contax645 focusing issue

[siba]

hello, I spotted a post on a similar issue on this site (from william, nov 2006), but I wanted to be more specific.
Basically, my contax645 + p45 back is not focusing correctly. What I see as sharp on my focusing screen is infront of what is truly in focus. I did a test to make sure, and from 4m (12ft) with my 80mm at f2 (fully open) the focus is 50cm (just under 2ft) off. What is actually in focus is 50cm behind what I am seeing as sharp on my focusing screen. I say on my focusing screen because to make sure it wasn't a problem with my prism I did the experiment also with the prism off and looking at the focusing screen with a loup - even though one can even see the fifference without a loup.

Autofocus focuses correctly, i.e. it looks out of focus through my prism, but is sharp on my p45 back. So, it is not a digital back issue. To be honest, I don't really know how autofocus works. Maybe someone can enlighten me; perhaps it will enable me to understand this problem better.

When I got the camera a couple of years ago I presumed that the autofucus was bad, and I stopped using it. My focusing problems I put down to me just not being a very good focuser. Gradually, though I became conscious that there was something wrong outside of my own capabilities, and during a shoot a month or two back, where I shot everything wide open, the twigs in the tree behind the subject were all in focus while the subject was out.

The problem only becomes a big issue when shooting small apertures because the depth of field compensates when shooting "normally". And it is particularly evident when shooting a person full body, because that 50cm difference means none of the face is in focus at all.

So it took me a couple of years to find out that my focusing problems are in my camera and not in me. On the one hand this came as a huge relief. On the other hand I don't know what to do about it. Can anyone help?

The focusing screen needs to be a couple of mm closer to the mirror for accurate focusing, but I don't feel like tampering with the housing before using up all my other options.

I bought the split focusing screen hoping that it was a focusing screen issue, but the problem remained the same.

Sorry for the extremely long post, but I wanted to make it as clear as possible.

Cheers for any advice, especially if anyone has had the same problem and fixed it.

Stefan

[Morgan_Moore]

How the AF works.. there is a semi transparent area in the mirror that allows light through the mirror onto a lilltle flap behind that mirror that reflects the light into a sensor in the body of the camera, the sensor does some calulations and realises when the image is most contraty and therefore most likely in focus.

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If the AF works and the viewfinder is 'off' then the mirror is seated correctly (at 45degrees) but the ground glass that you are looking at is in the wrong place it is sitting too low or high  - the ground glass needs to be placed very accurately for a P45 res - a hair thickness off will throw the focus out

I am not familiar with contax having only owned mamiyas and blads but..

-it (the GG) may just not be seated right - ie someone has taken it out and put in upside down on just not all the way back in

- it (the GG) may be sitting on some sand or grit or a hair

- it may be that you have some kind of mask to show croplines that is mounted underneath the GG instead of ontop

-the GG may be a replacement that is the wrong type for that model of camera

if none of the above is true you will probably need to get the camera seriviced and recalibrated somehow or at worst buy a new (!) body

it is good news that the AF works because a missalignment of the mirror would probably be a more complex problem

ps most contax chat is on the MF board

SMM

[siba]

thanks morgan for your reply. Thank you for explaining the autofocus.
The fact that you feel the mirror is at the correct angle is positive.
But, unfortunately the simple solutions do not rectify the problem. There's nothing between the housing and the focusing screen frame -  and as I mentioned, I bought a new focusing screen to try to resolve the issue, but the problem has remained. The only thing I can imagine is that there is a fault in a batch of cameras, but why can't I find a link to this problem anywhere on the internet?

[Morgan_Moore]

thanks morgan for your reply. Thank you for explaining the autofocus.
The fact that you feel the mirror is at the correct angle is positive.
But, unfortunately the simple solutions do not rectify the problem. There's nothing between the housing and the focusing screen frame -  and as I mentioned, I bought a new focusing screen to try to resolve the issue, but the problem has remained. The only thing I can imagine is that there is a fault in a batch of cameras, but why can't I find a link to this problem anywhere on the internet?
[a href=\"index.php?act=findpost&pid=169852\"][{POST_SNAPBACK}][/a]

Well the p45 is a tool that requires high precision - haigher precision than the C645 was designed for

Everything in manufacturing is made to a tolerance - a degree of allowed size variation on components

Your contax could be 'in tolerance' ie not faulty but at one end of the 'tolerance scale' your p45 could also be not faulty but at the other end of the tolerance scale

And you would have a system that is both working and not working at the same time

I had a Mamiya 645AFD that just didnt focus with my kodak pro back even after being serviced

(BTW my solution was to bracket the focus when I shot portraits - and finally to buy and H1 and different back)

So, you could

Try getting everything serviced all at the same time - you will need to find somewhere very specailist maybe one of the dealer who post on the MF board can help

OR

This is the fun part

File down the edges of your GG so it sits lower or get an engineering mate to do this

Well you either need to file it down or 'pack' it up I cant work that out at this time of day dependant on whether A is greater or less than B

You can calculate the difference between A and B by seeing how much the lens must be extended or retracted to gain correct focus, its probably about between 1/4 and 1/10 of a mm

Here are some pictures to help explain what I mean..

