Canon AV-1 focus issue

[blockend]

A strange one.. When I cock the shutter of my AV-1 the image becomes blurred. It will focus, dimly, but the distances do not correspond with the distance scale on the lens. When I fire the shutter the image clears, and viewfinder sharpness matches the focus scale. Cocking the shutter again makes the problem reoccur.

I've checked whether the focusing screen or mirror have gone out of alignment, and mirror box and lens levers, but all appear to be normal. Any ideas?

 

[MattKing]

Is the lens stopping down when you cock the shutter?

 

[AgX]

Have you checked the mirror both in cocked and uncocked state? Does it in both cases rest on its arrest?
Is the blur uniform, or is there a gradient from top to base?

 

[blockend]

Is the lens stopping down when you cock the shutter?

I think so, I was shooting at f4 so it would only darken the 24mm one stop. However the issue wasn't the finder darkening, it was the focusing being way out. A subject 12 feet away would only become sharp when the lens scale registered 3 or 4 feet. Then the shutter would fire, the viewfinder cleared and focus was a mile out. True focus with a clear finder (and un-cocked shutter) would match the scale. I've never come across this on any camera.

My guess was the Fresnel screen had slipped somehow, or the mirror wasn't seating properly, but both seem ok compared other A-series Canons.

 

[blockend]

Have you checked the mirror both in cocked and uncocked state? Does it in both cases rest on its arrest?
Is the blur uniform, or is there a gradient from top to base?

I'm not sure. There is a film with 24 frames exposed currently, so I'm reluctant to keep firing repeatedly to check what might be going on. I'll swap the film to the A-1 later and give the AV-1 a workout.

 

[AgX]

For a speedy check you can uncouple film-transport from cocking, by this spoiling only one frame.
If you in addition cover the lens or mount at releasing, you even would not spoil anything...

 

[blockend]

Okay so I finished the film this morning, and the phenomenon occurred on about every third frame. With the cassette out of the chamber I fired off fifty frames and couldn't get a repeat. I swapped lenses to see if the problem was lens related, everything fine. My best guess is the mirror was sticky from lack of use, and has freed up subsequently - I use the camera about twice a year. If the mirror wasn't seating correctly, it could conceivably alter the infinity position through the viewfinder. I'll monitor the issue.

edit: back to the drawing board. The frames that were out of focus were also blank.