Strange Vignetting on RB-SD with 6x8 back

<p>Hi... I'm new. I am wondering if someone have experience with vignetting (strange) on Mamiya RB-SD with 6x8 back. <br>

<br />First, the setup. This is a new-to-me RB-SD with SD rotating adapter and 6x8 motorized back. As this is a RB-SD, there is no "8" on the light baffle. Mamiya support has told me all SD are 6x8 capable right out of factory that they don't put "8" on baffle to distinguish one from the other.<br>

I saw several discussions on this topic on this forum but mine is a bit strange in that one symptom, was never mentioned.<br>

In vertical orientation, the image is 74.5mm in height. No obvious vignetting but the top edge is not well defined. It's perfectly level though and no rounded corners. I think this is supposed to be 76mm in height per spec. (this part was never discussed anywhere)<br>

In horizontal orientation, all edges are clearly defined. Vignetting appears on both lower corners but all edges are sharp and clearly defined. Vignetting is larger on left. On right, it's just barely clipped at the corner. <br>

Looking at the film back, there is nothing obviously out of place.<br>

Looking at the rotating module, there's nothing bent or out of place. It is an SD type.<br>

Looking into the camera body, there's nothing bent or out of shape. Baffle does NOT have "8" but it is an SD. To me, it looks like the bottom side of the inside seem to sit higher than I think it should but again, nothing is lose or out of place, and I have no other RB to compare against.<br>

I will be grateful for any input or experience anyone can share. Thank you.</p>

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<p>Haven't we seen these exact same pictures before, in another thread?</p>

<p>Put the camera on a tripod and level it. Remove the lens. Trip the mirror so that it is up. Remove the film insert from the back and leave the shell door open. Turn the camera so that the open film holder is facing the light source. With the focus back into the body look at the film frame edges from the lens mount. You should be able to see the frame edges from the center of the lens opening without looking at an angle. Gradually turn the focus outward while observing the frame opening. The vignetting you are seeing on the film will be 1mm or less in the light path of the camera.</p>

<p>Can't see it? Try a combination square against the film rails of the film holder at the corners. Adjust the square as needed to determine the cause.<br>

<a href="http://www.amazon.com/Empire-Level-E255-Combination-Graduations/dp/B000ETWQ4U/ref=sr_1_8?ie=UTF8&qid=1352588212&sr=8-8&keywords=square">http://www.amazon.com/Empire-Level-E255-Combination-Graduations/dp/B000ETWQ4U/ref=sr_1_8?ie=UTF8&qid=1352588212&sr=8-8&keywords=square</a><br>

The back adapter or film holder not sitting centered on the camera body, a slight sag/deformation of the bellows, or light seal protruding into the light path are the most likely causes. </p>

<p>Do you have other lens and film holders, if so try them to see if they show the same problem.</p>

<p>That was extremely helpful. Thank you very much.<br /> I wonder if I can trouble you with one more thing?<br /> Doing the test as described, I can confirm, I cannot see the bottom edge of the film gate, so I started to look at the causes.<br /> I took the rotating assembly off. Something is very odd about this. <br /> I measured the opening on the rotating back. The long side measures 74mm. I then measured the opening of the film back. 74.5mm. Meaning part of it will always be obscured. Also, if I look at the rotating assembly in the vertical orientation, it looks like the opening is skewed lower (sagging) Also the bottom edge is so close to all other parts that it may be slightly being obscured causing top of my image not having a crisp edge.<br /> Does SD rotating back adapter supposed to look like this? Do I possibly have some botched repair job with mismatched parts??</p><div></div>

<p>It has been close to 20 years since I've had a RB in my hands. </p>

<p>It is obvious that the film frame opening is not centered on the revolving adapter. Can the film holder be removed, turned 180° and reattached and if so does that center the image frame?</p>

<p>Are the revolving adapter's alignment pins 90° to the adapter? The top right one in your picture looks like it may be slightly bent but that could just be the camera angle.</p>

<p> </p>

<p>No, on turning 180 degrees. It only goes one way. There are two pins that will prevent mounting it the wrong way.<br>

No, it's not bent... it's just my poor lighting.<br>

Pins are not bent. They are perpendicular to the aluminum body.<br>

Hum....</p>

<p>It is obvious now the fault is somewhere in the body/revolving_adapter. I just ordered another body/adapter because I wanted a second body anyway. In a few days, I'll be able to compare two bodies side-by-side.</p>

Have you seen the results produced by other 6x8 backs on the same model camera? Or could anyone who has (i never used a 6x8 on the RB) chime in?<br>It could also be - i suspect - a thing about Mamiya wanting to wring out too much (6x<b>8</b>) out of a camera that wasn't made to deliver that, getting away with it, but only just, producing what you have reported.

<p>No... I haven't seen any scans except for my own. I was hoping someone on this forum might have the same setup. I've heard from someone who says he has the same one saying edges are clear but no visual confirmation.<br>

Yes, it is entirely possible Mamiya tried this when body was really not designed to do that in the first place. The fact that 6x8 really is 6x7.5 shows it. <br>

Hopefully a second body will show me conclusively.</p>

<blockquote>

<p>The fact that 6x8 really is 6x7.5 shows it.</p>

</blockquote>

<p>Everybody's "6x8" is only 6x7.5 or 6x7.6, so that doesn't signify anything really. MF format sizes are always rounded up: 645, 6x6, and 6x9 are almost as bad. Only 6x7 is close to being accurate, and even that varies between brands.</p>

<p>What I wonder is which lens you used on your RB67? I gather that some of them cover 6x8 better than others. Might be a factor in what you're seeing.</p>

<p>I used 127mm K/L.<br>

I thought about the lens factor but if that was the case, it should show up as corner being rounded. I see just 1mm or less being clipped at the bottom end. <br>

I should get my second body sometime next week. It'll be interesting to see two side by side....</p>

Re nominal format dimensions: the "6" in 6x[...] always is 'short' too. Considering that it will be about 56 mm instead of nominal 60, true 56 x 74.5 matches the nominal 6x8 aspect ratio just about perfectly.

<p>I believe I now know the reason.<br>

The second body arrived. They are identical to each other visually. Unfortunately, this body is defective so I could not shoot with it.<br>

However, in process of comparing both bodies carefully, I noticed, with back not secured (the sliding locks in open positions), the back slides along the track just about 1mm. I am able to secure this in any position of this 1mm play. This play was larger on the new body.<br>

I then pushed the back in the direction that will give me more room at the bottom (top if you are looking at the negative image) then locked. Took test shots and processed.<br>

The film is in wash right now so I can't scan but seems that was the issue. Manufacturing tolerances. Apparently, Mamiya barely got the size so if the back shifts a little, it results in the issue I am having.<br>

When I get a working second body, I can do more conclusive testing.</p>

<p>Hey Taka,<br>

This is NORMAL. <br>

I have Mamiya RB system too and get exact same "vignetting" with 6x8 back. Took me a while to figure out.<br>

The 6x8 instruction manual alludes to it by giving various image area dimensions for each lens/focus distance combo.<br>

There is nothing wrong with your camera system.</p>

<p>Andre,<br>

Thank you. <br>

Would you mind sharing your instructions manual? I looked everywhere and could not find it.</p>

<p>Andre has kindly sent me the information. Based on this, what I am seeing appears to be normal. Thanks Andre and everybody for helping me out.</p>