Back focusing with Hasselblad 500 c/m.

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phass

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Hi all.
I’m having back-focusing, around 6 cm at distance of 1 m when I put A16 back.
Any thoughts about this?

Cheers.
 

4season

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I’m having back-focusing, around 6 cm at distance of 1 m when I put A16 back.
Any thoughts about this?

Maybe rotted foam behind the mirror causing inaccurate focusing. But this would affect all film backs, not just your A16.
 

Sirius Glass

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CLA time
 
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phass

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Thank you for prompt response, I should of mention that I have A12 which is not backfocusing.
Apology for not mentioning this.
 

Light Capture

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Thank you for prompt response, I should of mention that I have A12 which is not backfocusing.
Apology for not mentioning this.

If your other back is working correctly, the issue is likely with film plane on A16 back.
Back sides that pressure plate is pushing against are bent. It's standard factory recommended procedure during back service to check and calibrate depth of the sides to the front surface of the back.
Distance should be 3.55mm +/- 0.05mm.
Other issue could be if there is an issue with the insert. Also inserts for A12 and A16 are different. I didn't do any testing so far if swapping them would change anything.
If sending this out for service, it should be mentioned. Not everyone will do this check routinely or have a method to measure it precisely.
 

Philippe-Georges

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Maybe rotted foam behind the mirror causing inaccurate focusing. But this would affect all film backs, not just your A16.

In a 500C/M, there is no foam near the mirror when it is down, only when the mirror is up for exposure, there is just a hook to hold it down, which can be off too.

But, as it seems to occur only with that particular filmback, then it is interesting to have close look at the underside of the A16 back.
In the two holes, where the two hooks of the body must fit, there should be thin pieces of (hard-) sheet metal up against the front frame of the filmback.
These are meant to trim the right position of the filmback.

These tiny pieces, hardly a few tenths of a mm thick, can easy be lost, particularly when disassembling the front frame for maintenance like (DIY-) replacing the baffle foam of the slide slit.
It looks like a trifle but it seems to be important...

On the other hand, the frontframe of the filmback can be a little off too (if not the whole filmback it self).

BTW, the construction of the inner mechanics holding the film, in particular the position of the film plane/pressure plate, aren't that easy to be knocked off, mainly the transportation system is frequently disordered, but that one is located in the inside of the shell (which is a nasty flaw of Hasselblad).

But, as a disclaimer, this is a very personal guess and I might be wrong as I haven't seen the film back!

Anyway, a thorough CLA might be indicated...
 
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@Philippe-Georges:

There are 3 foam pads between the mirror and the plate that holds the mirror in the mirror frame. (see the attached picture from the service manual found all over the internet. #4 = pads, #6 = mirror frame, #5 = mirror)
It seems that almost no one knows about them and they are frequently confused with the well known mirror damping foam strip at the focusing screen, but they are
equally important. When they deteriorate, the mirror is not held against its 3 bearing points anymore. This points can be seen as two little nubs in the front of the mirror and
one in the back. The frame #6 has the nubs on it and the mirror rests against them. If there is a gap, the pads are bad. It is best checked when cranking the camera
and the mirror comes down. It moves a bit skewed until it locks in the mirror-down position.

Remarks: This is probably not the cause for phass' problem, but important to mention with the older cameras, since they will be going bad with time, no matter what.
It can drive the users crazy. I bet that a lot of hasselblad owners have this problem but do not know that this pads even exist.
I had a lot of misfocused slides and then i remembered that there are those pads, had them replaced and now my focusing is razor sharp again.

The two photographs show the position of the nubs. If the mirror is in tight contact with them, you are fine. If there is a gap, you are in for service.
Mine has been serviced recently, but before, i could fit a strip of a business card between the mirror and the nub....

edit: typo
 

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Hi all.
I’m having back-focusing, around 6 cm at distance of 1 m when I put A16 back.
Any thoughts about this?

Cheers.

The film blame is not precisely lining up the focus in plane, but corrected that, without a good technician will be difficult.
 

