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Glen Brownson

Krasnogorsk K3 focus problem S16 conversion?

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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

 

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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

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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!

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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.

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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.

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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.

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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

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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.

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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.

post-70721-0-05374300-1504814536_thumb.png

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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.

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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.

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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.

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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

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Hello,

I would like to ask for help / advise on the similar issue as the OP of this thread was dealing with. 

 

I bought myself a modified super 16 K3 which is my first film camera and I shot 2 reels with it so far. 

 

I didn't notice that at first as I was amused by how nicely the scanned images looked on the monitor. After some time when I was revisiting the footage I realised that while zooming out from an object that was in focus, once it got completely zoomed out, the whole picture got out of focus. 

 

Could this be caused as I read it here by the conversion to S16? Or maybe some kind of a problem with the meteor 5-1? 

 

Here's the selection of those clips with the zoom out to blur issue:

https://youtu.be/6t9BRVD3e2k

 

I will be very grateful for any useful advice!

I hope it's just some newbie problem that I might not see yet. 

 

Thank you,

Vojtech

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4 hours ago, Vojtech Koblizek said:

Could this be caused as I read it here by the conversion to S16? Or maybe some kind of a problem with the meteor 5-1? 

I would say that is exactly the problem. I'm not against converting cameras, actually I have converted a few including a K3, but you need to know that as soon as you remove the gate to widen it or replace the original mount you are changing the focal flange distance of the camera.

Some people assume that this technique of zooming in, focusing and zooming back should always work and that is not the case. Not every lens is a "Parfocal Lens" to begin with, actually most modern still photography lenses are not parfocal and for that reason the image doesn't stay in focus when you zoom out. I understand the Meteor is a parfocal lens, but a lens must be perfectly collimated to the camera in order to perform as expected. I imagine you bought a camera from eBay that was already converted. Converting the camera is not difficult, but not everyone can set "focal distance flange" to the right distance. This doesn't mean you are not going to get images in focus, this just means that the lens is not going to perform well as a parfocal and if you try to set focus using the witness marks on the lens images are not going to be sharp in focus neither.

You don't have to touch the spinning mirror shutter or the focusing distance to convert the camera, so if you see images in focus in the viewfinder, they should be in focus on your film. Try to fine tune the diopter to a point where you can see the grain on the focusing screen and you should be fine. If you really need the lens to work that way or to use witness marks on the lens take the camera to a technician and she/he can shim or machine the mount in order to get the focal flange distance right.

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Hello Ruben,

Thank you for your reply!

I am aware of the term parfocal lens but as you mentioned the Meteor 5-1 is a parfocal lens therefore I assumed that's not the problem. Unless it got hit in the past and the elements are damaged now.

 

7 hours ago, Ruben Arce said:

Try to fine tune the diopter to a point where you can see the grain on the focusing screen and you should be fine.

Ok, I will do that even though I was setting this up before shooting those reels. I also sent the video I posted here to the previous owner of this k3 and he told me that it might be an infinite focus problem on the meteor 5-1 lens.. 

I also have 2 prime lenses a Pentacon 135mm ; f2.8 and a helios 44-2 which I used for like 2-3 shots on that reel too but didn't notice the infinite focus issue (if this is the case) because I wasn't concetrating on it.

Is it possible to test the infinite focus just through the viewfinder or do I have to shoot an actual reel and wait for it to get it developed and scanned? 

 

Regards,

Vojtech

 

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I doubt the lens is damaged. What that person told you is pretty much the same I'm telling you. When the "Focal Flange Distance" changes the collimation is lost and a parafocal lens won't perform as expected. That doesn't mean the lens is damaged is just that the light rays are not converging at the exact distance they should. This issue is known as "Back Focus" on ENG cameras where the focal flange distance can be adjusted by a piece on the lens. In most cinema cameras that is not possible. A technician makes sure Focal Flange Distance is correct and your lenses should perform as expected.

As I said before what you see is what you get. You mention you tried a couple of prime lenses and you didn't have any problem with those. What you see on the viewfinder is what film is going to capture. If you use the meteor on the same way, adjusting focus on a given position your images should be in focus, as soon as you zoom in or out that is going to change.

Lenses sometimes lose the ability to focus to "Infinity" when the Focal Flange Distance is not correct (Depending on the current distance between the mount of the lens and the film plane) if the FFD is shorter you can focus to infinity, but you lose power on the close end.

