimum focus. No PP other than 100% crop.

Focused on the allen bolt at center with RF images aligned (100% crop)

L1011906 by Chris Bail, on Flickr

Same as above, cheated and focused a little forward

L1011907 by Chris Bail, on Flickr

Focus on the light switch slider with RF images aligned (100% crop)

L1011883 by Chris Bail, on Flickr

Same as above, cheated and focused a little forward

L1011884 by Chris Bail, on Flickr


Completely usable as is, so long as I remember to cheat the focus a little...but I'd like to figure out if this is fixable. (Focused on the the sign on the right and cheated the focus forward a little)

Leica M8 - Pentax-L 43mm f1.9 by Chris Bail, on Flickr

 

[johannielscom]

Before you do anything, switch out the adapter with other ones you have, or borrow other adapters to test. If yours is a Leitz adapter it should be within critical specks, if it's a non-Leitz adapter it may likely be the culprit.

Trying it on a screw mount Leica to see if it is spot-on there might be a nice idea.

 

[john neal]

sounds like you need a smidge more thickness in the adapter ring, but it will be a very small amount, so another (less worn?) adapter may do the thick. I have found that some of the off-brand adapters (Kobalux?) are slightly thicker than the Leitz ones, which can give problems with LTM lenses that are correctly shimmed (!)

 

[Chris Bail]

Before you do anything, switch out the adapter with other ones you have, or borrow other adapters to test. If yours is a Leitz adapter it should be within critical specks, if it's a non-Leitz adapter it may likely be the culprit.

Trying it on a screw mount Leica to see if it is spot-on there might be a nice idea.

I tried the lens on my M8 with all three of the CV adapters I have, with similar results (on the M8). I don't have any Leitz adapters to check it with yet.

I also tried the lens on my CL and my IIIa, and found it hard to tell if everything was lining up at infinity, It appears that the lens is doing the same thing, but it is not nearly as dramatic. Maybe it's the shorter base lengths of those two cameras, but it looks to be lined up fairly well. (not perfect, but pretty close).

I'll try putting a loupe up to the eyepieces of the other two cameras and check again in better light. I'd say that it is doing the same thing on the other two cameras, but the difference in the images is so small that I'm finding it hard to see.

Thanks for the input!

 

[Chris Bail]

sounds like you need a smidge more thickness in the adapter ring, but it will be a very small amount, so another (less worn?) adapter may do the thick. I have found that some of the off-brand adapters (Kobalux?) are slightly thicker than the Leitz ones, which can give problems with LTM lenses that are correctly shimmed (!)

A smidge more, or a smidge less? I can simulate a smidge more by loosening the lens in the mount slightly, correct?

I figured I needed a smidge less, as the cam isn't traveling far enough back to make the images line up at infinity.

As it is now, the lens already screws onto the adapter slightly past vertical, as do both of my Canon LTM lenses, though those don't show any focus issues, and infinity lines up correctly.

 

[ferider]

I had to think about this for a second, Chris. I believe you need to thin the adapter a little. Before you go and do this, confirm by doing the opposite: unscrew the lens from the adapter just a little, and see if the back-focus gets worse. If that's true, the adapter is too thick.

You can either buy a thinner adapter (Voigtlander type I adapters are a little thinner than type II), or use some sand-paper and sand down an existing adapter until infinity aligns. Should be easy since you have an M8 .... I had to do this a couple of times with film test rolls ....

Roland.

 

[Platinum RF]

focus shift?

 

[n5jrn]

I had to think about this for a second, Chris. I believe you need to thin the adapter a little. Before you go and do this, confirm by doing the opposite: unscrew the lens from the adapter just a little, and see if the back-focus gets worse. If that's true, the adapter is too thick.

I think it's the other way 'round. One focuses closer by moving the lens further away from the film or sensor; that's why extension rings work. So if he unscrews the lens a bit, he will be able to make the lens focus at the plane where the rangefinder aligns the images.