Philippe-Georges

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@Philippe-Georges:

There are 3 foam pads between the mirror and the plate that holds the mirror in the mirror frame. (see the attached picture from the service manual found all over the internet. #4 = pads, #6 = mirror frame, #5 = mirror)
It seems that almost no one knows about them and they are frequently confused with the well known mirror damping foam strip at the focusing screen, but they are
equally important. When they deteriorate, the mirror is not held against its 3 bearing points anymore. This points can be seen as two little nubs in the front of the mirror and
one in the back. The frame #6 has the nubs on it and the mirror rests against them. If there is a gap, the pads are bad. It is best checked when cranking the camera
and the mirror comes down. It moves a bit skewed until it locks in the mirror-down position.

Remarks: This is probably not the cause for phass' problem, but important to mention with the older cameras, since they will be going bad with time, no matter what.
It can drive the users crazy. I bet that a lot of hasselblad owners have this problem but do not know that this pads even exist.
I had a lot of misfocused slides and then i remembered that there are those pads, had them replaced and now my focusing is razor sharp again.

The two photographs show the position of the nubs. If the mirror is in tight contact with them, you are fine. If there is a gap, you are in for service.
Mine has been serviced recently, but before, i could fit a strip of a business card between the mirror and the nub....

edit: typo

I was not aware of these tree foam pats between the mirror, thank you for that information.
And yes, this might not be the cause of Phass's problem.

I once lost one of these tiny pieces while replacing a foam baffle in a film back, I copied one out of the pressure plate of a Polariöd cassette. These sheets of thin metal are very handy for 'emergency interventions'...
 

4season

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This is probably a stupid question, but when you loaded the film insert, did you ensure that the film sat beneath the silver metal tab "C"?
Hasselblad A12.jpg
 

Philippe-Georges

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This is probably a stupid question, but when you loaded the film insert, did you ensure that the film sat beneath the silver metal tab "C"?

This is indeed a pertinent question, but it is quasi impossible to insert a film holder loaded like this in a Hasselblad film back shell.

Not only the backing paper wil be that 'convex' that it hinders the film holder sliding in the shell, but the paper will be crumpled if not torn.
This is actually the main purpose of that tab, holding flat the backing paper, and down the pressure plate for facilitating the insertion.

Once the insert is properly fitted in place and the film back is closed, the key can be turned in to locking position and the pressure plate will be released, and the tab will the let go the paper...

But ofcourse, Phass will have experienced it.
 
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phass

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Thank you all for helpful info.
The distance between front plate of the back and internal rails is in 3.55 +- 0.05 mm.
And back paper was loaded correctly.
Is any way to calculate the thickness of shims to move front plate of the back. Though, I do not see the way to move the front plate without creating a light leaks.
Cheers.
 

Philippe-Georges

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Compare the condition of that 'bad' film back with a good one, perhaps an eventual difference will point in the direction of a solution?

Good luck.
 
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correction/shimming withouth being able to measure the flange distance of the whole system is dangerous.

i do not know exactly, but as far as i can see, the magazine has no means to adjust the distance except for pressing the film ledges of the inner frame. if someone knows better, i would like to know how it is done.
the manual says nothing about shimming.

for information: i used a caliper with its depth probe to measure the whole camera from the flange lugs (where the front bayonet plane is defined) down to the corners of the pressure plate of the magazine, which should be in the film plane.
the total distance is 74.9mm. but this requires great care to get the angle of the probe right and it is very easy to displace the pressure plate with the measuring force. (or to scratch something!!)
but it was the only way for me to check things because i did not have any measurement fixture at that time.

@phass: have you checked the issue with the mirror foam pads and the gap between the nubs? (the one where i postet the photographs)
check it anyway, in my case it was intermittent and it could look as if one magazine is the culprit while it was just coincidence.
 

Sirius Glass

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Just take it to an authorized Hasselblad repair place where they can put the body on a jig and put the body back into the shape of the factory specifications. Why are you making things hard on yourself?
 
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@Sirius Glass: They are all serviced by my local technician, but i wanted to be able to check it myself after a hard bump or something like that. So i made a jig for the flange distance and some time in the future, i
plan to build the full jig to measure flange distance and the mirror angle too.

