Jump to content


Photo

Krasnogorsk K3 focus problem S16 conversion?

Krasnogorsk K3 focus problem S16

  • Please log in to reply
13 replies to this topic

#1 Glen Brownson

Glen Brownson
  • Basic Members
  • PipPip
  • 14 posts
  • Other
  • London

Posted 01 September 2017 - 09:10 AM

I am having trouble focusing my K3. I've just got some footage back, almost all of it is, at least very soft, a lot of it is clearly out of focus.

 

This is the first batch I have filmed since the camera was converted to S16. The conversion used a re-centered lens mount.

 

The previous footage filmed was much better (not 100% sharp but acceptable).

 

I have used 3 lenses, all with the same problem, Meteor 16-79, Belomo (Peleng) 8mm, and a Sun Actinon 80-200mm. 

 

The image in the viewfinder doesn't seem to come into focus as it should. The zoom lenses are quite poor, especially at the extremes, but do seem better in the middle focal lengths.

 

Could this be an issue with the new lens mounting ring being too thick or thin, causing the focusing plane to have shifted?

 

Any other thoughts / experience?

 

Thanks

 

Glen

 


  • 0


#2 Simon Wyss

Simon Wyss
  • Basic Members
  • PipPipPipPip
  • 1337 posts
  • Other
  • Basel, Switzerland

Posted 01 September 2017 - 12:21 PM

I shudder at the lenses, the camera, and the conversion. Why does everybody need to destroy a camera’s orginal mechanical and optical geometry? 16mm is 4:3.

 

Orthodox film technician here. Orthodox sounds like a toothpaste, though.


Edited by Simon Wyss, 01 September 2017 - 12:21 PM.

  • 0

#3 Glen Brownson

Glen Brownson
  • Basic Members
  • PipPip
  • 14 posts
  • Other
  • London

Posted 01 September 2017 - 01:50 PM

I think I know why you shudder. The Sun Actinon lens, in particular, seems to be of low quality.

 

However, I am not over-burdened with money and this camera / lens set up allows (or hopefully allows if I can cure the problems) for me to produce super 16 film with lenses that should give me flexibility to shoot a wide range of subjects. The standard 16mm footage I shot was satisfactory, but why shouldn't I want to get more from my film? Plenty of cameras, including high-end cameras have been successfully converted to S16.

 

The K3 is cheap, rugged and there is a wide range of M42 mount lenses that are within my budget.

 

If you can tell me where I can find a S16 camera & lens combination that is far superior for a similar price I'd be very interested to hear it. Decent Bolex & lenses are out of my budget.

 

The K3 was produced cheaply, in high numbers and lets be honest, not to the highest of quality. There are many issues with the camera that are well known, so lets not pretend it's the Rolls Royce of the camera world and I have ruined a sacred camera.

 

I do get where you're coming from, and I appreciate that this set up is not the creme de la creme of cine cameras, but If I can get it to function reasonably well it will suit my needs perfectly.

 

Thanks for your input - my response is not intended to sound rude!


  • 0

#4 Dom Jaeger

Dom Jaeger
  • Basic Members
  • PipPipPipPip
  • 1492 posts
  • Other
  • Melbourne, Australia

Posted 01 September 2017 - 07:46 PM

Depending on who did the conversion it's possible that either the lens mount or the gate is a bit off.

But your comment that the image in the viewfinder doesn't come into focus as it should makes me think your problem is in the viewfinder. Since you have used the camera for a while you should know how to focus the eyepiece for your eye in order to focus on the ground glass, but it's possible you're not aware of this. It's the first step with any reflex camera, and the most obvious reason you can't get a sharp image in the viewfinder. If you can't get a sharp image when eye-focussing then you don't know if the image being recorded is sharp and your footage will be soft. You need to get the ground glass surface texture into focus.

If you've already tried that then we can move onto other potential causes.
  • 0

#5 Glen Brownson

Glen Brownson
  • Basic Members
  • PipPip
  • 14 posts
  • Other
  • London

Posted 02 September 2017 - 12:48 AM

The viewfinder ground glass is focused ok.