Note that since this was manufactured as an SLR lens (Pentax do not make lenses for rangefinder cameras, do they?), it really wasn't super-important to have a given plane of focus align with a certain precise position on the focusing helicoid. My guess is that issues like this pretty much come with the territory when using SLR lenses on an RF camera.

 

[ferider]

I think it's the other way 'round. One focuses closer by moving the lens further away from the film or sensor; that's why extension rings work. So if he unscrews the lens a bit, he will be able to make the lens focus at the plane where the rangefinder aligns the images.

You forgot that the Cam moves with the adapter thickness:

1) adapter thickness makes no difference for 50mm lenses (actually 51.6)
2) for tele lenses backfocusing means the adapter is too thin
3) for wides backfocusing means the adapter is too thick

Complicated ? Best is to confirm with the experiment I suggested.

Roland.

 

[n5jrn]

You forgot that the Cam moves with the adapter thickness:

1) adapter thickness makes no difference for 50mm lenses (actually 52)
2) for tele lenses backfocusing means the adapter is too thin
3) for wides backfocusing means the adapter is too thick

Complicated ? Best is to confirm with the experiment I suggested.

I'm not the OP, so I can't confirm it with an experiment, but I am at a loss to see how the focal length of a lens would make any difference: racking a lens away from the film/sensor plane always makes it focus closer. Or am I missing something here?

 

[ferider]

I'm not the OP, so I can't confirm it with an experiment, but I am at a loss to see how the focal length of a lens would make any difference: racking a lens away from the film/sensor plane always makes it focus closer. Or am I missing something here?

Yes. You rack it away, you rack the focus cam away too, and the viewfinder will show you closer focus. Leica rangefinder cams are calibrated to move back and forth by the same amount a 51.6 mm lens would move, to be exact.

Makes sense ? Thanks.

 

[n5jrn]

Yes. You rack it away, you rack the focus cam away too, and the viewfinder will show you closer focus. Leica rangefinder cams are calibrated to move back and forth by the same amount a 51.6 mm lens would move, to be exact.

Makes sense ? Thanks.

Sorry, it still doesn't make sense to me. I suspect we're talking at cross-purposes somehow, or I'm not grasping some key aspect of what an M-to-K adapter must do. I suspect a little playing with such an adapter myself would enable me to figure out what you're trying to say, but I don't have one.

So forget it. It will be easy enough for the OP to test for himself which one of us is correct.

Best regards.

 

[Chris Bail]

I had to think about this for a second, Chris. I believe you need to thin the adapter a little. Before you go and do this, confirm by doing the opposite: unscrew the lens from the adapter just a little, and see if the back-focus gets worse. If that's true, the adapter is too thick.

You can either buy a thinner adapter (Voigtlander type I adapters are a little thinner than type II), or use some sand-paper and sand down an existing adapter until infinity aligns. Should be easy since you have an M8 .... I had to do this a couple of times with film test rolls ....

Roland.

This makes sense. I'll try this experiment and sand down the adapter if necessary.

I'm pretty certain this isn't focus shift, as the problem, focus is showing closer than it really is, is consistent from near to infinity and shows up consistently with the lens stopped down as well.

Thanks everyone for the input, and I'll update with whatever I find out for posterity.

 

[Chris Bail]

I performed the experiment, and backfocus seemed worse, so I proceeded with sanding the adapter.

I went ahead and used a piece of very fine grit sandpaper laying flat on the table to sand down the face of my 35mm CV LTM adapter, rinse, repeat, etc, until the images lined up at infinity.

The images now line up properly at infinity, but infinity focus is off when the lens barrel indicates infinity. I now have to focus a little closer than infinity to have it properly focused. Back focus is now evident at every point from near to infinity, with the same amount of throw (as far as I can tell) to cheat the lens to focus on what I want.

So, what I think I've done, is move the lens elements closer to the sensor, right along with the part that interacts with the cam. Therefore, infinity lines up, but is not actually in focus, and I can compensate by focusing closer like I had been for other points of focus. Essentially, I've lost a little bit on the near focus (because I can't compensate and focus closer), and have a slightly thinner adapter, but still have the problem of the lens not interacting with the RF cam right.

