I think that my FM2 is back focusing. How to fix?

[rpavich]

For whatever reason my beloved FM2 is back focusing quite a bit. Taking a "head / shoulders" portrait with a 50mm at about a meter away results in about 1" or better of back focusing.

Now, I use a single lens the whole roll so I'm swapping out the lens to make sure it's not that but in the mean time, can anyone tell me how to determine if it's the camera and if it is, how to fix it?

I've dropped it a few times, not big drops (not 5' down to the pavement or anything) but I've bumped it off of the car seat onto the floor a few times. I'm not sure if that is what caused it.

 

[Ian Grant]

It'll be the position of the focus screen, it won't be the lens as it's manual focus. Yoiu need to reposition the screen.

Ian

 

[rpavich]

It'll be the position of the focus screen, it won't be the lens as it's manual focus. Yoiu need to reposition the screen.

Ian

Thanks Ian..that helps.

Is there some test that I can do to determine how much it's off and how to adjust it? I'm guessing that it's just shimming with some thin material.

Other than taking pictures and developing them, how can I test and adjust in real time?

 

[derelict]

I suspect my MX is doing this as well. I need to scan in some frames to double check. I would love to know how to remedy this as well.

 

[Ian Grant]

Thanks Ian..that helps.

Is there some test that I can do to determine how much it's off and how to adjust it? I'm guessing that it's just shimming with some thin material.

Other than taking pictures and developing them, how can I test and adjust in real time?

I doubt it needs adjusting, it's more likely just not seated properly after it's been dropped. Same goes for the MX. I had an MX where the screen wasn't seated properly I just took it out cleaned and refitted.

Ian

 

[rpavich]

Ok...so I took the lens off and poked at the frame that holds the screen in and the screen is not being held

I doubt it needs adjusting, it's more likely just not seated properly after it's been dropped. Same goes for the MX. I had an MX where the screen wasn't seated properly I just took it out cleaned and refitted.

Ian

Thanks Ian..I'll start there.

 

[darkroommike]

Some cameras with interchangeable screens will permit mounting the screen in either the correct Fresnel down or incorrect Fresnel up position, on a film camera always start by checking the screen.

 

[derelict]

Some cameras with interchangeable screens will permit mounting the screen in either the correct Fresnel down or incorrect Fresnel up position, on a film camera always start by checking the screen.

Expound upon this? I do not think that there is a wrong way due to the tab on the Pentax focusing screen but it is worth a look.

 

[rpavich]

Some cameras with interchangeable screens will permit mounting the screen in either the correct Fresnel down or incorrect Fresnel up position, on a film camera always start by checking the screen.

Mine was doing fine up until I just noticed it wasn't. Either I never noticed this before or something happened to it to change it.

 

[rpavich]

found this:

 

[ciniframe]

Not to say this is the problem but, I had a brand new Fujica ST701 (cheap camera, I know) in the 70's that had the focus off. On this camera the angle of the mirror was set by a small set screw that was fixed by a dab of sealant. Adjusting this caused the screen and film plane focus to finally come into agreement. I did the adjustment with a 200mm lens to provide greater accuracy. I have no idea how the FM is constructed so this may not apply at all. Just a thought.

 

[darkroommike]

Expound upon this? I do not think that there is a wrong way due to the tab on the Pentax focusing screen but it is worth a look.

I just had a look at the screen on my FE and the tab you grasp is off to one side a bit and cannot be put in upside down (and unlike I said earlier the Fresnel foes in in an "UP" position, closet to the prism. The frame that holds the screen is notched so that it's almost (never discount human ingenuity) impossible to put the screen in wrong. I have seen other camera systems where there is no tab or the tab is centered, making it easier to put it in the wrong way.

 

[rpavich]

I just pulled my screen out and cleaned it and put it back in. it was sitting crooked when I pulled it out..its' possible that it got slightly cockeyed. When I put it back it was sitting straight before I closed the tray that it sits in.

 

[wiltw]

For whatever reason my beloved FM2 is back focusing quite a bit. Taking a "head / shoulders" portrait with a 50mm at about a meter away results in about 1" or better of back focusing.

Now, I use a single lens the whole roll so I'm swapping out the lens to make sure it's not that but in the mean time, can anyone tell me how to determine if it's the camera and if it is, how to fix it?

I've dropped it a few times, not big drops (not 5' down to the pavement or anything) but I've bumped it off of the car seat onto the floor a few times. I'm not sure if that is what caused it.

you are suffering the 'focus recompose' focus shift. If you shoot with 50mm lens at 5' from subject, to frame a 3.5' x 2.3' area (portrait orientation) and you focus on the eyes and then shift the center of frame to place the head in the upper part of the frame, you are shifting the angle by about 15 degrees (the frame captures about 40 degrees in the long dimension). That causes a shift of the plane of focus, and an apparent focus shift of over 2", which cannot be adequately masked by the DOF of the lens even when it is shot at f/4. Get closer for an even tighter framing, and the DOF shrinks even more, and the angular shift is greater, making for even more apparent focus error.

This is a drawing of the induced error:

 

[rpavich]

you are suffering the 'focus recompose' focus shift. If you shoot with 50mm lens at 5' from subject, to frame a 3.5' x 2.3' area (portrait orientation) and you focus on the eyes and then shift the center of frame to place the head in the upper part of the frame, you are shifting the angle by about 15 degrees (the frame captures about 40 degrees in the long dimension). That causes a shift of the plane of focus, and an apparent focus shift of over 2", which cannot be adequately masked by the DOF of the lens even when it is shot at f/4. Get closer for an even tighter framing, and the DOF shrinks even more, and the angular shift is greater, making for even more apparent focus error.

