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09-12-2009, 06:31 AM   #1
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Pentax LX focusing problem

Hi there!

I'm new here, I registered here to ask if someone know what's wrong with my LX camera.

The focusing is a bit "off". Focusing at something 1 meter away, when it's sharp in the finder, the lens meter scale says 1.6 meters. The problem is more apparent when focusing at something farther away. When using SMC Pentax-M 50/f1.7, focusing at a house across the street from me (more than 15 meters away), the lens is set on infinity focus, but the split prism/micro prism shows it unsharp. I managed to take a photo through the lens to illustrate this.



What can be the cause of this?

09-12-2009, 08:13 AM   #2
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That is part of the symtoms of the "sticky mirror syndrom"

The mirror return bumper has a rubber pad on it that has degraded with time (common with LX's). The mirror is now further away from the lens then it should be, and this changes the focus.

You might also notice the focus at the bottom of the viewfinder is not the same as the focus at the top.

You might want to send you LX in for a CLA.

I had mine done by Eric Hendriksen. Here is his link:

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09-12-2009, 08:59 AM   #3
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Well wait a minute, not necessarily. If the resulting photos are also out of focus, then it's the mirror, but if the photos are in focus and it's just the distance scale that's out of whack, then the lens is the problem.
09-12-2009, 09:03 AM   #4
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As KungPOW said it's part of the sticky mirror syndrome. However it can be helped in a couple of ways, the best of course being sending it in for a CLA (Cleaning Lubrication Adjustment). My LX has the same problem so I have done a bit of research on where to best do a CLA in Sweden and LP Foto in Stockholm seems to be a good place.

If you are, like me, a cheapskate you can temporarily remedy this on your own. The problem as said is that the mirror is further away from the lens than it should be, now a small piece (about 2x2mm) of normal rubber washing glove will help that problem. And you should not need to glue it because the rubber is already sticky enough to keep it on its own. This in combination with fine tuning the focus did the trick for me. The Pentax LX service manual will show how to adjust the focus, it's quite easy.

09-12-2009, 10:47 AM   #5
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QuoteOriginally posted by KungPOW Quote
The mirror is now further away from the lens then it should be, and this changes the focus.


Correct me if I'm wrong but I just don't get how could the position of the mirror influence the focus. The focus is determined by the distance between your subject and the image plane (here the focusing screen). The image what is projected onto the matte screen cannot, in my opinion, be focused by moving the mirror closer or further, because the mirror only changes the direction of the light beam, it doesn't alter the distance between the optical elements.

I'd think that this problem is more likely caused by improper installation of the matte screen. Unfortunately I have the same problem with my newly aquired ME Super

I examined the matte screen closely and I saw 2 tiny screws on its sides. Do you think that those are for adjustment?
09-12-2009, 11:55 AM   #6
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KungPOW and Jimfear, your replies were most helpful. I had heard the expression sticky mirror before, but didn't know quite what it was. The seller had specified no sticky mirror problems had been discovered, so when I first got it, it didn't occure to me that the information might have been wrong.
But now I've looked it up, and it seems like these symptoms most probably are caused by sticky mirror.
And thanks Jimfear again for refering to a photo store so close to where I live. I've bought an ME Super there once, shouldn't be hard to find my way back there again.

09-12-2009, 12:28 PM   #7
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Yes, an out of alignment focus screen can also cause focusing problems. But the most common reason for focus issues with an LX that has not had the mirror bumpers replaced, is the alignment of the mirror

If the Mirror drops 1° the focal length on the lenses center axis, to the focusing screen will increase 0.57mm. This does not change the focal length to the film.

The result is that you can no longer focus to infinity, and the distance scale will be off.
10-08-2009, 09:55 AM   #8
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QuoteOriginally posted by KungPOW Quote
You might also notice the focus at the bottom of the viewfinder is not the same as the focus at the top.
I recently acquired a LX.
Tested with Carl Zeiss 85/1.4 & 35/2.4. The results with the 35/2.4 looks pretty okay, but not with the 85/1.4 as I found the main subject is out of focus although it was in-focus from the view finder.

Contacted the seller and he advised I should perform the Diopter Correction. However I do not think it has anything to do with Diopter as it is for focusing convenience for eyeglass wearers.

From the instruction manual, somehow I do not understand what it means by turning the adjusting screw until the Border Lines of the center spot come in sharp & crisp. The FA-1 Viewfinder has a horizontal split with prism around it.

What is the "Border Lines"?

I mounted a M50/1.4 and focus about 6 feet from the subject. From the viewfinder, the center is on focus, but the top & bottom are out.

Turn the focus ring clockwise, the top is became clear, but the center is out as well as the bottom. I then turn the focus ring counterclockwise so that bottom is clear.

Appreciate someone can advice if the problem is due to the mirror or something else.

Thanks.

10-08-2009, 06:23 PM   #9
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You need to adjust the diaopter to match your eyesight even if you don't wear glasses.

The "border lines" are the edges between the mat-glass and the micro-prism area. I try to adjust my viefinder until I can clearly see the texture of the focusing screen.

The viewfinder itself does not have the split prism. That is on the focusing screen, and it is removable.

The problem you discribe could either be the mirror is out of alignment due to sticky mirror syndrom, or the focusing screen is out of alignment. Or both.

The problem will be more aparent with longer lenses then shorter. That explains your 35 vs 85 observation.