You can test the lenses viewing trough the viewfinder yes, and you can get usable in sharp images just by focusing at certain position and not using the zoom. If your lenses are not capable of focusing to infinity I would consider that a problem, but still you can survive without it.

There is no way you can fix this issue. If you truly need the camera to be precise, get it serviced, which is a good idea any ways.

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Posted (edited)

Ok,

thank you for the explanation. 

I've tried the lenses and the result is that the Meteor 5-1 isn't in focus while set to infinity and also the Helios 44-2 has this issue.. Only the Pentacon 135mm f2.8 performed well.

I had completely open aperture while testing the focus.

I also did fine tune the viewfinder's diopter to the point where I see sharp edge of a building, street lamp and some other objects.

 

On 5/7/2019 at 8:54 PM, Ruben Arce said:

If your lenses are not capable of focusing to infinity I would consider that a problem, but still you can survive without it.

I wanted to ask you if you'd know how come that only the 2 lenses are not able to focus to infinity and the 3rd is? They all have m42 mount. I understand that if the FFD isn't calibrated that it might cause this issue of not being either in focus on the close end or inifnity as you explained to me. But why would that happen with 2 of the 3 lenses I have o.o

 

I am not sure if I'll be able to find anyone here in Czech rep. who'd be able to service the camera but I got a contact from the previous owner on some guy which I'll call and we'll see. 

Anyways thanks for your time and help, I appreciate that. 

 

Regards,

Vojtech 

Edited by Vojtech Koblizek
missing information

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Make sure to focus the viewfinder diopter on the grain of the ground glass, rather than on a sharp image. Best is to defocus the lens completely and point to a bright background, so all you see is uniform white, and then focus the eyepiece to see the grain of the ground glass. This way you will be sure to see what is in focus at the film plane (since that ground glass surface should be exactly the same distance from the mount as the film plane, just reflected off the mirror). Because the film plane and the ground glass are locked together, even if the camera flange depth or lens back-focus is out, whatever you see on the ground glass surface is what will be on the film.

It sounds to me like the flange depth of your camera may be a little out of tolerance, probably because of the S16 conversion. It only takes less than half the thickness of a human hair to throw out the focus scale of a lens. There are of course two settings that need to be exact - the flange depth of the camera and the back-focus of the lens - so it's possible that the camera flange depth is OK and 2 of your lenses have their back-focus out of tolerance, but I think that's less likely.

The most likely reason your f2.8 135mm lens doesn't seem affected is because back-focus is less critical with longer lenses. So at 135mm an error of a hair's thickness will barely shift the focus scale, whereas at 15mm the scale will shift considerably. This is why a parfocal zoom that is not seated at the correct back-focus distance will get softer as you zoom out to the wide end.

Also, the slower (smaller) the aperture, the more tolerance there is for variation in the film plane distance behind the lens (much like depth of field in front of the lens is increased), so an f/2.8 lens will be less affected than an f1.8 lens.

 

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Vojtech,

I think Dom Jaeger already  answered your questions and I totally agree with what he mentioned. I have seen those machined adapters that they use to recenter the lens to the S16 area, and I can tell you they are not exactly manufactured at the best standards. The one that I had when I converted my camera was rough and thicker than he original one. I used sand paper to reduce the distance and at some point I was able to focus to infinity, obviously I'm not saying that's the right way to do it, but if you really need the lenses to focus at infinity and you cannot send the camera to a technician that's an option.

I don't want to spread misinformation, so maybe someone else can confirm or deny, but I know that when the flange distance is longer than it should be (45.46mm in this case) lenses won't focus at infinity, on the other side if the distance is shorter than it should be the lens will focus at infinity and you lose focusing space on the short distance.

   

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24 minutes ago, Ruben Arce said:

I don't want to spread misinformation, so maybe someone else can confirm or deny, but I know that when the flange distance is longer than it should be (45.46mm in this case) lenses won't focus at infinity, on the other side if the distance is shorter than it should be the lens will focus at infinity and you lose focusing space on the short distance.

   

 

Yes that's right.

One way to remember is to note that adding extension tubes to a lens reduces the focus range to macro, so it's the same sort of thing - when a lens sits further away from the film plane it focusses closer, and in some cases won't reach infinity.

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hey having a related issue, anyone have some math advice? Original thread here 

 

 

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