SMM

[Morgan_Moore]

Further comments.

This shows the importance of a really good dealer when buying an MF back - and very careful testing of what you are purchasing

It demonstrates that the tolerances of old MF camera gear may not be up to the job

Sinar backs have a foil stack the cam be adjusted to tune each back to a specific body - thats why I bought a sinar

Hassy H bodies are most likely built to tighter tolerances than older MF designs - that s why I bought an H1

Although I do not own the 'closed' H system there is great avantage to this in terms of service beacuse you can send the whole rig to one maintainer and tell them to 'just make it work'

The same would be true when purchasing a Leaf back or camera or sinar back or camera

S

[siba]

I can see that there could be a tolerance issue and that with the digital back there's no room for fault. But that wouldn't explain the fact that the autofocus works and so when the camera is in actual true focus it looks out of focus. That's there whether I have the back attached or not.

The kit I bought second hand three years ago, and I only really started using it once I bought the digital back, so there was no option to return it.

I will very soon be delving into what you call the fun part. There seems to be no other option.

Anyway cheers for your thoughts

Stefan

[Morgan_Moore]

Before you start filing make sure you shouldnt be packing

this depends on wether you are getting front or back focus

I would set the camera up tethered

focus using the GG and see if it is front or back focus

S

[siba]

it would be a lot easier if I only had to add distance, but, as I mentioned in the initial post, I need to get the focus plane closer - in other words, I need to lower the focusing screen. Quite by how much I don't know. Probably a mm or even two. I'll go slowly.

[Morgan_Moore]

it would be a lot easier if I only had to add distance, but, as I mentioned in the initial post, I need to get the focus plane closer - in other words, I need to lower the focusing screen. Quite by how much I don't know. Probably a mm or even two. I'll go slowly.
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You can tell the distance by the position of the lens

if you have to turn the lens an 1/'8th of a turn to bring it into focus you may find that it extends or contracts by maybe 1/2 a mm

Then you need to do 1/2 mm

its simple physics keeping that A must Equal B and the relative position of the lens

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If it is beyond two mm Id would be concerned about it working !

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There is another thing to think about

When they make these cameras they masybe calibrate them somehow

So there might be a little screw or set of allen heads that adjust the height of the focus screen

This is why it may be worth a trip to a camera engineer who knows what they are doing (although mamiya UK couldnt do it for me and my Mam)

SMM

[Let Biogons be Biogons]

Before you start filing or packing or doing any other potential damage to your camera and it's components, send it to the Contax Service representative for repair (ToCAd in the USA, TRITECH in Germany, etc.).  For about $200 they will set it right, and do it the right way.  No risk, no trial and error, no inaccuracies, no fumbling around, no hack jobs, no irreparable damage.  You'll get a complete clean, lube and adjust.  $200 is a small amount to keep the camera working as it should, the way it was designed to work (and less than it would cost you to replace the focusing screen after you've ruined it).

[siba]

Thanks for the tip with Tritec. Yes that's certainly the more sensible thing to do. I just wrote to them so we shall see. For anyone else needing their services here is the link: http://www.tritec-service.de.

The only thing is that I can't imagine being without my camera for a month, and that's usually what it takes if I'm going to be sending it off to another country. I'm based in the Czech republic and there's no easy way of renting a contax body.

Maybe there's someone local who's more qualified to do the necessary alterations.

[siba]

Hi morgan. You wrote

"You can tell the distance by the position of the lens

if you have to turn the lens an 1/'8th of a turn to bring it into focus you may find that it extends or contracts by maybe 1/2 a mm

Then you need to do 1/2 mm

its simple physics keeping that A must Equal B and the relative position of the lens"

I've just had a look and obviously if I'm focusing close then I have to turn the lens quite a bit to adjust the focus, while if I'm further away then I only turn the lens slightly.

Am I missing the point?

[Morgan_Moore]

Am I missing the point?
[a href=\"index.php?act=findpost&pid=170536\"][{POST_SNAPBACK}][/a]

Well if A doesnt equal B that would have an effect of moving the back element of the lens to be misplaced by A minus B

Say this is 0.1 mm it could have an effect with a portrait focusing on the ears instead of the face or with a streetscene being backfocused by 5m the street sceen is nearere to infinifty so a certain percentage error will equate to more meterers of BF

I doubt that the image is always out by maybe 0.5m - that would not make sense

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Getting professional service is of course a sensible option but

- I understand you now have have a spare GG so ruining it may not be a problem

- There is of course the time factor

- When I had my Mamiya serviced it did not solve the problem - they tried to blame the problem on the non mamiya back


S