Focus tolerance on H'blad 500 C/M

[anyhuus]

I've testet my H'blad focus calibration, as I suspected it was out of alignment.

What I did was to photograph at a sharp angle and at a distance of approx 1,5 m,

a strip of paper with parallell lines each on cm a part. One of the parallel

lines had a marking, and I focused on that using the split-image on the

acute-matte ground glass. (The procedure is described in Barry Thornton's book

"Edge of Darkness"). Checking the negative, it seems that actual focus is about

3 cm behind what was in focus on the ground glass.

 

Furthermore, when trying to focus on infinity, the split image does not come

together completely before the lens stops at infinity.

 

I sent the camera to service in order to check the focus alignment. The say that

this camera is well within tolerances wrt. focus, and have done nothing other

than check it with their instruments.

 

My question is, what can I expect in terms of tolerances of focus alignment?

Backfocus of 3 cm at a distance of 1,5 m? What happens with this deviation as

distance increases? Does the deviation increase proportionally, or become less

of a problem?

Should I just avoid the wide open apertures, and let DOF save the day?

[paul_gardener]

Correct setting of camera body, filmback and lenses can only be done according to factory standards by a qualified technician who has the right tools to check and adjust the settings.

 

The words "is well within tolerance" give me an uneasy feeling.

The camera is either spot on or is not spot on.

 

The following procedure should be observed:

 

Body: check condition of all foam strips for older cameras or if the body has the larger mirror check the mechanism especially the condition of bearings.

Set the mirror at exactly 45 degrees.

After that set the groundglass to correct focus.

Before any attempts are made the body should be checked for correct alignment of front to back.

 

The lens should be checked for infinity adjustment with a collimator.

 

Finally the film back should be checked and adjusted so that the film plane is aligned properly.

 

Film backs are often overlooked.

From experience all film backs that have been used for 4-5 years will need adjustment.

 

The error you describe 3 cm at a distance of 1,5 m is too much and is not as your technician describes well within tolerances.

 

In the USA besides Hasselblad USA of course, your camera will be in good hands with David Odess in Massachusetts.

[johan_de_groote]

"The words "is well within tolerance" give me an uneasy feeling. The camera is either spot on or is not spot on."

 

Well within tolerances with 3cm backfocus is a bit off. But everything has tolerances, "spot on" doesn't exist.

[michael_briggs2]

As Johan wrote, there must be a tolerance because the manufacturing process can't set the lens register to exactly the expected distance. The reasonable tolerance can be set by the camera's designers based on the equation for the depth of focus (not field). Starting from the fastest lens that will ever be used on the camera, and the circle of confusion for most critical application, they can calculate the depth of focus. Then they should set the tolerance on the lens to film distance as a fraction of this, assigning some of the tolerance to the camera, to the lens, etc.

 

The equation for depth of focus is +/- CN. With circle of confusion 0.03 mm and faster lens aperture N = 2, the depth of focus is +/- 0.06 mm.

 

An fixed distance error in positioning the lens converts into various errors depending on focused distance. This could be figured out from the equation for focusing a lens (e.g., the Lens Tutorial at http://www.photo.net/photo/optics/lensTutorial): 1/So + 1/Si = 1/f. But you didn't tell us the focal length of the lens that you used. Guessing 80 mm = 8 cm the calculations are:

 

Correct should be 1/So + 1/Si = 1/f with f = 8 cm, So = 150 cm, which gives Si = 8.451 cm.

 

Actual was 1/So + 1/Si = 1/f with f = 8 cm, So = 153 cm, which gives Si = 8.441 cm.

 

The difference is the error in the positioning of the lens: 0.009 cm = 0.09 mm = 0.004 in. So the implied error is very small. It is just outside of my estimate of the depth of focus. This is based on my guess that the lens is 80 mm.

 

The implication for another distance: So = 20 m = 2000 cm.

 

Expected is 1/So + 1/Si = 1/f with f = 8 cm, S = 2000 cm, which gives Si = 8.032 cm.

 

But actual Si is off by 0.009 cm, to be Si = 8.023 cm, so 1/So + 1/Si = 1/f gives So = 2775 cm, which is off by 775 cm. But at this distance the depth of field is typically large, so even this offset may not be noticeable.

