zenith universal turret viewfinder misfocused

[Cruseiros]

Hy i've bought a universal vf in a street market for about 10$ (15€)
but it seems to be misfocused on some focal lengths does anyone know a good site or can gime me instruction to right focus that?
Fortunately 135mm seems to be godd focused ( i've just bought a 135mmF3,5 canon)
but other lengths 28

 

[ZorkiKat]

Are you referring to what is seen through the eyepiece as being "misfocused"?
Do you see a blurry image at all settings except for 135mm?

Jay

 

[Cruseiros]

exactly: looking trought the eyepiece ( as you will do to compose a photo) i see everything blurry at every settings.

The various lenght sets has a courious indicaton on the external ring (that is used to turn the optics) that seem to be a kind of focus but it desn't work ( e.g. 28mm focal lenght has 3 indications : 1m, infinite, and 28mm).

I've tried time ago to dissassembly it to clean the optics from the inner side: nothing change but luminosity increase

Inside optic group ( the part that doesn't turn) is composed by a couple of prism.
If I've time I'll shoot some photo with the digital camera and post them.

 

[wolves3012]

The scale around the outside is for parallax correction, not for focus. If you disassembled the optics, did you replace the two prisms correctly and did it work after that?

 

[Glauke]

I've had the same problem with mine. All the five lens were out of focus, and there is no way to register the focus correctly. There was another problem: the two prisms were not fixed in their position, shaking the vf it could be heard the sound of their movement.
The secret lies in the eyepiece mount: it can be turned to set the right focus, but sometimes it could be not enough, due to a bad construction of the body.
Since i wear glasses, i finally resolve the problem with the substitution of the eyepiece lens, with one with more relief.
Late, when i will back home, i'll post some pics to explain the problem and the possible solutions.

 

[Cruseiros]

yes i try many positions during reassembling but only one of that works.
Inside to keep prisms in contact there is a cilinrical metal spring. but it seems to be in mint condition so I dindn't try to change it.

Thanks i' ve not tought about parallax correction :-(

 

[Cruseiros]

thanks glauke i'm waiting for photos :-D

 

[Glauke]

Here the finder:

 

[Glauke]

Without eyepiece. The back prism can be see through the eyepiece hole. In my vf this prism isn't fixed in its position, even the presence of the small barrel spring between the two glasses

 

[Glauke]

and here is the original eyepiece lens: note the double-screw barrel with the small lens inside blocked by the flat holed screw

The idea is you can adjust the focus simply turning the eyepiece barrel, but not always it runs.

 

[Glauke]

The lens barrel simply cannot be screwed enough to reach the right focus. Second, the prisms are not fixed in their position.
I think that both problems are due to a bad construction of the vf body: in theory, the lens barrel can go screwed until it touched the glass prism, fixing it and reaching the focus, but in my vf the barrel just stopped against the border of the body. In other words, the prism seat is a bit too narrow and deep to get the lens barrel in the right position.

The simplest solution is to manually enlarge the prism seat, so the barrel can be screwed more deeply. It's not hard, since the body is made of cast aluminium

 

[Glauke]

The second (and hardest) solution is to change the eyepiece lens with another one with a greater focal lenght. It is not difficult, since the barrel can accept much greater lens than the original.
Personally, since i wear glasses, i choose to build a completely custom barrel, with a much greater lens, which resolve all the problems: focus, fixing prism and eye relief
This is the pic of the new barrel, with a rubber O-ring on the prism side

 

[Glauke]

My vf.
Sorry for the quality of the pic (webcam). If any question, ask!

 

[Cruseiros]

Ok but it's strange that 135mm lenght is right focused

 

[Glauke]

Ok but it's strange that 135mm lenght is right focused

Simply because long focals have more tolerance to the wrong focus distance. To be precise, the distance between the lens and the film depends by the distance of the subject. If the subject is at infinity, the focus distance must be exactly the lens focus lenght. If the subject is more close, the focus must be longer.
The variation of the lens-film distance need to get the right focus from infinity to closer distance depends to the lens focal lenght. That's why, for example, Jupiter 11 needs a straight movement of about 2 cm from infinty position to 1 m position, when an Orion 15 needs just 0,8 mm.
The vf is the same: 1mm of off-focus means negligible error with 135mm and completely blurred image with 28mm. Other focals should be in the same condition: 135mm seems perfect, 85mm quite good, 50mm slightly out of focus, 35mm clearly off, 28 completely out

 

[Cruseiros]

ok perfectly explained thanks :-D

 

[Glauke]

Glad to help!