 

I have removed the re-centering lens mount ring and compared it to the original. The new ring is slightly thicker than the old - meaning it pushes the lens further away from the film. It's very slight (I don't have a micrometer to measure it at the moment).

 

When I put the original ring back on, the lenses seem to focus better, so this could be my problem.

 

I should be able to get the ring machined to correct thickness in a few days.


  • 0

#6 Dom Jaeger

Dom Jaeger
  • Basic Members
  • PipPipPipPip
  • 1492 posts
  • Other
  • Melbourne, Australia

Posted 02 September 2017 - 02:05 AM

If the lens mount is a little thick and the camera flange depth is too long you should still be able to adjust the lens focus scale to get a sharp picture. The zoom won't hold focus as you zoom out, but you should still be able to set sharp focus on near objects at a particular focal length, unless the flange depth is quite a bit out. You should notice that at say 6 ft the lens needs to be set past that mark on the focus scale. The discrepancy will get worse as the focal length gets shorter, but be barely noticed at 200mm. If the flange depth is really way out you will only be able to focus on very close objects (as if you're using an extension tube).

If you're having trouble getting anything sharp in the viewfinder then there is something else happening. For example if the lens mount is skewed you might only see a sharp image on one side, or if the mirror is no longer spinning flatly the image will oscillate etc.

To properly check a reflex camera like this a technician would use a collimator and a reflective backing plate in the gate to check that 1: a correctly set lens is focussing at the film plane and 2: the film plane and the ground glass are both set to the same depth (with the ground glass depth being reflected off the mirror). It's a pretty quick and simple check for a technician, and saves lots of headscratching and conjecture about what might be out of tolerance.
  • 0

#7 Simon Wyss

Simon Wyss
  • Basic Members
  • PipPipPipPip
  • 1337 posts
  • Other
  • Basel, Switzerland

Posted 02 September 2017 - 02:25 AM

my response is not intended to sound rude!

 

Certainly not, it would rather be me to feel awkward with opposing someone’s wish to use Super-16. That’s at least the case with America.

 

My advice: concentrate your investments. One good camera, entirely in order. A good normal lens. A good wide-angle lens. Even at peak performance USSR amateur equipment won’t sell anymore. A Ciné-Kodak Special is better serviceable, will serve you longer, and has a pre-shoot reflex finder. Techs can pull the aperture plate from its holder, if it needs to be Super-16. Dovetailed it can be swapped.


Edited by Simon Wyss, 02 September 2017 - 02:26 AM.

  • 0

#8 Glen Brownson

Glen Brownson
  • Basic Members
  • PipPip
  • 14 posts
  • Other
  • London

Posted 03 September 2017 - 12:33 AM

Dom

I'm happy that the lens is not skewed, focus is even across the image.

The degree of focus does change according to the focal length though.

First step is to get the new lens mount machined to the same thickness as the old one - as the problem has started since it was fitted seems a logical place to start!

Thanks for your help.


  • 0

#9 Glen Brownson

Glen Brownson
  • Basic Members
  • PipPip
  • 14 posts
  • Other
  • London

Posted 07 September 2017 - 03:03 PM

I measured the difference in thickness between old and new mount with a vernier.

0.1mm

Would this be enough to make a difference?

 

One way to find out.

 

0.1mm machined off inner facing side of flange. Machined this side as removing any on the lens side of the flange would mean the lenses would not stop in the right position - for example the zoom leaver may now stop at the 12 o'clock position rather than the 7 o'clock (as seen from viewfinder).

 

This was tougher than expected. Not sure what material the ring is made of, but it doesn't seem to be ally or mild steel, the cutting bit had a hard time!

 

Replaced the ring and tried the lenses - it seems to have done the trick!

 

Only time will tell, but seems better from an initial check.

 

I did have some photos, but could not get them to attach, will add to later post if I can work out what I'm doing wrong.

 

Thanks to Gary Paley of TDC Ltd for carrying out the machining.

Attached Images

  • Screen Shot 2017-09-07 at 21.00.52.png

  • 0

#10 Glen Brownson

Glen Brownson
  • Basic Members
  • PipPip
  • 14 posts
  • Other
  • London

Posted 07 September 2017 - 03:09 PM

Photo of ring in lathe with cutting bit.