 

Possibilities: camera repair shop made mistake and accepted camera slightly out of tolerance. Your test was slightly off, and camera, while perhaps not perfect, is within tolerance. etc.

 

Is the focus off in actual photographs of real subjects taken at fast apertures? If so, I'd send the camera back to the same or a different repair place. If you don't have an actual problem, maybe you don't need to worry about it. You could try repeating your testing. etc.

[michael_briggs2]

Does anyone know what the register tolerance is for a Hasselblad 500 C/M? Is 0.09 mm within tolerance?

[anyhuus]

Thanks all for the answers. I've tested both an 80mm and a 150mm lens, and the error seems abut the same (the third line behind the line I actually focused on on the test-strip is the one in focus on the film. This is the same between both lenses. Also the fact that I can't focus on infinity on the ground glass seems the same between the two lenses).

 

I startet to suspect something was wrong after a session of portrait photos, where a lot of the pictures were out of focus. The distance was around 3-4 meters, 1-2 stops down from wide open. But the focus error is much larger than 3-4 cm, more like 30-40 cm. I don't understand it. I am certainly questioning my skills as well as equipment. To rule out the equipment, I sent it to service. However, I cannot quite put it to rest as it seems that 3 cm off is too much esp. if I use extention rings to make closeup portraits where only the eyes are to be in focus.

 

(How would this deviation materialize when using extention rings?)

 

The service man insists that this deviation is actually quite good, and all we can expect from a camera this old (1974). It makes me feel uneasy as well, esp. if the only thing that is out is the ground glass, and it should be very easy with the right tools to adjust that so it at least have the same focus as the film plane.

[marek_fogiel]

One of the easiest problems to eliminate, could be an incorrect positioning of the

ground glass itself. Remove it from the body and make sure there's nothing that can

obstruct an accurate placement of the ground glass, place it back carefully and redo

the test, maybe you'll be lucky. Another way of proceding would be to focus on a close

distance target by measuring the distance with the tape (you have to measure up to

the film plane), setting the focus on the lens using the distance scale, and doing this

test too - it should give you the idea if the problem is with the mirror/ground glass or

rather with the distance between the lens and the film plane.

[johan_de_groote]

Thanks Michael for that explanation.

[paul_gardener]

The distance scale of the lens is totally irrelevant.

That scale is noting more than an indication as to what distance the focusing point is set.

 

Infinity settings of lenses are adjusted to leave "some" room.

In other words a lens set at infinity will almost certain focus past infinity.

The extra bit is necessary to be able to compensate for temperature.

 

From a physics or philosophical point every setting has a tolerance.

For a photographer exact focus is spot on.

[paul_gardener]

@ Marek,

 

The method you desribe holds a strong risk that you will compensate any wrong settings with other incorrect settings.

 

That is why I decribed the way a qualified repair company and Hasselblad repair centers go about setting mirror and focusing screen.

 

All these settings are in vain when the filmbacks have not been checked to have their filmplane set to factory standards.

[polka]

Michael Briggs gave the correct answer (in fact, differentiating his formula, one finds more precisely 0,085mm) which is the error between the path to the film and the path to the groundglass. Now, what would be the consequences of such an error ?

 

Independently of focal length and for any distance reasonnably outside the macro range (distances attainable with the focusing ring, for instance), this systematic focusing error "e" yields a confusion circle with a diameter "c" depending on the diaphragm "D", following formula :

 

c = e / D = 0,085 / D (mm)

 

Thus for instance for aperture 2,8 :

 

c = 0,085 / 2,8 = 0,03mm

 

This should be considered acceptable (or not ?). It depends... but then, when stopping down, it gets better fast (at 8 you already get c = 0,01mm).

 

Paul

[paul_gardener]

@ Paul Koppf,

 

Any idea about the tolerance Hasselblad permits for the setting of the film plane?

[polka]

I would not care about what H... "permits", because "me, the customer" MIGHT PERMIT !!! (actually not, I do not "permit" their prices to empty my purse), and if I felt that the pictures I take are not as sharp as I would like, after - indeed - checking it is not my fault but theirs, I would ... (well the issue is not pertinent in my case :-).

 

Paul (and after all, why not ask "them" ? they should know... but want to tell ?)