The best thing to do is send the camera in for a CLA. I had my LX done by Eric Henriksen. Very happy with the results.

his website: Home
10-09-2009, 08:11 AM   #10
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A good way to determine if the mirror alignment has gone off is to see if the focus in the viewfinder is not the same at the top of the finder from the bottom.
When my first LX went out of whack, if the center (this is looking through the finder, not the resulting image) was in focus, the top and bottom was slightly out of focus.
The mirror rests on a small adjustable arm that has a rubber bumper on it. I used a small piece of rubber from an old motorcycle inner tube to replace the bumper and then adjusted the height of the arm until the top, center and bottom of the viewfinder came into focus. I cemented the new bumper in with contact cement.
When I acquired two more LX bodies later on, I sent all three to Pentax to have all the perishable parts replaced with new and to have them all calibrated.
10-09-2009, 06:06 PM   #11
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Thanks guys for your help.
It is indeed the mirror alignment has gone off since the focus in the viewfinder is not the same at the top of the finder from the bottom.
Until decide sending for CLA, I try to DIY the mirror alignment first.
10-09-2009, 11:35 PM   #12
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Since the holder for the rubber bumper is an L shape bracket, it form a small U shape when is mounted to the side of mirror box, I found 2 methods to that may solve the mirror alignment and sticky mirror problem.

Avoid totally removing the sticky stuff as it act as adhesive.

1)With a cutting board, sharp cutter & steel ruler, stick some PVC electrical tape on the cutting board. Cut a strip or two of about 3mm width and about 10-15mm length. Wound it around the holder of the rubber bumper to form thickness of about 0.5mm thick. Check the lens if can focus to infinity when the focus ring comes to a complete stop. If not, unwind the PVC electrical tape and cut another strip longer or shorter to ensure you achieve infinity. Have not tried this method but it seems logically.

For method 2, I done it and tested okay. Prefer using the O-Ring since it is made from rubber.

2)Using an O-ring with 2mm inner diameter with thickness preferably 0.5mm. (since I cannot find a shop that carries 0.5mm thick, I bought a 1mm type and cut to 0.5m on one side with a shaving blade) Loop it around the bumper holder with the 0.5mm thickness facing the mirror. The sticky stuff will hold it in place, if not; use double sided tape between the O-Ring and holder.
01-19-2010, 04:49 PM   #13
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QuoteOriginally posted by Septrain Quote
Since the holder for the rubber bumper is an L shape bracket, it form a small U shape when is mounted to the side of mirror box, I found 2 methods to that may solve the mirror alignment and sticky mirror problem.

Avoid totally removing the sticky stuff as it act as adhesive.

1)With a cutting board, sharp cutter & steel ruler, stick some PVC electrical tape on the cutting board. Cut a strip or two of about 3mm width and about 10-15mm length. Wound it around the holder of the rubber bumper to form thickness of about 0.5mm thick. Check the lens if can focus to infinity when the focus ring comes to a complete stop. If not, unwind the PVC electrical tape and cut another strip longer or shorter to ensure you achieve infinity. Have not tried this method but it seems logically.

For method 2, I done it and tested okay. Prefer using the O-Ring since it is made from rubber.

2)Using an O-ring with 2mm inner diameter with thickness preferably 0.5mm. (since I cannot find a shop that carries 0.5mm thick, I bought a 1mm type and cut to 0.5m on one side with a shaving blade) Loop it around the bumper holder with the 0.5mm thickness facing the mirror. The sticky stuff will hold it in place, if not; use double sided tape between the O-Ring and holder.
That seems like a good solution- but I would hesitate to use electrical tape, no matter how convenient- perhaps Scotch Magic tape instead?- as with electrical tape the glue migrates and weeps quite quickly. Where might this end up? Cleaning this glue from out of a mirror box would be my worst nightmare.
01-19-2010, 06:20 PM   #14
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QuoteOriginally posted by h734790 Quote


Correct me if I'm wrong but I just don't get how could the position of the mirror influence the focus. The focus is determined by the distance between your subject and the image plane (here the focusing screen). The image what is projected onto the matte screen cannot, in my opinion, be focused by moving the mirror closer or further, because the mirror only changes the direction of the light beam, it doesn't alter the distance between the optical elements.
No. Of course, in a perfect camera the distance from flange-to-film = distance flange-to-focusing-screen (via the mirror.)

If the mirror's not resting in its correct position - in the case on a sticky-mirrored LX, where the mirror is resting at a sharper angle than it should be - the flange-to-mirror distance is, of course, going to be longer than it should be, thus making the flange-to-focusing-screen distance longer than it should be, and therefore making out-of-focus-images when the scene appears in-focus.

Given that the OP says that when the lens is focused at infinity gives sharp pictures, but the microprism is out of focus, it's a fair bet that the mirror's sitting farther back than it should, as the flange-to-screen distance has been increased, resulting in the lens appearing that it's unable to focus on objects at infinity.

Septrain, keep in mind that the farther away your focus, the greater the depth of field - this is why the problem of you not-sharp-across-the-screen gets everything in focus the farther away you focus. It does sound like a focus screen problem, not a curvature-of-field problem (certainly not for a Pentax 1.4 50mm, surely!)
01-20-2010, 07:14 AM   #15
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I had this exact problem with my Mamiya 645 Super. The mirror was resting in the wrong place, lower than it's supposed to I think. I had it repaired.
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