I think it must be the lens. Not the elements, as on the stock, unsanded adapter, it focused great at infinity, but the part that runs along the cam.

Anybody here have any idea who can work on one of these lenses? Is this something that can be easily adjusted? (The bit of metal around the rear element that interacts with the RF cam)

Thanks for the input up to this point. I was really hoping it would be as simple as sanding down the adapter...sadly, it seems it isn't.

 

[Landshark]

I'd disagree that the thickness of the adapter has no effect on focus. It physically moves the lens in or out. If the adapter had no effect on focus, the changing of thickness would have no effect. Which it does.

I'm not familiar with the particular lens.
But! With many lenses the focusing ring can be adjusted independently of the focus.
If this is true of the 43/1.9 there will be three ~1mm set screws around the ring.
If there's a rubber grip they're frequently hidden under the grip.

If it has the screws.
Set the lens to infinity, slightly loosen the screws and move the focusing ring in the direction it needs to go and snug up the screws.

 

[Freakscene]

focus shift?

The Pentax 43 doesn't have appreciable focus shift. Focus shift occurs when you stop down and focus shifts from the point of focus due to residual spherical aberration. What this OP is describing is a lens that doesn't focus correctly.

I found the same thing with my Pentax 43 on a Voigtlander Type II adaptor. Getting a Leitz adaptor fixed the problem. I couldn't find a Voigtlander Type I adaptor.

Marty

 

[ferider]

I performed the experiment, and backfocus seemed worse, so I proceeded with sanding the adapter.

I went ahead and used a piece of very fine grit sandpaper laying flat on the table to sand down the face of my 35mm CV LTM adapter, rinse, repeat, etc, until the images lined up at infinity.

The images now line up properly at infinity, but infinity focus is off when the lens barrel indicates infinity. I now have to focus a little closer than infinity to have it properly focused. Back focus is now evident at every point from near to infinity, with the same amount of throw (as far as I can tell) to cheat the lens to focus on what I want.

So, what I think I've done, is move the lens elements closer to the sensor, right along with the part that interacts with the cam. Therefore, infinity lines up, but is not actually in focus, and I can compensate by focusing closer like I had been for other points of focus. Essentially, I've lost a little bit on the near focus (because I can't compensate and focus closer), and have a slightly thinner adapter, but still have the problem of the lens not interacting with the RF cam right.

I think it must be the lens. Not the elements, as on the stock, unsanded adapter, it focused great at infinity, but the part that runs along the cam.

Anybody here have any idea who can work on one of these lenses? Is this something that can be easily adjusted? (The bit of metal around the rear element that interacts with the RF cam)

Thanks for the input up to this point. I was really hoping it would be as simple as sanding down the adapter...sadly, it seems it isn't.

Sorry, Chris, agree, don't think it can be solved via simple adapter collimation.

Don@DAG or John@Focalpoint do work on non Leica lenses.

Roland.

 

[Chris Bail]

The thickness of the adapter certainly made a difference in focus. I'm thinking that the lens is off. Not the optics, as the lens focuses on infinity, when the focus mark on the lens barrel is at infinity. (On the adapter before I sanded it) but the RF doesn't show that infinity is in focus.

Sanding down the adapter made the RF look correct, but infinity focus was off, because the lens element was then a little closer to the sensor.

What I think I need to figure out, is how to adjust the part of the lens that interacts with the cam...after getting myself a genuine leitz adapter and finding someone else's digital M to try it all out on maybe.

 

[Chris Bail]

Thanks ferider. I'll get in touch with DAG.

 

[n5jrn]

The thickness of the adapter certainly made a difference in focus. I'm thinking that the lens is off.

That makes sense to me. If the correlation between where the RF cam thinks the lens is focused at and where the lens actually is focused at is off, you can affect the focus by messing with the adapter but the place the RF thinks the focus at will shift, too, and things will still be off.