I absolutely agree with you. If something is wrong, give it to someone who knows what he does.
 
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phass

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phass

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correction/shimming withouth being able to measure the flange distance of the whole system is dangerous.

i do not know exactly, but as far as i can see, the magazine has no means to adjust the distance except for pressing the film ledges of the inner frame. if someone knows better, i would like to know how it is done.
the manual says nothing about shimming.

for information: i used a caliper with its depth probe to measure the whole camera from the flange lugs (where the front bayonet plane is defined) down to the corners of the pressure plate of the magazine, which should be in the film plane.
the total distance is 74.9mm. but this requires great care to get the angle of the probe right and it is very easy to displace the pressure plate with the measuring force. (or to scratch something!!)
but it was the only way for me to check things because i did not have any measurement fixture at that time.

@phass: have you checked the issue with the mirror foam pads and the gap between the nubs? (the one where i postet the photographs)
check it anyway, in my case it was intermittent and it could look as if one magazine is the culprit while it was just coincidence.
I keep this film back as back up one. I accidentally found its problem.

I was able to check the mirror path by comparing the distance from the object - the film plane with the lens scales (I have 80 mm CF and 50 mm CF) which eliminates the mirror/the focusing screen error.


Just take it to an authorized Hasselblad repair place where they can put the body on a jig and put the body back into the shape of the factory specifications. Why are you making things hard on yourself?

My original setting - body+back is fine, so I'm very hesitant to mess with them. I also know that all shops are for several moths back logged. Perhaps in the future I'll do something about it, unless I was lucky to got a frankenstine.
 

Sirius Glass

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Sorry, I missed you question. This back is from 1991.

The age does not matter. I have older film backs. The back needs to be serviced by a Hasselblad repair person.
 

Sirius Glass

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I keep this film back as back up one. I accidentally found its problem.

I was able to check the mirror path by comparing the distance from the object - the film plane with the lens scales (I have 80 mm CF and 50 mm CF) which eliminates the mirror/the focusing screen error.




My original setting - body+back is fine, so I'm very hesitant to mess with them. I also know that all shops are for several moths back logged. Perhaps in the future I'll do something about it, unless I was lucky to got a frankenstine.

Where are you located? If we knew we could suggest less backlogged places. US is much to vage.
 
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phass

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Light Capture

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Thank you all for helpful info.
The distance between front plate of the back and internal rails is in 3.55 +- 0.05 mm.
And back paper was loaded correctly.
Is any way to calculate the thickness of shims to move front plate of the back. Though, I do not see the way to move the front plate without creating a light leaks.
Cheers.
I would agree with you that if your other back and body are working fine, there is no issue there. Especially if you were able to check and measure focal flange distance.

There are couple more possibilities. Hasselblad factory jig for back and body have flat surface. Body and back mating surface is sitting on corresponding flat surface and flange distance is calibrated/measured from flat reference surface.
If there are slight distortions on the back or body mating surface they will measure fine but if that distortion is mismatched on a body/back pair there could be an issue with focusing. It's not very likely but it's possible especially if there was a distortion or drop of some kind at some point.

Film back is calibrated with a flat surface with hole in the middle and rails are measured in reference to the whole mating surface of the back.

Other possibility is that there is distortion somewhere on insert. Check the spring tension on film pressure plate. Is it weak or strong? Is pressure plate pushing evenly on the rails and touching them all around? If there is some resistance when locking the insert in, flap retraction mechanism might be bent and slightly affecting pressure plate position on the right side.

Loading backing paper on spools or a roll of film if you have a waste roll and observing how the front surface looks can also help.

Another option is removing the front plate on the back to see if there is anything between the plate and back body. There could be something pushing small area of the plate out.
 

Sirius Glass

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I am from Ohio.
Cheers.

I suggest David Odessa or Samy's Camera, 431 S. Fairfax Ave., Los Angeles, CA 90036, Telephone: 323-938-2420 ask for Mike
I use Samy's because it is close by and I get great service.
 
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