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Setting SLR Mirror Angle With A Collimator

ic-racer

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In most all SLRs the mirror pivots at the upper portion of the film frame. Mirror angle has very little effect in that area, as the pivot is not moved with the mirror angle adjustment.
In the viewfinder, the top of the film frame is the bottom of the viewfinder image.

So, adjusting the mirror angle affects focus at the top of the viewfinder image.

Assuming the lens has allready been set for infinity focus, if the top of the viewfinder is blurry at infinity, the mirror angle could be off.

A collimator makes a good infinity source.

In this case the top and bottom of the viewfinder image are checked with the green star projected by the collimator.

Each camera has a different means of adjustment.

Yashicas have a screw, accessible from the front, that alters the mirror angle.


 
In this case the collimator (which normally projects downward) is bounced off a mirror on the base of the device to make it more convienient to view the image with one's eye on the viewfinder of the camera.

Camera lens set on infinity.

In this case the mirror angle is correct.

Yes, the focus screen could be askew, but this technique is more for when one replaces the mirror, or fiddles with the mirror box and needs to put it back together correctly.

 

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First, I want to say thanks for these posts. When I was just getting started repairing cameras, I didn't have too much trouble finding service manuals and disassembly guides. But what really seemed to be lacking was content on camera fundamentals. How do cameras work. How do you check that they are working properly, etc. We need more posts like this.

Now I'll add my two cents...In addition to uniform focusing, the mirror angle can also affect parallax. If the mirror is slightly low or slightly high, the image projected to the focusing screen will be shifted vertically. I actually read in some National Camera article that you can use this as another way to check mirror angle. Setup your camera on a tripod and point it at a scene with details on the top and bottom edges (I use a test chart on the wall). Put a ground glass in the film plane and open the shutter. Observe the edges of the image at the film plane. Close the shutter and observe the image in the viewfinder. The vertical framing should match (but the image in the viewfinder will be slightly cropped on most cameras).

The one skepticism I had on this method was the accuracy of the viewfinder image. We know that most cameras don't have 100% viewfinder coverage. It's typically around 90%. And one of the reasons for the smaller coverage is to cover up inaccuracies in the image pipeline. The makers basically said, we can guarantee whatever is in the viewfinder will be in the final image, but the borders a crapshoot. So if that's the case, I'm not sure I can use the difference between the viewfinder framing and the image projected on the film plane to set something as precise as mirror angle. Thoughts?
 
If you set mirror angle regularly, the easiest technique is to use a 45 degree camera holder. This way, all you do is put the camera on the holder without a lens and center the star target in the collimator finder. This takes the camera viewfinder out of the equation. Of course, it may take a bit of work to construct a 45 degree jig but worth it if you do this procedure regularly. The photo shows two jigs that I use, one in the foreground is for 35mm cameras and the other is for 645 format.
 

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I was hoping you would chime in! I did see you referenced 45degree jig in a prior thread, now I know.

BTW do you have a micrometer lens base? It is hard to find any pictures of them outside the cine world.
 

Good points. On the Yashica in question, that would be screw #3 (upward/downward) mirror position.

What I have done in the past for framing is to take a picture of a dry-erase board with a hand-drawn exact outline of the viewfinder image on the board. First did that in the 1980s checking/confirming the moving parallax frame on my Rolleiflex TLR.

In the 1980s, in my young, absolutist, concrete thinking ways, I even put the negative with the frame outline in my enlarger an fixed my easel blades exactly on the viewfinder image...wouldn't want to include anything in the image I didn't actually see.
 
ic-racer said:
I was hoping you would chime in! I did see you referenced 45degree jig in a prior thread, now I know.

BTW do you have a micrometer lens base? It is hard to find any pictures of them outside the cine world.
Click to expand...
Yes, I do have the micrometer lens base. Here is a photo of it with one of the lens mounts removed showing the spindle.
 

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The camera in question, by the way, was a Yashica FX-3 in which I re-affixed the mirror. The two-sided tape I used may have not been the same thickness as the original as the top of the viewfinder screen was obviously not in focus after the repair. Previously I'd have had to wait until daylight to go outside and fiddle with it. The collimator allowed me to do this at night right in the workshop.
 
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