Attached Images

  • Screen Shot 2017-09-07 at 21.01.34.png

  • 0

#11 Dom Jaeger

Dom Jaeger
  • Basic Members
  • PipPipPipPip
  • 1492 posts
  • Other
  • Melbourne, Australia

Posted 07 September 2017 - 09:47 PM

In terms of focal flange depth 0.1mm is a huge error. A typical tolerance would be more like 0.01mm. On an 18mm focal length shifting the lens forward by 0.1mm would move the plane of focus from say 10 ft down to 5 ft.


  • 0

#12 Will Montgomery

Will Montgomery
  • Sustaining Members
  • 2018 posts
  • Producer
  • Dallas, TX

Posted 08 September 2017 - 10:58 AM

To properly check a reflex camera like this a technician would use a collimator and a reflective backing plate in the gate to check that 1: a correctly set lens is focussing at the film plane and 2: the film plane and the ground glass are both set to the same depth (with the ground glass depth being reflected off the mirror). It's a pretty quick and simple check for a technician, and saves lots of headscratching and conjecture about what might be out of tolerance.

 

In terms of focal flange depth 0.1mm is a huge error. A typical tolerance would be more like 0.01mm. On an 18mm focal length shifting the lens forward by 0.1mm would move the plane of focus from say 10 ft down to 5 ft.

 

Bingo. Have you seen the thickness of a lens shim? They are so incredibly thin but are used to correct these issues by the tiniest adjustment. Hardest part is having the collimator.


  • 0

#13 Jon O'Brien

Jon O'Brien
  • Basic Members
  • PipPipPip
  • 157 posts
  • Other
  • Brisbane

Posted 08 September 2017 - 06:46 PM

Not about Krasnogorsk, but here's a somewhat related thing. With C mount lenses and how they screw in, for instance on a Bolex H16, isn't this a somewhat imprecise method to mount a lens to get precise FFD? I'm thinking specifically of a Switar 16-1.8 lens for a H16 S-16. Because this is a mildly wide angle lens for S-16, I'm wondering if any focus problems will be difficult to detect looking at the ground glass, and won't be seen until I get the first reel back. 


  • 0

#14 Dom Jaeger

Dom Jaeger
  • Basic Members
  • PipPipPipPip
  • 1492 posts
  • Other
  • Melbourne, Australia

Posted 09 September 2017 - 12:57 AM

A C mount lens screws down to a solid flange, so the accuracy is fine, it's just a bit of a small mount for heavy lenses like zooms and tight focus issues can cause a lens to unscrew.

But while the mount accuracy is up to the job, many C mounts simply aren't correctly set, usually due to people fiddling with them, or sometimes because they were set to a work with a camera that was out. Worn focus threads can also cause enough play to shift the focus noticeably depending on the direction the focus ring is turned.

You're right that it can be hard to judge critical focus on wider lenses just by eye with small or dim viewfinders. Ideally a camera/lens combination should be checked by a technician, especially if bought on eBay with no guarantees. That way you can be sure both the camera flange depth and lens back-focus are correctly set, so if unsure about eye-focus you can rely on the scale.

Edited by Dom Jaeger, 09 September 2017 - 01:00 AM.

  • 0



Also tagged with one or more of these keywords: Krasnogorsk, K3, focus problem, S16

CineTape

Abel Cine

Visual Products

The Slider

Gamma Ray Digital Inc

Tai Audio

Willys Widgets

rebotnix Technologies

Technodolly

CineLab

Glidecam

Paralinx LLC

Aerial Filmworks

FJS International, LLC

Rig Wheels Passport

Metropolis Post

Ritter Battery

Media Blackout - Custom Cables and AKS

Broadcast Solutions Inc

The Slider

FJS International, LLC

Broadcast Solutions Inc

Metropolis Post

CineLab

rebotnix Technologies

Visual Products

Tai Audio

Rig Wheels Passport

Willys Widgets

Technodolly

Glidecam

Gamma Ray Digital Inc

Paralinx LLC

Aerial Filmworks

Media Blackout - Custom Cables and AKS

Abel Cine

Ritter Battery

CineTape