I know what you mean but I'm not. I wasn't focusing and recomposing. (and that was just one example) I had all my shots from close ups to long distance shots do the same.

 

[rpavich]

Well I did a test. I put the camera on a tripod and pointed it down at a 45° angle at a ruler sitting on my table.
Then i took out the viewfinder screen and cleaned it and put it back.

Then i shot a few frames.

Result: Its about 1" off.

I ordered a focusing screen to use on the film plane to test and adjust/shim...that arrives Saturday.

 

[ic-racer]

It'll be the position of the focus screen, it won't be the lens as it's manual focus. Yoiu need to reposition the screen.

Ian

Yes, and don't forget mirror angle can also be a culprit. Mirror angle can change with time on its own, whereas focus screen position can't change on its own...needs a curious screwdriver wielding owner to happen.

 

[rpavich]

One question; I see that the focusing screen is held in with tension from two spring loaded clips on the holder; it appears that it cannot go higher so my question is; when shimming where do I shim?

One the upper side of the screen and only at the clip locations?

 

[rpavich]

Yes, and don't forget mirror angle can also be a culprit. Mirror angle can change with time on its own, whereas focus screen position can't change on its own...needs a curious screwdriver wielding owner to happen.

this is really starting to look like it's not something I can do myself. I guess I have to get a quote from a repair man.

 

[derelict]

I took a look at my MX and think it is off as well. I am going to do the tripod/ cable release/ ruler test as well to check it. I scanned in a roll I took a few weeks ago and some of the shots were okay while some that should have been spot on were off by a bit.

It was CLA'd by Eric (means something if you are a Pentax shooter) so I might send it back to him to check.

 

[rpavich]

I took a look at my MX and think it is off as well. I am going to do the tripod/ cable release/ ruler test as well to check it. I scanned in a roll I took a few weeks ago and some of the shots were okay while some that should have been spot on were off by a bit.

I'm beginning to think that it was always this way since I've owned it (bought it used off of Ebay.) I see enough "missed back focus" shots in my negs that I think it was and I was random enough in my focusing that some turned out good and so it didn't alarm me. (or that the depth of field was deep enough so that I didn't notice)

 

[derelict]

I'm beginning to think that it was always this way since I've owned it (bought it used off of Ebay.) I see enough "missed back focus" shots in my negs that I think it was and I was random enough in my focusing that some turned out good and so it didn't alarm me. (or that the depth of field was deep enough so that I didn't notice)

I think that that is what I am finding as well. At f/8, it is hardly noticeable. When I am making a low light, wide open, it becomes more noticeable.

I will do my testing this weekend and should know for certain by mid next week whether I am imagining things or if it is truly off.

 

[ic-racer]

I had a Rolleiflex 2000F in the 1980s that was off. The image on the film was in focus behind where the viewfinder indicated. A photograph of a ruler and the simple lens equation [1/p + 1/q = 1/f] gave me an estimate of how high the focus screen needs to be raised. I took 20 layers of tape, measured that and divided by 20 to find out how thick my tape was. I used the appropriate amount of tape on the corners of the focus screen.

Subsequently I try using simpler cameras that are easier to repair (with the same Zeiss lenses). So the service manual diagram is from a Yashica FX3. I knew that camera's lens was collimated to the film [internally the lens is set so it's infinity stop provides perfect focus on the film plane at distant objects]. So, I adjusted the mirror so the distant horizon [infinity] was in focus on the focus screen.

 

[yessammassey]

I had this same problem with my FE2. I tried reseating the focusing screen, adjusting the angle of the reflex mirror, and did this all while comparing the focus through the VF with the film focal plane. To achieve this, I taped a spare FE2 focusing screen onto the film rails and - using a loupe to observe the focusing screen - focused on a ruler with the mirror and shutter locked up with a cable release. I focused on a ruler tick mark, looking through the film focus plane, then released the shutter, observed focus through the viewfinder, adjusted, and repeated, until the two views were in agreement.

I blew through a few rolls of film, making different adjustments, and testing the results. Lots of ruler photos. Even though the VF and film plane focus were in agreement, the mis-focus was always there in the photographs.

I finally got rid of the FE2! Now I have an FM. It has a non-user-removable focusing screen, so I know no previous owners have abused it. Its focus is spot on.

Hope you have better luck than me! Sorry I can't offer any better advice!

 

[Jeff Bradford]

I had this same problem with my FE2. I tried reseating the focusing screen, adjusting the angle of the reflex mirror, and did this all while comparing the focus through the VF with the film focal plane. To achieve this, I taped a spare FE2 focusing screen onto the film rails and - using a loupe to observe the focusing screen - focused on a ruler with the mirror and shutter locked up with a cable release. I focused on a ruler tick mark, looking through the film focus plane, then released the shutter, observed focus through the viewfinder, adjusted, and repeated, until the two views were in agreement.

I blew through a few rolls of film, making different adjustments, and testing the results. Lots of ruler photos. Even though the VF and film plane focus were in agreement, the mis-focus was always there in the photographs.

I finally got rid of the FE2! Now I have an FM. It has a non-user-removable focusing screen, so I know no previous owners have abused it. Its focus is spot on.

Hope you have better luck than me! Sorry I can't offer any better advice!

I had a similar issue with my FE2. After changing the focusing screen didn't solve the problem, I adjusted the spring tension of the pressure plate. Now it appears to be spot-on.