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Consistent missed focus on Arri SR2 - Zeiss Super speed MKI


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Hey everyone,

I have recently started to venture into 16mm film and bought an Arri SR2 paired with a set of Zeiss Super Speed MKI (9.5, 12 and 16)

After testing a few stock options I shot a music video and just received the scans back. Both the test shoot and the final images from the music video consistently exhibit similar focusing issues - always focused a little too short (about 30cm to be exact).

Could the camera / ground glass be behind this issue?

Could the lenses be behind this issues? I noticed the 9.5mm and the 12mm to mostly correlate with the soft shots.
 

I do however understand that I am mostly likely the be the culprit, but the consistency of this issue really surprised me.
The ground glass is relatively clean and bright, and seemed tack sharp every single time.
I did take the time to make sure each shot were properly focused, but despite many precautions, many shots came back soft, even while having the camera on sticks.

I plan on having the camera and the lenses serviced but thought I would ask more seasoned ACs for advice. 
 

Cheers!

Val

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Hi!

I can think of three different potential reasons:

a) Did you correctly adjust the viewfinder to your eyesight? (When you do have a camera with attached EVF, then the EVF might also be the problem.)

b) Are you sure that the lenses have been correctly mounted (not tilted etc.)?

c) Could it be that the camera‘s lens mount or the lenses are defective?

 

BTW: Did you check the images on the film with a magnifier? (It could also be a problem with the transfer device.)
 

Did you try printing a Siemens star (e.g. Siemensstern-50mm-56.jpg ) or another focusing aid and pinning it onto a tree? (Alternatively, you could also use a newspaper with large headlines.) Then put the camera onto a tripod and try focusing from a distance of 2m, 4m, 6m and 10m (measured with a yardstick or measuring tape) and check whether the distances on the lens‘ focus ring show the correct distances when you think that you have correctly focused.

Hope this helps

Jörg

Edited by Joerg Polzfusz
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if this is repeatably happening you need to have your camera and lenses serviced. You will need your flange depth checked as well as your fiber screen mount checked. Your lenses will need to be collimated. 

Maybe someone here can recommend a tech or service house in London that can help you. 

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The eyepiece of the camera is similar to a Lensmeter in the sense that you must first adjust the eyepiece to to your eye. Not everyone's distance or near vision is the same. So focus the lens onto an object that is precisely the same amount of meters the lens reads, then focus the camera eyepiece until the image on the ground glass is crystal clear. Try that first, then move on to step two.

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The camera itself most likely needs attention and is a task that only a competent camera tech can undertake with confidence.

Any mismatch between the viewed groundglass texture and the film surface will be more apparent with wide-angle lenses. The wider the aperture, the more apparent the focus issue will be.

The adjustable eyepiece dioptre itself which enables the eye to focus on the groundglass screen is not the issue. 

The likely causes of your problem may be :- 

the position of the textured surface of the groundglass may be incorrect relative to the film plane. 
the rotating shutter mirror surface may no longer be correctly positioned due to wear of a bearing surface. 

Edited by Robert Hart
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Thanks for all the knowledge and advice everyone - I really appreciate!

I had already check the eyepiece and had it adjusted to my sight - hence the surprise when every shot I framed through it was always tack sharp in focus. As highlighted by @Robert Hart, I suspect the issue might come from a mismatch either from the ground glass or mirror position. 

I feel like the next step would be to get both lenses and camera body properly inspected and serviced.

I was planning on getting the SR2 upgraded to super 16 / PL mount and this would also require to have both the mirror and the ground glass changed and calibrated. Hopefully this should solve this issue! 

I will keep you all in the loop to let you know what solved the issue.

Thanks again for all your precious help!

Val
 

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You seem to have arrived at the correct solution already, but I'll add my 2 cents.

There are three calibrations that are critical to reflex film cameras working as they should:

1. The camera flange depth

2. The ground glass depth, as reflected off the mirror - which should match the nominal flange depth

3. The lens back-focus collimation, which should match the nominal camera flange depth

If you only use eye-focussing, then the first two calibrations are the only ones really essential, at least for lenses that aren't zooms or anamorphics. If you want your focus marks to line up, and be sure that they will reach infinity, then you also need the lenses collimated. Professional crews always use lenses with marks they can trust, so they can tape out if needed and also as a double check for eye-focus. Wide angles especially can be hard to judge sometimes just by eye. 

To properly check all these settings on a professional grade camera like an SR2, you really need a technician with the right depth gauges and an auto-collimator. They should be able to check the camera and a set of lenses in under an hour. 

 

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On 4/30/2023 at 1:19 PM, Dom Jaeger said:

1. The camera flange depth

2. The ground glass depth, as reflected off the mirror - which should match the nominal flange depth

3. The lens back-focus collimation, which should match the nominal camera flange dept

 

Hey Dom,

Thank you for your insight, I think you might be right and I will ask the technician to take these point sin consideration while servicing the camera.

Thanks again for the help everyone! I will let you know as soon as I receive the diagnostic.

All the best,

 

Val

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