[polka]

OK, let me give a less impertinent answer :

 

All of us are focusing our cameras using in the end the same "device" : our eyes. And

the accuracy of focusing depends thus on the finest details that our eyes can separate -

in their best conditions of viewing ; this, "the grain of our retina" is remarkably invariant

in our species ; its value is grossly 45" , forty five seconds of arc.

 

When you use this parameter to evaluate the best sharpness that you might expect

when focusing your H... (or equivalent MF camera), you find following formula :

 

c = 0,06 / D (mm)

 

Thus taking a picture at aperture 2,8, your eyes alone would limit the accurary of

focusing to approx. 0,02mm.

 

You notice that it is better than the value given by the other formula ?

 

Well in my opinion, an "exception camera" like a H... should not allow worse

mechanical tolerances to add with (and supersede) these kinds of unavoidable human

limitations.

 

Paul

[paul_gardener]

Hasselblad and their technicians at their service centers will tell you all you want to know about factory standards. They are obliged to do so for all equipment that is sold with EU approval: The CE certificate.

 

The tragedy lies in the fact that virtually all filmbacks that have been used for a period of 5 years or more , even if this was not heavy but moderate use, will show that the filmplane alignment needs to be corrected..

 

My Hasselblad technicican with several decades of Hasselblad experience tells me he has not found any filmback used for six years or more that is still correctly aligned.

Hasselblad keeps 0,01 mm as max. tolerance for the filmplane of the filmback.

Meaning the rest of the system is as good as or better than this maximum accepted deviation from the factory standard.

 

I mentioned tragedy because there are surprisingly little complaints from customers about images being out of focus.

I have seen professional photographers using filmbacks that were several decades old.

These old bangers needed to be pressed against the body because they did not align properly.

 

With Anders camera something is terribly wrong, the focusing screen itself could be assembled incorrectly, the body may have been subject to mechanical stress due to a fall, the mirror could be misaligned due to bad foam, adjustment of focusing screen can be set incorrectly to name but a few possibilities.

 

Nothing wrong with buying used gear but keep in mind it pays to have newly acquired gear checked and serviced by a qualified technician.

[anyhuus]

Paul:

I never thought of the backs as a source for the error. I get very nervous when you say something is terribly wrong with my camera - esp. after H... service says that one cannot expect better than what it is now.

 

I have bought another 500 c/m camera - considerably newer (early 90'ies), and tested that in the same way as the old 500 c/m from '74.

 

What i do know now is:

 

1. the old camera focus screen will not focus entirely on infinity (lens focus ring stops before infinity is visibly in focus on ground glass. This is independent of lens.

 

2. on the old camera the filmplane focus about 3-4cm behind focus screen on 1,5 m distance. This is independent of lens.

 

3. the new camera focus screen is precisely in focus when lens focus ring stops at infinity - independent of lens.

 

4. on the new camera the filmplane focus about 0-1cm behind focus screen on 1,5m distance - independent of lens.

 

What I do not know, is

 

1. if filmplane is in focus on infinity - even if focus screen isn't

 

2. to which extent filmbacks are a factor.

 

Will try to systematically test film backs as well now. (I've got 5 of them. With two cameras and four lenses in addition I suppose there will be a lot of film spent testing all combinations. Maybe I can combine with EI and dev.time tests as well :-)

[paul_gardener]

Hello Anders,

 

Sorry, I did not want to upset you but rather explain what is necessary to go about setting up camera alignment for correctly focused images.

 

My conclusion something is wrong with the first camera was based on your information that it is impossible to focus at infinity with two different lenses.

That indicates a problem with the mirror adjustment and or the alignment of the focusing screen.

Besides that the camera should be checked for correct back to front dimensions.

 

Now you have a second body to compare with it may be helpfull to exchange the focusing screen from the first camera for the one in the second body.

An incorrectly assembled screen can also be the cause of the problems with the first body.

 

With the information you have now you may be able to find out if the service center your camera will be serviced at follows the correct procedure.

Official Hasselblad centers will give your camera a good reliable check up.

[anyhuus]

Hi Paul,

 

Thanks for taking the time to provide me with a thorough answer and good advice. I appreciate that.

You're right. With two cameras I now have means to more systematically try to narrow down what the problem actually is.