Thread focusing?

Na Horuk · 04-09-2012, 06:51 AM

I'm not sure where to ask this, so I'll post here and hope this has the least chance of being wrong

My old Zenit SLR has a little thread in front of the pentaprism and the thread has a small ball on it. The point is, that when you look through the viewfinder, as you focus, the ball becomes invisible. When you are out of focus, you see it again. What is this type of focusing called? Is this something only ancient SLR's used?

I guess the process is mostly psychological, as your brain will tend to look at the biggest field that it recognizes and that will always be the in-focus parts. But when you are out of focus and everything is blurry, your brain will focus on the ball on the thread.

**RKKS08** · 04-09-2012, 12:31 PM

Are you speaking about a plain round microprism field? Focusing screens with microprism, but without split image were available for most professional bodies of the film area (Pentax KX, MX, LX, MZ-S). You could find them more often in medium format film cameras.
The microprism field would be visible as a circle with a kind of moiree, which would nearly or completely vanish if in focus.

However, this wouldn't explain what you call a "thread".

Na Horuk · 04-09-2012, 03:42 PM

Here is a shot. Notice that neither the camera itself is in focus, nor is the subject the camera is aimed at. Its not a crack and its not on the lens. Its hard to focus on it, though. Hope the pic makes it clearer.

lol or am I misunderstanding something horribly here? I am trying to connect some bits and pieces of information I was told about this quite some time ago.

**RKKS08** · 04-10-2012, 05:16 AM

I think this is either a crack or dirt (or both) somewhere along the optical path. It's nearly in focus now. As far as I know, items on the mirror would not show, but dirt on the focusing screen does. And I don't know how a damage of the mirroring surface of the prism - or dirt on the other 2 surfaces of the prism - would show. Anyway, I don't think it is a "feature".

By the way, which Zenit is it? I still have a broken Zenit am2, which I got from eBay because of the attached Helios 44K-4. But it has a very bright viewfinder with a standard horizontal split screen with microprism ring, and an additional added brighter plain area. It does not show anything like your picture.

Na Horuk · 04-10-2012, 12:55 PM

Its a Zenit 3M. It looks like a piece of string with a knot and slightly frayed at the bottom.

04-09-2012, 06:51 AM	#1
Na Horuk Veteran Member Join Date: Feb 2012 Location: Slovenia, probably Photos: Gallery \| Albums Posts: 11,186	Thread focusing? I'm not sure where to ask this, so I'll post here and hope this has the least chance of being wrong My old Zenit SLR has a little thread in front of the pentaprism and the thread has a small ball on it. The point is, that when you look through the viewfinder, as you focus, the ball becomes invisible. When you are out of focus, you see it again. What is this type of focusing called? Is this something only ancient SLR's used? I guess the process is mostly psychological, as your brain will tend to look at the biggest field that it recognizes and that will always be the in-focus parts. But when you are out of focus and everything is blurry, your brain will focus on the ball on the thread.

04-09-2012, 03:42 PM	#3
Na Horuk Veteran Member Join Date: Feb 2012 Location: Slovenia, probably Photos: Gallery \| Albums Posts: 11,186 Original Poster	Here is a shot. Notice that neither the camera itself is in focus, nor is the subject the camera is aimed at. Its not a crack and its not on the lens. Its hard to focus on it, though. Hope the pic makes it clearer. lol or am I misunderstanding something horribly here? I am trying to connect some bits and pieces of information I was told about this quite some time ago. Attached Images Last edited by Na Horuk; 04-09-2012 at 03:53 PM.

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04-09-2012, 12:31 PM	#2
RKKS08 Site Supporter Join Date: Mar 2010 Location: Frankfurt am Main Posts: 1,241	Are you speaking about a plain round microprism field? Focusing screens with microprism, but without split image were available for most professional bodies of the film area (Pentax KX, MX, LX, MZ-S). You could find them more often in medium format film cameras. The microprism field would be visible as a circle with a kind of moiree, which would nearly or completely vanish if in focus. However, this wouldn't explain what you call a "thread".

04-10-2012, 05:16 AM	#4
RKKS08 Site Supporter Join Date: Mar 2010 Location: Frankfurt am Main Posts: 1,241	I think this is either a crack or dirt (or both) somewhere along the optical path. It's nearly in focus now. As far as I know, items on the mirror would not show, but dirt on the focusing screen does. And I don't know how a damage of the mirroring surface of the prism - or dirt on the other 2 surfaces of the prism - would show. Anyway, I don't think it is a "feature". By the way, which Zenit is it? I still have a broken Zenit am2, which I got from eBay because of the attached Helios 44K-4. But it has a very bright viewfinder with a standard horizontal split screen with microprism ring, and an additional added brighter plain area. It does not show anything like your picture.

04-10-2012, 12:55 PM	#5
Na Horuk Veteran Member Join Date: Feb 2012 Location: Slovenia, probably Photos: Gallery \| Albums Posts: 11,186 Original Poster	Its a Zenit 3M. It looks like a piece of string with a knot and slightly frayed at the bottom.