The focus is always wrong when using my 50/1.2 with my pentax lx

[arthurq]

when i shoot wide open, the subjects in my photos always turn out blurry.

it's as if the lens is focused a few inches off from what seems to be in focus in the viewfinder.

i dont notice this with my other lenses, but they have much smaller apertures, so it would be hard to tell if the focus was slightly different than what the viewfinder indicated.

I know that rangefinders can have problems with matching the true focus point of a lens with the rangefinder patch. but an SLR?

I guess what I'm asking is, is it likely a problem with the lens or the camera? how can i check or repair it?

 

[marcr1230]

If anything it's a problem with the body
The distance from lens to mirror to prism might be just enough different then lens to film
I suppose it could also involve how the film is held

The problem reveals itself due to the thin DOF of the lens wide open

 

[erikhaugsby]

Do you have another lens with which to check focus? It could be a problem with the focus screen, as they are interchangeable it might have gotten bumped out of alignment.

 

[mfogiel]

The truth is, it is most likely not the problem of the lens or of the camara, but of the combination of your lens with the focusing screen. The fact is, each lens will interact differently with each type of focusing screen, even if all the distances are properly calibrated.
I always shoot a series of test shots with each lens on the body I intend to use it on. In case of fast lenses, shooting at apertures from wide open to f5.6 is also recommended, as there is often the focus shift to account for as well.
This is why I like Nikon F3, because you can get 37 different focusing screens for it, and hope that one or the other will match your lens.
Have a look at this thread: http://www.rangefinderforum.com/forums/showthread.php?t=137012

 

[Ljós]

Great camera, the lx! However, as you probably are already aware, it was often plagued by sticky-mirror-syndrome:

"A small rest can be seen screwed to, and protruding from the wall of the LX mirror box when the mirror is locked up. The surface of this rest is covered with a pad of elastic material and generally if the shutter is sticky this pad will also show signs of deterioration and have become slightly compressed causing focus inaccuracies. When the mirror rest becomes compressed the mirror is effectively supported at a lower position which alters the plane of focus at the finder screen relative to the film plane. The effect can be seen when photographs captured at wide apertures show a plane of focus in front of the subject which was selected as the plane of focus through the finder or if infinity focus cannot be obtained through the finder.


An advance indication of 'Sticky mirror' syndrome is often experienced as the slow creep of focus errors in prints and slides."
http://users.tpg.com.au/distudio/pentax_lx_problems/

Might be worth checking this possible source of the problem.

Greetings, Ljós

P.S.:

 

[John Hermanson]

When you focus on the horizon (or something 100' away), does it come into focus when the focus ring is at the infinity mark? If not, mirror angle in the camera may be wrong (but then all pictures with all lenses would be out of focus). John

 

[sevo]

Is infinity accurate? Are other distances accurate by scale?

 

[ChrisN]

Great camera, the lx! However, as you probably are already aware, it was often plagued by sticky-mirror-syndrome:

"A small rest can be seen screwed to, and protruding from the wall of the LX mirror box when the mirror is locked up. The surface of this rest is covered with a pad of elastic material and generally if the shutter is sticky this pad will also show signs of deterioration and have become slightly compressed causing focus inaccuracies. When the mirror rest becomes compressed the mirror is effectively supported at a lower position which alters the plane of focus at the finder screen relative to the film plane. The effect can be seen when photographs captured at wide apertures show a plane of focus in front of the subject which was selected as the plane of focus through the finder or if infinity focus cannot be obtained through the finder.


An advance indication of 'Sticky mirror' syndrome is often experienced as the slow creep of focus errors in prints and slides."
http://users.tpg.com.au/distudio/pentax_lx_problems/

Might be worth checking this possible source of the problem.

Greetings, Ljós

P.S.:

Spot-on. I had the same problem with my LX. You can "fine-tune" the position of the pad that the mirror rests on - it is mounted on an arm that has an adjustable pivot.

 

[charjohncarter]

ChrisN is right. Half my cameras are off, but they don't have a f1.2 lens. Here is a discussion of how to test for either.

http://photo.net/learn/focustest/

 

[rick oleson]

That would certainly do it.

I've seen a similar problem when using autofocus cameras with large aperture, manual focus lenses: in that case, it's the AF focusing screens that are designed for brightness with slow zoom lenses and not expected to serve for critical focusing. To optimize brightness at small apertures, the AF screens are composed of microlenses that concentrate their field of view on the central f/4 (or so) of the cone of light coming from the lens, with the result that they are blind to the shallow DOF of larger apertures and what you see in the finder has greater DOF than what you get on the image. But, that would not apply to the LX. The mirror stop is where I would look.

 

[Pioneer]

Have the mirror rest replaced and it should be fine. I am generally quite happy with my LX and 50/1.2, when I do my own part. Remember, that is a very, very narrow depth of field and is not always easy to hit, even when everything is right with the equipment.

 

[Greg_E]

Have you locked it in bulb mode and checked at the film plane with a small ground glass to see if the lens is really in focus? Then you could adjust the lens and maybe put a piece of tape on it so the focus can't move, release the bulb. Now that the mirror is down you could experiment to see if raising the mirror slightly will correct the out of focus issue through the viewfinder. The mirror stops might be worn as suggested above, and slightly raising the mirror with some sheets of paper should confirm this.

 

[ChrisN]

That would certainly do it.

I've seen a similar problem when using autofocus cameras with large aperture, manual focus lenses: in that case, it's the AF focusing screens that are designed for brightness with slow zoom lenses and not expected to serve for critical focusing. To optimize brightness at small apertures, the AF screens are composed of microlenses that concentrate their field of view on the central f/4 (or so) of the cone of light coming from the lens, with the result that they are blind to the shallow DOF of larger apertures and what you see in the finder has greater DOF than what you get on the image. But, that would not apply to the LX. The mirror stop is where I would look.

Again, spot-on Rick. I've had this experience with the 50/1.2 on my Pentax dSLR. The cameras are designed for auto-focus, which is independent from the mirror. I don't think the "focusing screen" is placed at the correct critical height to ensure accurate agreement with the required sharp focus at the sensor.

Luckily the 50/1.2 focuses well enough with the LX; I stick to auto-focus lenses with the dSLR (and fine-tune the focus with the available adjustment for each lens).

 

[rick oleson]

Hi Chris! I encountered this when I put my 50/1.2 Nikkor on a Canon DSLR. After much frustration, I finally fixed it by replacing the screen with a manual focusing type. Took some careful shimming to get the screen positioned well enough to satisfy f/1.2.....