Canon 7 doesn't seem to focus correctly

[Chad]

I can’t seem to get my Canon 7 to focus correctly.

I measured out exactly 2 meters from an object to the front of the lens. I set the focal length on the lens to 2 meters and then adjusted the horizontal focus screw until the object was in horizontal focus. I then focused on the moon and the lens focal length reads a little over 20 meters (not infinity). See attached picture.

So I reversed my procedure and adjusted the horizontal focus to be aligned (when viewing the moon) when the lens focal length was set at infinity. As expected, when I later set up my camera at a measured 2 meters from an object, the lens focal length read a little over 2 meters (about 2.1 m).

The camera returned similar results regardless of which lens I was using.

What can I do about this? Do I have to live with the focus being correct at one distance and not another?


My second focusing problem is that the vertical focus adjustment screw seems to have zero effect. I have screwed it in and out as far as it would go and the ghost object stays slightly above the actual object. Is the screw that I have marked in the picture the vertical adjustment? Why would the vertical adjustment screw not be effectual? Is there a fix for this?

Any guidance would be greatly appreciated.


thank you

~Chad

 

[jlw]

ManGo is correct (and so is Brian, who types faster than I do) in saying that the focusing scale is calibrated for distance from the film plane, not the front of the lens. The film plane is indicated by that black horizontal engraved line just behind the light-meter readout on the camera top.

However, the RF setting you got by setting infinity focus to match the moon should be correct. That setting should produce correct focus on film at 2 meters also. Try it again measuring your 2-meter distance from the film plane instead of the front of the lens, and your focusing scale should be pretty close to 2 meters.

If it's not exactly 2 meters, though, don't assume your RF setting is faulty -- focusing scales usually are engraved pretty accurately, but they're not necessarily perfect. Your best bet is to lock the shutter open using the T setting and check on the actual film plane, using a magnifier. You can make a good 'focus checker' by putting a piece of frosted transparent tape across the base of an 8x magnifier and setting it across the film plane. Make sure the magnifier base rests on the inner film rails, not the outer ones (it's the inner ones that locate the film; the outer ones locate the pressure plate.)

About the vertical adjustment: Apologies for being obvious if I misunderstood your post, but that screw-with-two-holes item indicated by the pointer is not the vertical adjustment screw; it's only the cover for the vertical adjustment screw. Remove it, and the actual screw is farther down. It's a silver-colored screw in a silver-colored surround. You'll need a very small straight-bladed jeweler's screwdriver to adjust it. This adjustment is very touchy -- the image will jiggle frantically whenever you try to turn the screw, so you can't just look through the viewfinder and turn it until it's good. You have to "sneak up" on the correct adjustment by turning the screw a tiny amount, checking, and turning again.

The reason the adjustment is so delicate is the way the adjustment works. The secondary mirror (behind the small RF window) sits on a stainless-steel spring, which is restrained by this screw. When you turn the screw, you're jacking the mirror up or down by allowing the spring to move up or down. Since the spring is naturally jiggly, the image jiggles as you turn the screw. The good news is that once you reach the adjustment point, the pressure of the spring helps lock the screw against vibration etc., so the alignment should stay correct for a long, long time without any further adjustment.

A bit of pedantry in case you're interested: The main improvement in the rare Canon 7sZ (unofficial designation for the final version of the 7s model; only about 4,000 made) was a different vertical adjustment mechanism that doesn't jiggle, making adjustment a bit easier for the repair technician. Instead of a mirror-on-a-spring, it uses a prism in a rotating drum to adjust the vertical position. The drum is on the side of the beamsplitter block behind the main viewfinder window, so on this model the adjustment cover is moved to the left, over the first 'n' in 'Canon.'

 

[Chad]

ManGo, Brian, jlw, thank you very much for your advice.

1. I thought that the horizontal adjustment screw was the only one tha had a cap over it. So the thing with two holes in it is just a cap also? Thats great to find out.
2. I didn't know that I should be making my measurements from the film plane. Also, thank you for the tip about the black line being the film plane.

The distance between the front of the lense and the film plane pretty much accounts for the roughly 10 cm difference I encountered between a measured 2 meter and the lens focal length reading.

Thanks again, I'll readjust tonight and report my finding.


Chad



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[Chad]

*Update*

First I adjusted the vertical focus so that both images were inline. Easy fix once I knew how to do it right. Next I adjusted the horizontal focus so that a measured 2 meters (from the film plane this time) corresponded exactly to 2 meters on the lens. Later I focused on the moon and the lens read a much more accurate value, just under infinity. See attached picture.

Thanks again everyone.


Chad

 

[Chad]

It's best to set the RF with the lens set to infinity, then adjust the RF to coincide with it. Infinity to a rangefinder is about 300ft. What you have is certainly close enough for the 35mm lens, but if you stick a 50/1.2 or 50/0.95, you might have problems.

This seems counter-intuitive; wouldn't you want the RF to be most accurate at close range? 😕



Chad

 

[Chad]

Most repair manuals recommend adjusting at infinity. That is where the lens comes to an absolute stop. Meauring close-up is more prone to error. If your measurement had been exact, the lens would go to infinity with the RF and not stop short. There are other adjustments in the RF mechanism, which control near and far adjustment, but I've never had to adjust them using the infinity technique. I use a Canon 50/f0.95 lens with my Canon 7, and do not have a problem with the close focus.

I see, well I'll readjust then.


Thank you Brian.


Chad

 

[jlw]

Chad, you won't need it to do the simple adjustments you're doing -- but if you're interested, you can download a PDF version of the Canon 7 factory repair manual from the kyphoto website:

http://www.kyphoto.com/classics/repairmanuals.html

For the download you'll need to enter the user name and password shown in the box at the top of the page -- it's just a little trick to prevent automated downloads.

The manual doesn't have as much how-to information as most of us would like, but there are exploded diagrams of all the camera's mechanisms and some useful specs and procedure lists.