Alignment of IQ back

[cunim]

OK, I think I understand it. Disclaimer: This is my own amateur analysis, and may be totally wrong.

The problem is flex between the extension box for longer lenses and the camera body. The camera itself is not misaligned.

To position the 120 and 150 mm Apo Digitars, Arca supplied extension boxes that mount to the rear of the 3Di.

That puts the heavy back on a long lever arm. The camera body flexes on top, where the box/back assembly attaches to it. This results in a bit of "droop" in the sensor relative to the optical axis. Droop is, of course, equivalent to backwards tilt.

With a 72 Apo Digitar - no extension box - there is no flex and everything lines up nicely. Same with the longer lenses on the monolith. No extension boxes, no flex, and no misalignment.

Conclusion: If you are using a 3Di with extension boxes and a heavy digital back, expect a bit of tilt misalignment.

Note: I see pictures of new Arca R-mount Apo Digitars which have spacer tubes between the lens and camera instead of on the rear.

Lenses Schneider-Kreuznach APO DIGITAR N 5.6 120mm for Rm3D

Much less weight hanging off the spacer that way so I expect flex is not a problem. Anyone using the tube spacer?

 

[jlm]

with the extension on the lens, the extra cantilever load would be picked up by the tilt mechanism

 

[darr]

The problem is flex between the extension box for longer lenses and the camera body. The camera itself is not misaligned.

... The camera body flexes on top, where the box/back assembly attaches to it. This results in a bit of "droop" in the sensor relative to the optical axis.

What type of material are the extension boxes made from?

I ask because I use multiple extension boxes with my ALPA gear (macro work) and have never experienced flex with the extensions or camera body.
It is difficult to believe AS did not test their camera + extension boxes just for this very thing.

Has anyone else anywhere (forums, blogs, etc.) ever discussed this idea before?

 

[cunim]

What type of material are the extension boxes made from?

I ask because I use multiple extension boxes with my ALPA gear (macro work) and have never experienced flex with the extensions or camera body.
It is difficult to believe AS did not test their camera + extension boxes just for this very thing.

Has anyone else anywhere (forums, blogs, etc.) ever discussed this idea before?

I think they are aluminum but, be clear, the boxes are not flexing. Appears to me that the flex is in the interface of the digital back adapter to the 3Di. And let's be careful about deciding that is a problem. Some flex may be within the design spec for a light camera which has so many adjustment features built into the frame. I see no flex whatsoever in the Monolith, for example, but try carrying that around. My lovely Alpa Max was also very rigid (yes, I miss it) but less convenient to use than the 3Di, and that fine focussing capability sure is nice.

Also, please remember these are observations done by one pixel peeping eye and by one person. Before taking them seriously, I would like to see comments from a few other 3Di owners who have tried something similar and found it to be an issue. I have also asked Martin his impressions. This may just be the ravings of an amateur who lacks the experience to know when he has it good (me).

 

[cunim]

By asking about the box material, Darr got me thinking.

I have two AS-Mamiya adapters. Both are held tightly in the Monolith and the 3Di. I have two identical extension boxes (for the 120 and the 150 apo digitars). Both rock slightly in and out at the top when inserted in the 3Di. I haven't checked if they rock in the Monolith but I will.

That's the problem. The camera is fine. The thin aluminum extension boxes are a bit warped. The rocking at the top is minor, but it is enough to tilt the sensor plane a bit and affect images made with the longer lenses.

If anyone else out there has extension boxes for AS R mount lenses, can you let me know if you see a similar effect? Mount the box in the camera and press gently in at the top. Release. Does it move a tiny bit in and out? Now try the same thing with your camera back adapter in the same slot. Is it held more tightly?

Reason I ask is to find out if this is a design issue (affecting all boxes) or if I just happen to have a couple of lemons.